Software-Defined Networking and Open Networking

Software-Defined Networking and Open Networking
Software-Defined Networking and Open Networking
Understanding Foundational Concepts and Constructs v1.1
Victor Lama
Dell Network Solutions
Enterprise Campus and Data Center
G500 Banking & Securities
Initial Release
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Table of Contents
Introduction............................................................................................................................................... 5
General SDN and Open Networking Concepts .....................................................................6
What is SDN? ..............................................................................................................................6
What is Open Networking? ......................................................................................................6
What exactly is disaggregation and why is this development in the networking
industry significant?................................................................................................................... 7
Is there any special requirement for installing a third party operating system on Dell
Networking bare-metal switches? .........................................................................................8
How does ONIE work? .............................................................................................................8
So-called off-the-shelf “merchant silicon” features highly in discussions about Open
Networking. What is it and why is it relevant?...................................................................... 9
Explanations of SDN typically consist of references to a control plane, a data plane
and a management plane. What are they? ........................................................................... 9
Discussions around SDN network design oftentimes focus on what are called leaf
and spine or Clos architectures – what are they? ............................................................ 10
Are Dell’s Open Networking solutions considered SDN? ................................................ 11
What are some of the abstractions used in SDN? ............................................................. 12
SDN is oftentimes equated with OpenFlow. Is that the only “southbound” protocol
used by SDN controllers? ....................................................................................................... 17
What is the Northbound API used for in an SDN Network? ............................................ 19
SDN-based solutions are oftentimes categorized in terms of underlay and overlay
networks. What are they and what is the difference? ..................................................... 20
What problems does SDN solve and what are some real-world use cases? .............. 22
Dell OS10 and Third-Party OS Vendor Solutions ................................................................25
Dell OS10...................................................................................................................................25
What are Dell Networking OS10’s most salient features and innovations? ......... 26
Control Plane Services.................................................................................................... 27
Switch Abstraction Interface ........................................................................................ 28
Common Management Services ................................................................................. 28
Cumulus Linux ......................................................................................................................... 31
What is Cumulus Linux? .................................................................................................32
Is Cumulus Linux an overlay or underlay solution? ..................................................32
Does Cumulus Linux run OpenFlow or any other “southbound” protocol for the
purpose of communicating with a centralized SDN controller? ............................32
Can Cumulus Linux be deployed in an NSX/VxLAN environment? .......................32
Can Dell switches running Cumulus Linux be deployed in an OpenStack
environment? ...................................................................................................................33
On which Dell Networking switches can Cumulus Linux be deployed? ..............33
What Services and Support model are in place for a customer who buys a Dell
switch with Cumulus Linux? ..........................................................................................33
Big Switch Networks (BSN) .................................................................................................. 34
Big Cloud Fabric (BCF) ................................................................................................... 34
Big Monitoring Fabric (BMF) .......................................................................................... 37
IP Infusion ................................................................................................................................ 38
Pluribus Networks .................................................................................................................. 39
The last several years have a seen a sea-change of innovation in data center
networking. An entirely new lexicon has emerged that includes terms such as
software-defined, underlays, overlays, programmatic control and centralized
controllers. They fall under the rubric of what is termed Software Defined Networking
(SDN) and they represent a major paradigm shift regarding the manner in which
networks are considered and deployed.
Without an understanding of some basic SDN concepts, appreciating the significance
of these developments can present a challenge to even the most seasoned IT
professionals. In fact, even networking professionals who don’t understand the
architectural implications of SDN and do not have a background in fundamental
software constructs may find themselves overwhelmed by the ecosystem of
software-based networking solutions, where they fit into the new architecture, and
the corresponding jargon that is used to describe their functionality.
The goal of this white paper is to begin the process of demystification by taking a
taxonomic approach to understanding the lingua franca of the world of SDN and
Open Networking, especially as it pertains to Dell. The reader can then expand that
knowledge through further reading.
For ease of reading, the information in this paper is presented in a question-andanswer format. Some questions are designed to help seasoned networkers refine
their knowledge, while others are meant for IT generalists, who are intellectually
curious and seek to broaden their horizons. To ensure that non-networking focused
IT professionals can appreciate certain architectural principles associated with SDN,
some basic legacy networking concepts will also be touched upon.
Section 1.0 of this paper offers some general knowledge regarding SDN and Dell’s
Open Networking initiatives. Terms and phrases that appear with relative ubiquity in
the course of SDN discussions are referenced and deconstructed in this section.
Section 2.0 focuses on specific vendor offerings from Dell’s third-party ecosystem of
Open Networking partners.
Note: This white paper may be updated periodically. It is advisable to look for the
latest version.
General SDN and Open Networking Concepts
The following section is an FAQ-formatted overview of some of the most important
foundational concepts around SDN and Dell Open Networking, their relationship to
each other, the architectural constructs they exploit, the innovations they offer, and
the lexicon used by computer scientists and vendors to describe all of the above. This
section will also cover some general networking concepts.
What is SDN?
For the last 25 years, TCP/IP networks have been built using well-known distributed
communication protocols, such as the Spanning Tree Protocol and the Routing
Information Protocol. As Professor Scott Shenker, one of SDN’s pioneers, explains,
without the necessary software abstractions in place, computer scientists have been
forced to “master the complexity” of writing distributed algorithms on vertically
integrated packet forwarders. SDN addresses these shortcomings by centralizing
control and providing a standardized interface to configure network forwarding state
in a dynamic and programmatic fashion. Stated otherwise, SDN is about creating
programmable networks.
This topic will be addressed in much more depth in subsequent questions.
What is Open Networking?
Within the context of Dell Networking, Open Networking refers to the disaggregation
of a network switch’s operating system (OS) from the underlying hardware. A switch
that does not ship with a vertically integrated OS from the hardware vendor is
commonly referred to as a bare-metal switch or a White Box. When the bare-metal
switch is provided by an established network hardware vendor, like Dell, it is referred
to as a Brite-box (a concatenation of white and brand name box).
In general, Brite-box vendors offer a “safer” approach to disaggregated networking.
By choosing Dell, one can take advantage of a global footprint, a world-class supply
chain, and a 24x7 follow-the-sun support model. As an industry pioneer in the
disaggregated model, Dell offers operating systems from several different vendors
that can be loaded onto a bare-metal Dell switch, such as Cumulus Linux, Big Switch
Networks Switch Light or IP Infusion’s OcNOS. Dell also offers its latest Linux-based
OS10 operating system as part of its disaggregated model.
What exactly is disaggregation and why is this development in the
networking industry significant?
The decoupling of an operating system from the underlying hardware is known as
disaggregation. This allows network architects the flexibility to deploy the operating
system and hardware platform of their choice, independent of each other. This is
similar to the manner in which an x86 server from Dell or HP can load an OS from
multiple vendors, like Microsoft or Red Hat. The opposite of this would be a vertically
integrated switch from vendors like Cisco Systems that come with the proprietary
NX-OS software, or a Dell Networking switch with Dell OS9 loaded.
Disaggregation in networking has opened up the market to fierce competition
among third-party software vendors whose objective is to bring differentiated
solutions to market before the other. Competition breeds innovation and better
economics for the consumer.
A software-driven network design also has its advantages in terms of agility and the
ease with which it can be adapted to support future network demands. Since the
network’s persona (its forwarding and processing logic) is abstracted from the
underlying hardware, the network can be redesigned by changing the software that
defines it. Meanwhile, all the networking hardware can remain the same.
Vertically Integrated
Traditional Switches
White Box Switches
Brite-Box Switches
Cumulus, Big Switch,
IP Infusion, etc
Cumulus, Big Switch,
IP Infusion, etc
Accton, Quanta, etc
Figure 1-1 – Vertically Integrated and Disaggregated: White Box and Brite Box
Is there any special requirement for installing a third party operating system
on Dell Networking bare-metal switches?
Yes, a piece of Linux-based software called the Open Networking Install Environment
(ONIE - pronounced “oh-nee”) is required. This boot loader or “shim” sits between
the network switching chipset and the operating system, thereby abstracting the
underlying hardware. ONIE enables a bare-metal network switch ecosystem, where
end users have a choice among different network operating systems from which to
choose. Among the founding members of the ONIE project are Cumulus, Big Switch
Networks, Dell and Broadcom. ONIE was contributed to the Open Compute Project
in 2013.
All Dell Networking switches with the “ON” designation are equipped to leverage
ONIE and load third party switch operating systems. Today, that includes 1, 10, 40,
and 100G (multi-rate) switches that are all built using commoditized hardware from
network chipset manufacturers, such as Broadcom.
How does ONIE work?
ONIE is a small operating system based on Linux that boots on a switch, discovers
which network installer images are available (whether on a local network or locally
stored), loads the image, and then provides an environment so that the installer can
load the network OS onto the switch. This is illustrated in the figure below. Take note
that this example is provided by Cumulus, but ONIE is leveraged for other third party
OS solutions, too.
Figure 1-2 – Open Networking Install Environment Behavior
So-called off-the-shelf “merchant silicon” features highly in discussions
about Open Networking. What is it and why is it relevant?
Merchant silicon refers to the Application Specific Integrated Chips (aka ASICs,
Network Processing Unit [NPU] or chipset) used for TCP/IP switching that are
manufactured by vendors, like Broadcom and Cavium. Over the last few years,
advancements in merchant silicon’s feature sets, functionality and scalability have
placed it on par with specially-designed ASICs, thereby lending feasibility to the
disaggregated model. In fact, Cisco Systems, which arguably has represented the
gold standard in proprietary switching ASICs, is now including a Broadcom chipset to
perform the foundational L3/L2switching in the Nexus 9000 Series, one of its
highest-performing data center switch platforms.
Spirited discussions can be had with regard to feature parity between merchant
silicon ASICs and proprietary ASICs, but one thing is certain: advances in the former
have allowed for a departure from the vertically integrated hardware-software model
and enabled innovation and competition among networking operating system
Explanations of SDN typically consist of references to a control plane, a data
plane and a management plane. What are they?
A network‘s architecture consists of three conceptual layers of functionality in which
the tasks of gathering intelligence (control plane), forwarding data (data plane), and
managing devices (management plane) are addressed.
When forwarding traffic, a switch or router has two tasks to perform that are handled
by the control and data planes. First, it has to understand the topology of the
network, meaning the manner in which network devices are connected to each other
and the role that each plays. For example, in a layer 21 network that is running the
Spanning Tree Protocol (STP), a switch will have to exchange Bridge Protocol Data
Units (BPDUs) with other switches to determine which one is the root bridge, which
path to the root bridge should remain enabled, and which ones need to be blocked
to remove any bridging loops.
Similarly, in a layer 32 network that is running, say, the Open Shortest Path First
(OSPF) routing protocol, messages will have to be exchanged between routers to
calculate a loop-free path across the networks in the domain. For example, each
“Layer 2” refers to an environment in which frames are switched between hosts on the same VLAN/subnet using only their
source and destination MAC addresses.
“Layer 3” refers to an environment in which frames are routed between hosts on different VLANs/subnets using their source
and/or destination IP addresses.
router on a multi-access network must determine which one is the Designated
Router (DR), which is the Backup DR (BDR) and which is a DROTHER. Then OSPF Link
State Advertisements (LSAs) will be exchanged between them to populate the OSPF
Link-State Database. Once the Dijkstra algorithm is run, the calculated shortest path
routes to all the discovered networks in the domain will populate the Routing
Information Base (RIB).
In short, the switch’s control plane provides the layer 2 and layer 3 data sets (network
and host reachability information) that inform the data plane. A network control plane
that has an instance running on every switch is said to be distributed.
Second, the switch has to process the user data it receives on the ingress interface
and make a forwarding decision. But it may first need to filter the incoming packets
according to a configured security policy. Then it will have to analyze the frame’s
header to glean the necessary source and destination address information and
consult the tables and databases that the control plane populated for guidance on
how to forward it. At that point, an encapsulation and packet header rewrite
operation may have to take place. Then the packet will need to be queued according
to a Quality of Service (QoS) policy as it’s scheduled for transmission at the outgoing
interface. In short, the mechanisms and processing logic used to forward frames exist
in the switch’s data plane, which is sometimes referred to as the forwarding plane.
Lastly, the management plane concerns itself with those protocols and mechanisms
used to manage the switch itself, such as Telnet, SSH, SNMP and NTP. The
management plane is sometimes described as a subset of the control plane.
Discussions around SDN network design oftentimes focus on what are called
leaf and spine or Clos architectures – what are they?
Clos architectures are not new. In fact, the name comes from the scientist, Charles
Clos, who designed them to scale telecommunications networks back in 1952. Clos
architectures lend themselves to efficient horizontal scaling, path resilience,
deterministic traffic flows and predictable latency and jitter. The availability of highdensity, low profile, fixed configuration, multi-rate switches have made Clos
architectures relevant once again.
A leaf-and-spine architecture replicates the internal architecture of a chassis crossbar
switch, which is referred to as the switch fabric because of the ability of an interface
to send data to any other interface (full mesh). Picture a matrix or the mesh of a
fabric with intersecting horizontal and vertical cross stitches.
One of the benefits related to this meshed fabric design involves resiliency and the
shrunken failure domain (“blast radius”) that results when one of the spine (core)
switches fails. In the legacy Fat Tree (multi-tiered) design with only 2 core (“spine”)
switches, a failure of one of them results in a loss of 50% of the network’s forwarding
 ToR switches are s6000-ON (40G x 32 ports) Switches
 Spine switches are s6100-ON (40G x 64 ports)
 All switches running Dell Networking OS9
Figure 1-3 – Sample Clos Network with 576 40G QSFP+ Servers
Compare that with a Clos network that leverages L2 or L3 multi-pathing between a
leaf switch and multiple (more than 2) spine switches, such as the one depicted in
Figure 1-3 above. If one of the 8 spine switches fails, the network will experience a
corresponding loss of only 12.5% of “backplane” capacity.
The following article offers an excellent overview (with historical context) of Clos
Are Dell’s Open Networking solutions considered SDN?
As one may suspect, the answer depends on interpretation. According to the Open
Networking Foundation (ONF), which describes itself as “an organization dedicated to
the promotion and adoption of Software-Defined Networking (SDN) through open
standards development,” SDN is defined as a set of standards-based software
abstractions that separate the control plane from the data plane and provide
programmatic provisioning of the physical and virtual switches. The ultimate
objective is to be able to create programmable networks.
Dell Networking’s Big Cloud Fabric (BCF) solution from Big Switch Networks is an
example of the canonical SDN architecture described by the ONF. On the other
hand, Dell Networking’s Cumulus Linux or OS10 offerings do not provide such
control and data plane separation, but do offer programmatic capabilities through
the Linux API and a wide ecosystem of existing Linux-based packages/tools.
Different interpretations of Open Networking and SDN notwithstanding, there is a
common thread that runs through the fabrics of all the aforementioned solutions:
the use of software abstractions to create tractable layers of modularity and
programmability. This accelerates innovation at each layer while maintaining a
consistent interface.
For example, a computer’s architecture is typically represented as consisting of the
following five levels of abstraction: hardware, firmware, assembler, operating system
(kernel) and processes (applications). Scientists and engineers have leveraged these
abstractions for decades to divide responsibility for development and to accelerate
innovation at all levels.
The OSI 7-layer model is another good example of layered abstractions. Each layer
can have its existing protocols further developed, and new ones added, without
impacting the layer above or below it; this is absolutely essential. Imagine how
complex (read: impossible) it would be to write an application or add functionality if
every change directly impacted – or depended on – the specifics of the physical
transport! The Internet, which has undergone profound changes at each layer, would
have never survived had the OSI’s data plane abstractions never existed.
Unfortunately, many such abstractions have been absent in the network control
plane. This has changed, however, over the last decade thanks to research and
development in academic circles. The result is a [r]evolutionary network engineering
paradigm known as SDN.
What are some of the abstractions used in SDN?
In the classic SDN model (Figure 1-4 below), the control plane is abstracted
(separated) from the data plane and centralized. Furthermore, within the SDN
controller itself, there are several layers of abstraction that enable a division of
responsibilities among them. The centralized control plane’s software is typically
hosted by a cluster of x86 servers (at least two) for redundancy.
The control program, the “northmost” software construct in the SDN controller stack,
is presented with an abstracted view (virtual topology) of the network by the Network
Hypervisor. The control program is acted upon by an operator or by an external
application through a northbound Application Programming Interface (API). A set of
network connectivity and service requirements are instantiated by the control
program and presented to the Network Hypervisor. The requirements articulated by
the control program may be related to a routing or traffic engineering application, or
perhaps a security-related primitive to provide tenant isolation, like VLANs or VRFs.
Regardless, the control program is only responsible for expressing the desired
outcome without having to actually implement anything. Instead, it relegates the
implementation to a lower level of software.
Control Programs and Network Applications
Abstracted Logical View – Virtual Topology
Network Hypervisor (Virtualization)
Global Network View
Open Flow
Open Flow
Network Operating System
Figure 1-4 – Classic SDN Architecture with Control Plane Abstractions
The Network Hypervisor translates (or compiles) the control program’s requirements
into low-level messages to program the physical network. Since the Network
Hypervisor has a global view of the network, it knows how to orchestrate among the
physical elements and it knows how to configure the network to execute the
requirements presented to it by the control program. The Network Hypervisor is also
responsible for discovering all the physical and virtual network endpoints and
maintaining state information regarding any topology changes of the underlying
network, including the addition or removal of switches, routers, and interconnecting
links and their capabilities.
The southbound interface on the SDN controller that interacts directly with the
physical and virtual switches is hosted by a Network Operating System (NOS). The
NOS has an authoritative view of the entire network, including all the endpoints, and
is responsible for conveying forwarding state from the Network Hypervisor to the
switches. It also keeps track of traffic statistics, topology changes and interface state.
The NOS pushes the desired forwarding state to the switches by leveraging a lowlevel protocol known as OpenFlow to program a switch’s flow tables. OpenFlow
leverages a secure communications channel between the controller and the switches
(TLS) and uses TCP as its transport (Port 6653). As a result, the network fabric gets
programmed with the forwarding rules that must be honored to implement the
control program’s requirements. These rules, also known as flow entries, populate a
switch’s flow-table(s), i.e., forwarding memory, which is stored in Ternary Content
Addressable Memory (TCAM).
The OpenFlow standard does not specify the low-level details of how it should be
implemented. Like most protocol specifications, the implementation details are left
to the vendor’s imagination. Moreover, the manner in which a switch’s flow tables are
populated is directly related to the question of scalability. As such, different operating
models have come into existence as the availability of a switch’s flow tables have
been made available to OpenFlow.
In the days of OpenFlow 1.0, data plane programming was accomplished “reactively.”
In practice, this means that the forwarding memory is treated like a cache that gets
gradually populated as packets arrive. If at the time of the packet’s arrival there is no
entry in a switch’s flow table, it will execute an OpenFlow “Packet-in” operation to
consult the controller, which would then reactively program a flow entry into the
switch’s TCAM.
Depending on the size of the network and its level of activity, the number of Packetin messages and responses could easily choke the communication bus between the
switch and the controller, thereby creating a bottleneck and associated scalability
limitations. There is also a concomitant CPU overload consideration to be made
regarding the controller’s ability to respond to so many queries.
Reactive flow control is an effort to work around the limitations of early OpenFlow
implementations and not an inherent requirement of the protocol itself. Because the
amount of forwarding memory (TCAM) exposed by these early implementations was
so small, SDN controllers could not program all necessary forwarding rules at once
and were in effect forced to cache them dynamically.
On the other hand, OpenFlow versions 1.3 and later allow for the utilization of
multiple flow tables as part of its pipeline processing schema. Therefore, the network
control plane can converge proactively and populate the tables with the forwarding
rules in a preemptive manner. As such, once a policy is configured (e.g., adding an
ACL from the CLI) or a network change occurs (e.g., a link comes up), the controller
proactively updates all of the necessary switches and forwarding memory with the
new forwarding rules.
A proactive OpenFlow implementation means that common case packet forwarding
operations do not even involve the controller and the data plane/control plane
channel is no longer a bottleneck, the controller’s CPU is no longer being
overwhelmed, and overall packet processing latency is reduced. Proactive forwarding
is only feasible with switches that expose multiple tables of forwarding memory.
In either case, upon receipt of a frame, the switch’s data plane will try to match the
information found in the header with the information found in its flow tables. If a
match is found, an associated action against the frame will be taken – either it will be
forwarded or dropped – and the relevant counters will be incremented. If a match is
not found, the frame will be punted to the controller.
As per the OpenFlow 1.5 specification, OpenFlow-enabled switches are required to
support up to 12 match-header fields (e.g., source/destination MAC,
source/destination IP, TCP ports, etc.), but up to 38 optional fields are defined,
including MPLS (Multi-protocol Label Switching) labels, TTL counters and QoS
In a similar manner to the way a server’s operating system compiles high-level source
program requirements into a low-level machine language to program a server’s CPU,
the Network Hypervisor compiles the control program’s requirements and the NOS
leverages OpenFlow to program the switch’s flow tables. This is why Dr. Martin
Casado, OpenFlow’s primary developer, once described his creation as something
akin to an x86 instruction set for a network. With this basic architecture and set of
abstractions in place, it is up to the industry to develop control programs and
applications that will exploit them. Therefore, OpenFlow should be viewed as a way
of implementing an SDN network; it’s a means to an end and not the end in and of
As alluded to previously, not all abstractions deal directly with separating the control
plane from the data plane. In the Open Networking (disaggregated) model, an
abstraction in the form of a Linux program called the Open Networking Install
Environment (ONIE) exists within the switching hardware itself. The ONIE bootloader
allows different third party operating systems to be loaded onto a bare-metal switch
that uses industry standard merchant silicon, such as Broadcom or Cavium ASICs.
That third-party OS may be the engine behind an SDN solution, as in the case of Big
Switch Networks – or perhaps not, as described earlier with Cumulus Linux. The
network shown in Figure 1-5 below is an example of a typical controller-based SDN
architecture. Notice that it is the exact same topology of switches that were used in
the Clos network in Figure 1-3, which happened to have Dell Networking OS9
running on them, and no controllers. However, because the Dell switches were ONenabled, all one had to do was uninstall OS9, install Big Switch Networks’ Switch
Light operating system for Big Cloud Fabric (BCF), and deploy the BCF centralized
controllers. With relatively minimal disruption, a legacy network is converted to a
next-generation SDN without having to remove or replace any switching hardware.
That is the power of abstractions!
 ToR switches are s6000-ON (40G x 32 ports) Switches
 Spine switches are s6100-ON (40G x 64 ports)
 All switches running BSN Switch Light OS
BCF Controllers
Figure 1-5 – Sample Big Switch BCF SDN Clos-based Architecture
Yet another abstraction, this time provided by Dell’s OS10 SDN/DevOps-inspired
Operating System, is known as the Switch Abstraction Interface (SAI, pronounced
“sigh”). The SAI takes the ONIE model to the next logical step by providing a standard
C-based API to program switching ASICs, thus allowing different vendor chipsets
(other than Broadcom) to be deployed with various different third party operating
SDN is oftentimes equated with OpenFlow. Is that the only “southbound”
protocol used by SDN controllers?
Although OpenFlow was the original SDN protocol used to provide programmatic
control of the data plane, SDN architectures have evolved since then. They now
include a litany of other design approaches and protocols that exploit the original
abstractions that the original SDN architecture defined. OpenFlow itself has been
considerably developed since its inception to include more functionality, such as the
ability to support multiple tables and a management protocol known as OF-Config or
the OpenFlow Configuration and Management Protocol. OF-Config is a companion
protocol to OpenFlow and leverages YANG modules (RFC 6020) and NETCONF (RFC
6241) to model and manipulate configurable system attributes and operational state
for both physical and virtual switches that run OpenFlow. Parenthetically, the OVSDB
protocol (Open vSwitch Database) is a configuration and management protocol for
An SDN controller from vendors like Cisco take an approach that deviates from the
classic SDN model. For example, Cisco’s Application Centric Infrastructure (ACI)
controller, known as the Application Policy Infrastructure Controller (APIC), does not
fully centralize the control plane (logically speaking) nor does its southbound
protocol assume a completely “dumb” switch.
In the view of Cisco’s developers, the built-in intelligence that has traditionally
defined a switch’s control plane has value to offer: specifically, in delivering
programmable networks by sharing responsibility with the controller as part of a
hybrid approach. Instead of using OpenFlow to program a switch’s flow tables with
low-level match-action instructions, Cisco’s APIC uses a higher-level protocol called
OpFlex to convey a set of policy-based requirements from the APIC controller to the
switch, which will in turn leverage its own policy engine to implement them.
Cisco’s approach illustrates the difference between an imperative programming
model and a declarative one. In the former, a control program will define a desired
end state as well the exact steps that need to be executed to achieve it; whereas in
the latter, which is the programming model leveraged by Cisco’s APIC, the desired
end state is defined without directives on how to implement it. Instead, the
implementation details are left for an intelligent forwarding device to determine. One
way to think about this is to consider the idea that the lowest compiled state for a set
of programming instructions has been moved further “south” in the declarative
model from the SDN controller’s Network Operating System (centralized control
plane) to the switch’s operating system (distributed control plane).
Moreover, service provider-oriented SDN solutions leverage a new MP-BGP (MultiProtocol Border Gateway Protocol) NLRI (Network Layer Reachability Information)
and address family known as BGP-LS (BGP Link State) to communicate IGP (Interior
Gateway Protocol - OSPF or IS-IS) link state information from an IGP speaker to a
centralized controller for the purpose of establishing MPLS-based Label Switch Paths
(LSP) within and across different domains as part of an end-to-end MPLS-TE (Traffic
Engineering) solution. Yet another extension to MP-BGP known as Ethernet Virtual
Private Network (EVPN) offers a new address family to convey L2 MAC address
information for endpoints as a control plane alternative to VxLAN’s (Virtual Extensible
LAN) default “flood-and-learn” approach.
SDx Central published a report in 2015 that includes some of the most popular SDN
controller solutions and the southbound protocols they use. As illustrated in the list
below, OpenFlow is just one of many southbound protocols to deliver programmable
URL to Report:
From the report:
Brocade SDN Controller – OpenFlow, OVSDB, BGP and NETCONF
Cisco APIC SDN Controller – OpFlex
Cisco Virtual Topology System – NETCONF, RESTCONF, MP-BGP EVPN
Ericsson SDN Controller – OpenFlow, OVSDB, BGP, NETCONF, PCEP, BGP-LS
Juniper Contrail – BGP, XMPP, NETCONF, OVSDB
NEC Programmable Flow PF6800 SDN Controller – OpenFlow
Avaya SDN Fx Controller – OpenFlow, OVSDB, OF-Config, NETCONF
Nuage Networks SDN Controller – OpenFlow, OVSDB, BGP
ODL Lithium – OpenFlow, OVSDB, BGP, NETCONF
VMware NSX (not in report) – User World Agent, netcpa, RabbitMQ, vsfwd
(NSX-vSphere). OpenFlow and ovsdb (NSX-Multi-Hypervisor)
Security, Orchestration, Automation, Virtualization, Load-balancing, etc.
App From
Third-party App
Client App
Business App
Northbound API
(REST, JSON, etc)
Network Operating System
Southbound API
Figure 1-6 Generalized View of Classic SDN Architecture
What is the Northbound API used for in an SDN Network?
During the early days of SDN, it was the Southbound API, and in particular OpenFlow,
that garnered the lion’s share of the industry’s attention. That’s understandable given
that OpenFlow was the foundational technology that enabled programmatic
networks. The implications of separating the control plane from the data plane and
leveraging abstractions to dynamically program the network were foreign concepts
that network engineers spent a good amount of time trying to grasp and appreciate.
As such, conversations around applications took a backseat and were nebulous at
best, since no one could point to a specific use case or a vendor that was actually
shipping a product that anyone had in production. Much has changed since then,
and the application layer that consumes the Northbound API has been demystified.
The Northbound API makes the network’s control information (Control Plane)
available to higher level abstractions, such as applications. Everything below the
Northbound API (i.e. the control and forwarding planes) still delivers the same
traditional networking functionalities. The network basically continues to forward
packets, as it always has, although the implementation is different. What is new,
insofar as the application is concerned, is the architecture. Instead of control
information being distributed across multiple network devices, thereby requiring
complex distributed algorithms, it is now abstracted from the hardware, centralized
and made available to the applications via the API. The network can thus be
programmed with software applications instead of command line interface (CLI).
The application could be designed to deliver traditional network services, such as
firewalls or load balancers and basic L2 and L3 forwarding, or complex orchestration
across cloud resources (storage, compute and network), like OpenStack. SDN
applications can instantly change network configuration to align it with business
objectives or customer Service Level Agreements (SLA), such as forwarding packets
over the least expensive path (think SD-WAN), dynamically adapting QoS based on
available bandwidth and user subscription, dropping unwanted packets and tracking
back to their source for containment, etc. Security, traffic engineering, multi-tenancy
management, and network monitoring are just a few examples of the applications
that are leveraged by the Northbound API.
Some vendors sell a controller with built-in applications, like Big Switch’s Monitoring
Fabric, Cisco’s ACI and vMware’s NSX. All of them, however, expose an API (RESTful)
that software developers can write to with the hope that a massive ecosystem of
software can be developed around it, thereby making their API a de facto standard.
1.13 SDN-based solutions are oftentimes categorized in terms of underlay and
overlay networks. What are they and what is the difference?
An underlay network is defined as the underlying physical network – the transport –
upon which virtual networks (overlays) forward data. Each network type has
implications with regard to the possession and exploitation of topological awareness,
forwarding intelligence and path selection.
Overlay networks put flow-based path selection at the endpoints, such as hypervisors
or their hardware-based analogs. In other words, the forwarding intelligence resides
at the edge of the network. Overlays can be used when one does not have control of
the underlying infrastructure. A good example of an overlay network is a Virtual
Private Network (VPN), like the one telecommuters use every day to access their
company’s intranet. A laptop with a VPN client installed is one endpoint of the tunnel
while the VPN concentrator at company headquarters acts as the other. The underlay
is provided by the ISPs that connect the two ends.
At the sending end, the traffic payload is encapsulated in another frame. A separate
tunnel (outside) header is appended to it whose source and destination IP addresses
are those of the tunnel endpoints. At the distant end, the original frame is de-
encapsulated and forwarded onto the private network. The original packet travels
through the physical underlay without its payload, including its original (inside)
header, ever being examined or acted upon by the intermediate switching nodes.
This is why it is said to have traveled through a tunnel: insofar as the original frame is
concerned, its headers were examined twice, once at the encapsulating end, and
again at the de-encapsulating end – with no intermediate hops in between.
Physical Underlay
Tunnel Endpoint
Tunnel Endpoint
Tenant-C Original Frame
Figure 1-7 – Overlay Networks in a Multi-Tenant Environment. The underlay
forwards traffic based on tunnel header information. The tunnel endpoint is a
hypervisor or a physical switch.
SDN and Network Virtualization have brought overlay networks inside the data
center. They are used in cloud-based XaaS applications for multi-tenancy network
isolation and to provide Layer-2 adjacency across routed boundaries. The two most
common overlay tunneling technologies used in SDN solutions today are VxLAN and
GRE (Generic Route Encapsulation). In each case, the hypervisor acts as the virtual
tunnel endpoints for virtual workloads. For physical servers with no hypervisor
installed, a hardware-based endpoint is used, such as the VxLAN Tunnel Endpoint
(VTEP) functionality available in the Broadcom Trident-2 (and later) chipset.
Take note that the underlay fabric has no knowledge of the existence of any private
networks that sit “behind” the tunnel endpoints. That relieves it from having to create,
store or change network forwarding state. That is of particular value in a dynamic
cloud environment with migrating workloads whose identity is routinely being
decoupled from its location courtesy of the logical tunnels. All the underlay has to do
is route packets between static endpoints, which is something that IP fabrics have
been doing very well for many years.
By the same token, the overlay network has no knowledge of the underlay network’s
topology, forwarding mechanisms, routing protocols, security and quality of service
policies, etc. It can be said that the two are orthogonal to each other, although the
underlay provides transport services to the overlay. Therefore, the transport fabric
needs to be reliable, robust, redundant and resilient.
There are SDN solutions available that are designed for a more collaborative
approach between overlays and underlays, in which the overlay is allowed to exploit
underlay information to conduct efficient path monitoring and construct highlyreliable overlay topologies. Cisco’s ACI (Application Centric Infrastructure) and Big
Switch Network’s Big Cloud Fabric (BCF) solutions are examples of this approach.
What problems does SDN solve and what are some real-world use cases?
As Professor Scott Shenker, one of the pioneers of SDN development, once
remarked, “[OpenFlow] doesn't let you do anything you couldn't do on a network
before.” Given that this comes from one of the most passionate and prolific voices in
SDN evangelism, this is not an altogether mundane comment.
So what exactly does OpenFlow-based SDN provide? SDN establishes a set of
clearly defined layers of functionality that comprise a set of principles upon which
to build networks. These principles are embodied in the following:
Separation of the control plane from the data plane
A centralized view of the entire network, including endpoints
Software abstractions that allow for modularity within the control plane stack
The concept of a network operating system that abstracts the installation of
state in network switches from the logic and applications that control the
behavior of the network
 A standardized programmatic interface (API) for the data plane
This clearly defined architecture sets the stage for the development of control
programs and applications that can exploit these constructs to solve the network
challenges that are addressed sub-optimally – if at all – today, as well as the
requirements for future use cases. With that, the network has been put on par with its
compute and storage counterparts.
In terms of the usefulness of an OpenFlow-based SDN is concerned, given the SDN
controller’s centralized view of the entire network, which includes the identity
(IP/MAC addresses) and location (switch/hostname/port) of all the physical and
virtual endpoints, deploying legacy networking functionality (segmentation, isolation,
quality-of-service, security and load balancing, etc.), as well as managing and
monitoring the network, can be simplified.
The so-called “killer app” for SDN is Network Virtualization. It’s worth mentioning that
the availability to leverage virtualized networking constructs is not new at all. For
example, Virtual Local Area Networks (VLANs) have been in existence for over 25
years. And network engineers have been deploying virtual device contexts for routers
and firewalls, Virtual Routing and Forwarding instances (VRFs), logical routers, datapath virtualization solutions, like Virtual Private Networks (VPNs) and Multi-protocol
Label Switching (MPLS), control plane virtualization and other forms of network
virtualization for many years. In fact, a well-known Cisco Press book titled Network
Virtualization by Victor Moreno and Kumar Reddy was published in 2006 – long
before anyone heard of SDN and OpenFlow.
However, within the context of SDN, Network Virtualization (NV) refers to the
creation of virtualized network entities (routers, switches, firewalls, and load
balancers) using one of two methods.
 The first is through software emulation plus overlays. This model is referred
to as hypervisor-based or overlay-based network virtualization, which can
use a centralized controller – i.e. vMware NSX/Nicira NVP, OpenStack, Big
Switch BCF P+V, etc.
 The second method is to virtualize the physical network itself via the classic
SDN model, which utilizes a centralized controller and a southbound API for
the data plane, such as OpenFlow – i.e. Big Switch BCF, NEC Programmable
Flow, Brocade SDN Controller, Cisco APIC, etc.
In either case, the virtual networks exist as part of a software abstraction that is
decoupled from the underlying physical hardware. The hardware acts as the actual
forwarding engine, the substrate upon which the virtual constructs are built. In the
overlay model, that hardware is typically a virtualized server running hypervisor
software, in which case the physical network is irrelevant. In the classic SDN
approach (sometimes referred to as an underlay model), the underlying hardware is
the physical network itself, in which case a network virtualization app/control
program can leverage SDN’s abstractions to create from it a shared pool of network
transport that can be “sliced” into different virtualized networks.
The compute analog to NV is Server Virtualization, where the familiar attributes of a
physical server are decoupled and reproduced in software (the hypervisor) as vCPU,
vRAM, vNIC, etc. And just like a virtual machine, a virtual network can be instantiated,
manipulated, saved and deleted with a few clicks of the mouse.
Control Program/App
Control Program/App
(e.g. OnePK)
Open API
(e.g. OpenFlow)
Overlays (e.g. VxLAN, NVGRE)
Figure 1-8 – Two Approaches to Network Virtualization: Classic SDN (left) and
Overlay SDN (right). Note that the underlay switches are inconsequential in the
overlay model.
There is a growing number of use cases and applicability for SDN, such as SD-WAN,
network monitoring and tap aggregation, service insertion (IDS/IPS, firewalls, loadbalancers), campus applications, automation and orchestration and others. The list
grows as more control programs/applications are written and the technology
Dell OS10 and Third-Party OS Vendor Solutions
The following sections will give a high-level overview of Dell Networking’s Open
Networking-enabled software solutions, including the Dell OS10 operating system.
They are part of Dell’s disaggregated software-hardware model. Given that each
operating system defines a certain architecture and design approach, the choice of
which one to deploy is a function of the specific architectural and operational
Dell OS10
In January of 2016, Dell Networking released the Base version (Open Edition) of its
next-generation data center networking operating system called OS10. The “10”
reflects the next numerical progression from Dell’s legacy OS9 software train, but the
two OSes are completely different. OS9 is a mature, tried and tested network switch
operating system with a NetBSD kernel and a wide range of “table stakes” data center
and campus networking technologies. As Dell Networking’s flagship, legacy operating
system, OS9 has a global footprint and a long history of success.
On the other hand, Dell OS10 is slated for release in general purpose enterprise
deployments (Enterprise Edition) by CY16Q4. It is new and very different from OS9.
Also, like the third-party software vendor solutions that will be described later in this
section, Dell OS10 leverages the Open Networking Install Environment (ONIE) that
comes with a Dell “ON” switch, such as the S4048-ON.
From the Dell OS10 FAQ Document – June 2016:
The OS10 Open Edition includes the Linux distribution, SAI API (NPU abstraction), and
CPS API (Control Plane Abstraction) over REST and Python wrappers. The Open
Edition enables native Linux applications (management, monitoring), third party
applications such as Quagga and Bird, and the integration of custom applications via
the CPS API.
The Open Edition is a development environment suitable for lab environments to
prototype, develop, and test new applications. The Open Edition is also suitable for
customers that require a high level of customization and have in-house capabilities
for both development and support for production environments.
The OS10 Enterprise Edition includes the Open Edition capabilities plus a full Dell
Networking protocol stack and management infrastructure providing a CLI to
configure and monitor the platform. The Dell protocol stack comes fully integrated
with the system software for the underlying hardware.
OS10 Enterprise Edition is designed for mainstream production networking
environments, especially for organizations transitioning to a DevOps operational
Figure 2-1 Dell OS10 Software Stack with Unmodifed Linux Kernel
2.1.1 What are Dell Networking OS10’s most salient features and innovations?
The OS10 Base Module has been available for free since March of 2016 and runs an
unmodified Debian Jesse Linux kernel. Linux is one of the most widely-used
operating systems and can provide deployment and operational homogeneity across
multiple IT layers, including networking, storage and compute. The OS10 Base
Module can leverage the wide array of software packages and tools that exist within
the Linux ecosystem today, like Quagga and Bird for routing TCP/IP, and it also
provides a rich environment for developing homegrown applications and leveraging
DevOps automation and management tools, such as Puppet, Chef and Kubernetes.
Dell OS10 is a very modular operating system that consists of several layers of
abstraction. Of these, the most notable are the Switch Abstraction Interface (SAI), the
Control Plane Services layer (CPS) and the Common Management Services interface
(CMS). Combined, they make application development and network programmability
(configuration and management) simpler and more streamlined. Recall that the very
purpose of software abstractions is to create tractable layers of functionality that can
be developed independently of each other, and without consideration for the
underlying complexities and detail. Recall the abstracted control plane in the ONF’s
SDN model that was covered in section 1.10. This modularity makes OS10 extremely
flexible in terms of its programmability and supported deployment scenarios.
2.1.2 Control Plane Services
The benefits of Dell OS10 Control Plane Services revolve around platform stability,
scalability, feature development velocity, extensibility, and integration with external
systems and applications. The CPS is the chief enabler of a network operating system
that is purposefully designed for cloud/DevOps/SDN environments. The CPS layer is
an inter-application framework that provides a stateful, distributed database service
that also leverages a publish/subscribe messaging paradigm. OS10 applications use
the CPS API to communicate with each other, just as custom-written applications
use the CPS API to communicate with the OS10 components and services. As such,
an application expresses a desire (subscribe) to receive state information regarding
one of the OS10 device’s subsystems, and then consumes that information in order
to take a prescribed action.
To understand how the CPS infrastructure operates, consider the example of a
Temperature Control (TC) application. This simple application will monitor an OS10based switch’s temperature and the speed of its cooling fans. The TC application will
subscribe to event information regarding the temperature of the switch by registering
its interest in such an event with the CPS layer. When the temperature exceeds a preconfigured threshold, the Platform Abstraction Service (PAS, which is a higher layer
abstraction of the actual hardware drivers) will publish that information. In response
to the notification, the TC application can speed up the rotation of the fans and/or
send out an urgent email to the network operations center team, or even manipulate
the data center’s environmental controls to compensate for the cooling system’s
The CPS API has been leveraged by Dell Engineering in partnership with other
vendors and clients to deliver innovative solutions. The following are two examples.
 Silverpeak Accelerated Route Convergence
The objective of the convergence application is to accelerate route convergence and
failover after a link failure. In the topology under test, the SD-WAN edge appliance
was connected to both a broadband Internet service and an MPLS cloud through a
Dell switch that was running OS10. Typically, in SD-WAN solutions, delaysensitive/tier 1 traffic travels over a highly-reliable and private MPLS link, while less
demanding traffic is sent over a public broadband connection. The application must
register with the CPS and subscribe to event information regarding the state of the
physical link that is connected to the broadband provider. Once the link is
deliberately failed, an event notification is sent to the CPS, which in turn publishes it
for consumption by the application. The application, in turn, will shut down the link
that the SD-WAN appliance uses for Internet-bound traffic, thereby triggering an
immediate reconvergence and traffic switchover to the MPLS link.
 OS10 Integration with Kubernetes Networking
The client had an existing Kubernetes container environment that was leveraging the
Tectonic distribution of CoreOS, which uses Flannel to provide an overlay network
for inter-pod communications. Dell provided a solution for Kubernetes networking,
where the customer was able do away with overlay networks and allowed them to
deploy a Kubernetes cluster on an existing L3 fabric. The solution is a simple app that
runs on a top-of-rack Dell OS10 switch. The app extracts the necessary network
information about the Kubernetes cluster that is stored in the distributed key value
database known as etcd. The CPS API is leveraged to program the container subnets
on to the kernel and the ASIC by pointing those subnets to the server that hosts the
particular subnet. Then BGP advertises those subnets to a peer router, thereby
ensuring that they are reachable.
2.1.3 Switch Abstraction Interface
Under the hood, Dell OS10 employs the Switch Abstraction Interface (SAI), which
provides a common interface for operating system software to communicate with
and program the underlying networking ASIC/NPU. That operating system software
can be the kernel itself or processes that are running in user space, while the chipset
may be from any one of several vendors whose SDK conforms to the SAI
standard. The SAI allows Dell to quickly integrate new ASIC/NPUs for customers
reducing the time it takes to provide them with the latest chipset technologies.
This is not trivial, as it represents the next step in the evolution of disaggregation. The
end-result is that ASICs from multiple vendors may be deployed (Broadcom, Cavium,
etc.) with the operating system software of choice, thereby allowing network
architects to build a network that meets their needs without any deployment
constraints imposed by proprietary solutions.
2.1.4 Common Management Services
Dell OS10 exposes an API for use by external management systems such as a
NETCONF client. NETCONF is a switch management protocol that was first
developed by the Internet Engineering Task Force (IETF) in 2006 and later revised in
2011. It provides mechanisms to install, manipulate and delete configuration files on
network devices. The impetus for its development was the realization by the industry
that SNMP was only being used as a means to monitor the network and not write to a
system’s configuration. Network engineers preferred vendor CLI and vendorprovided scripts over unwieldy SNMP workflows.
In OS10, the NETCONF server leverages the data modeling structures that are
defined by Yang data modules. Yang modules explicitly and precisely determine the
syntax and semantics of the data that can be externally managed and manipulated by
the NETCONF protocol. In other words, Yang provides a well-defined abstraction of
the different elements of a switch’s subsystems that can be configured by an external
management system, such as the attributes of physical and virtual interfaces, IP
routing protocols, platform management related items, etc. Yang also makes a
distinction between configuration data and operational state data for the purpose of
streamlining management.
Through the Hello process and the subsequent exchange of capabilities that occurs
during session initialization between a NETCONF client (network management
station) and NETCONF server (OS10 device), Yang advertises the specific elements in
the device’s subsystems that can be configured, monitored or otherwise manipulated
by the management station. Notifications and administrative actions that are available
to NETCONF are also made known at that time.
A concrete example of how OS10’s NETCONF agent can make life easier for network
operators involves the data stores that it defines. NETCONF defines several types of
data stores in the network device, including the startup, running and candidate data
stores. The running data store is where the device’s current running configuration
can be found and the startup data store includes the configuration that will be
executed upon starting the device. These two data stores are well-known to network
engineers. On the other hand, the candidate data store, although less familiar to
some, is comparatively the more interesting element in terms of operations. The
candidate data store holds a potential configuration that can be executed once it is
committed to the running configuration. This is a very handy tool to have at one’s
disposal when making configuration changes that may inadvertently cause an
outage. The candidate data configuration will be withdrawn after a set period of time
has lapsed if it is not committed, thereby restoring the network to a previously
operational state.
NETCONF also works symbiotically with the candidate data store to make network
configuration changes to many devices at once in what is described as network-wide
transactional operations. The change is first made to the each device’s candidate data
store and only after all devices agree to the feasibility of the new configuration will
the candidate configurations be executed. If something fails and the confirm-commit
message is not executed, the changes will rollback to a previous functioning state.
OS10 meets the demands that Cloud/DevOps environments make of modern
network operating systems, such as programmatic access to a device’s configuration,
automation, the ability to execute unordered configurations (relegating ordering to
the intelligence within the device itself), configuration validation, the ability to
execute network-wide configuration changes at once, a standardized data structure
to advertise configurable elements and the ability to separate configuration and state
Cumulus Linux
The Cumulus operating system for Dell Networking ON-based switches is most
suitable for enterprises whose personnel are familiar with Linux. The Cumulus
administrator interface is not quite an “industry-standard” command line interface
(CLI) to which network engineers have become accustomed, but it is similar. One
should be familiar with Linux file manipulation and basic command sets for managing
images, as well as with Linux-based networking constructs to be effective. In short,
there is indeed a learning curve for the “average” network administrator, but for
engineers who want to capitalize on the value of deploying a Linux operating system
in their environments and also possess the drive to venture slightly out of their
comfort zone, the learning curve is certainly manageable.
One of the biggest benefits offered by Cumulus is the ability to manage the network
with the same automation tools that are used to manage a Linux-based server farm,
such as Ansible, Puppet and Chef.
Figure 2-2 Cumulus Linux Network Operating System Architecture (from Cumulus
What is Cumulus Linux?
Cumulus Linux is a networking focused Linux distribution that is deeply rooted in
Debian. Cumulus Linux replaces the vertically integrated operating system that a
networking vendor would normally provide with their hardware. Switches running
Cumulus Linux provide standard networking functions such as bridging, routing,
VLANs, MLAGs, IPv4/IPv6, OSPF, BGP, access control, VRFs, and VxLAN overlays.
Is Cumulus Linux an overlay or underlay solution?
Insofar as Cumulus Linux is simply an operating system that runs on the physical
networking hardware itself, it can loosely be categorized as an underlay, although it
does not support an SDN architecture with a centralized control plane, as described
earlier in this paper. It does support network overlays by providing the ability to
enable the VxLAN gateway feature in the Broadcom chip.
Does Cumulus Linux run OpenFlow or any other “southbound” protocol
for the purpose of communicating with a centralized SDN controller?
No. According to Cumulus CEO JR Rivers, “The only way you can truly be successful
in meeting the customer needs around OpenFlow is to be truly focused on a great
OpenFlow agent that lives on the switch platform…In general, when customers want
to use OpenFlow, Cumulus will say, go talk to Big Switch.”
Can Cumulus Linux be deployed in an NSX/VxLAN environment?
Absolutely. The value offered by virtual networks comes from the fact that they are
abstracted from the underlying physical network. Therefore, a virtualization overlay,
like VxLAN, which is also the overlay engine behind NSX, is orthogonal to the
switching hardware and the operating system they run. The VxLAN Tunnel Endpoint
(VTEP) resides on the virtualized server’s hypervisor. This is where the
encapsulation/de-encapsulation of the tunnel header takes place. The underlying
network only needs to have the ability to forward IP traffic between those tunnel
There is an exception and it involves the case in which a server is not running a
virtualization platform and therefore has no hypervisor. In that case, the server will
require a physical switch to act as the VTEP to allow for communication between the
physical and virtual server environments. Broadcom’s Trident-2 chipset fully supports
that functionality from a hardware perspective, but the operating system running on
the switch will have to offer the ability to expose that functionality through CLI.
Cumulus Linux version 2.0 and later fully support VxLAN and NSX.
Can Dell switches running Cumulus Linux be deployed in an OpenStack
Yes. In fact, there is a detailed design guide on Cumulus’ website for deploying
Cumulus Linux with OpenStack.
URL to Design Guide:
On which Dell Networking switches can Cumulus Linux be deployed?
Dell Networking switches with the “ON” designation are capable of running thirdparty operating systems. The Cumulus Linux HCL is actively updated as more vendor
hardware platforms are tested and certified. As of the writing of this paper, the Dell
Networking S6000-ON, S4048-ON, S3048-ON, S6100-ON and the Z9100-ON are
on the Cumulus HCL.
What Services and Support model are in place for a customer who buys a
Dell switch with Cumulus Linux?
As always, the hardware is fully supported by Dell and that does not change at all in
the disaggregated model. Typically, the customer is responsible for installing the
operating system and configuring the switch. However, the Dell Networking SE,
along with their counterparts at Cumulus, are resources who can be leveraged to
give the customer a smooth out-of-box experience. As of today, Dell Services does
not provide any services SKU for a Cumulus Linux rollout, but services can be
purchased from Cumulus.
Big Switch Networks (BSN)
Contrary to what the name suggests, Big Switch Networks does not sell switches.
The company sells networking software that is loaded onto an Open Networkingenabled switch as part of a disaggregated deployment model. Dell offers two
solutions from BSN: Big Cloud Fabric (BCF) and Big Monitoring Fabric (BMF). Both
solutions involve a classic controller-based SDN architecture, and the operating
system for both is BSN’s Switch Light OS.
2.3.1 Big Cloud Fabric (BCF)
BCF is an SDN-based networking solution that closely resembles the classic SDN
model that was described in the previous section. The architecture includes a pair of
centralized controllers that present a northbound API for consumption by
orchestration and cloud management tools, such as CloudStack, OpenStack and
vMware. There is also a CLI and GUI interface for human interaction, but they are
both REST API clients that translate command inputs into REST calls.
Figure 2-3 Big Switch Networks - Switch Light OS Decomposed
The centralized controllers also leverage OpenFlow’s southbound API to program the
data plane with the necessary flow information to implement the policies that are
instantiated by the control program. Because the Switch Light OS completely
displaces any other software on the switch, it does not have to share the hardware
tables with any other protocol data structures. That, coupled with the fact that newer
versions of OpenFlow (1.3 and higher) allow for the use of multiple flow tables,
means that BCF can operate in proactive OpenFlow mode, thereby allowing the
fabric to scale.
The data plane is a leaf and spine (Clos) topology that – as of the writing of this paper
– scales to 32 leaf switches (48 x 10G ports typically) and 6 spine switches (32 x 40G
typically). The best way to think of the BCF architecture is to imagine a decomposed
chassis-based switch with a pair of supervisor modules (aka Route Processor
Modules, L2/L3 engines, etc.) and line cards. The BCF centralized controllers would
be the equivalent of the redundant supervisor modules (active/standby), the leaf
switches would comprise the 10G access line cards, and the inter-switch links and
spine switches would comprise the non-blocking backplane. See figure 2-4 below.
The L2/L3 boundaries of the Clos network are arbitrary and policy-driven, not
topology driven.
In other words, the inter-switch link ports are non-committal in terms of their
function; they may be part of a L2 or L3 forwarding mechanism.
Figure 2-4 Big Cloud Fabric from Big Switch Networks. Comparison to Chassisbased switch.
As mentioned in the previous section, network virtualization is one of the prime use
cases for SDN. In the BCF solution, a virtual network, which is comprised of L2 and L3
entities, is called a Tenant. The L2 broadcast domain (aka VLAN) of the Tenant
network is referred to as a Logical Segment and the L3 component is called a Logical
Router, which is also described as a VRF or Tenant Router. Tenants communicate
with each other and with external devices through another L3 construct known as a
System Router. Tenant routing – meaning between and within a segment – takes
place at the leaf/access layer, which acts as a distributed router in hardware in which
each leaf acts as the default gateway for any logical segment that it hosts. InterTenant and external routing take place at the spine.
Starting with version 2.6 of the Switch Light OS, BCF can integrate with vMware NSX
to provide visibility to the network underlay on which the NSX overlay tunnels “ride,”
as well as analytics to gage performance and reliability. By consuming an API
provided by vCenter, the BCF controller can learn of the activity occurring on the
overlay (i.e., which ESXi hosts are connected to which leaf switch and the virtual
networks that they host, the creation of VTEPs, visibility to the VMS connected to the
VTEPs, and virtual overlay network troubleshooting and analytics). This is what was
being referred to earlier in this paper with regard to the value of integrating between
the underlay and overlay networks.
BCF also integrates with OpenStack Neutron via a user space software agent for
KVM-based virtual switches that adds functionality and is said to enhance
performance of Open vSwitch (OVS).
Figure 2-5 Big Cloud Fabrics Architecture Overall View
2.3.2 Big Monitoring Fabric (BMF)
The BMF solution from Big Switch Networks involves the deployment of a similar
architecture to the BCF solution, but it is used as a means to interconnect a
production network to a separate fabric whose only purpose is to provide “pervasive
visibility and security.” In other words, the network itself is a packet broker. It uses the
same Switch Light OS with some modifications, redundant controllers and an
unfolded Clos architecture. The entire solution can also be thought of as a big switch
chassis to which the production network’s monitoring taps and SPAN ports are
connected, as well as the monitoring tools and performance analyzers. BMF
leverages the ability of OpenFlow to match a wide scope of abstracted control plane
information to capture traffic flows and forward them programmatically to a
repository (farm) of network monitoring tools.
Enterprises who want to test the SDN waters may leverage this solution as a low-risk
opportunity to introduce SDN into their environments and learn about how to
leverage its capabilities and how to manage and maintain them while keeping their
existing legacy production network in place.
Figure 2-6 Big Monitoring Fabric Architecture Overall View
IP Infusion
OcNOS is the name of the operating system from IP Infusion that can be loaded onto
a Dell Networking ON-based switch. The main attraction for this operating system is
the vast MPLS-based services that it provides. This enables Dell to offer
commoditized WAN edge and service provider based solutions. MPLS is a packetswitched data transport technology that is typically leveraged by enterprises to
connect a data center to remote sites or as a data center interconnect (DCI) solution.
MPLS can offer L3 or L2 connectivity, in which case, in its simplest form, an MPLS
service can be thought of as a big switch (L2) or router (L3) that sits between sites.
OcNOS offers a relatively newer MPLS-based control plane solution known as EVPN
– or Ethernet Virtual Private Network. One use case that is of particular relevance in
the field of virtual networking is as a control plane for VxLAN. By default (according to
IETF RFC 7348), VxLAN uses a multicast-based flood-and-learn approach (data plane
learning) to VTEP and endpoint discovery. The overlay broadcast, unknown unicast,
and multicast traffic (BUM) is encapsulated into multicast VXLAN packets and
transported to remote VTEP switches through the underlay using multicast
forwarding. Flooding in such a deployment can present a challenge for the scalability
of the solution. The requirement to enable multicast capabilities in the underlay
network also presents a challenge because some organizations do not want to
enable multicast in their data centers or WAN networks.
EVPN offers a standards-based control plane solution to the VxLAN data plane that is
more intelligent, efficient and scalable. It leverages a newly-established MP-BGP
address family (L2VPN) and NLRI for advertising MAC addresses and mapping them to
IP addresses. EVPN inherently supports multitenancy, privacy and route isolation.
One can find a solutions guide for VxLAN and EVPN using OcNOS at the following
Since EVPN is an evolutionary technology that relies on other foundational
technologies, to fully understand EVPN, one should familiarize themselves with the
following RFCs:
 RFC 4271 - Border Gateway Protocol 4 (BGP4):
 RFC 4760 - Multiprotocol Extensions for BGP4:
 RFC 4364 - BGP/MPLS IP VPNs:
Finally, the OcNOS management plane can support a variety of management
interfaces, such as “industry-standard” CLI, SNMP, REST, NETCONF and SAF IMM-OI.
Pluribus Networks
Open Netvisor Linux (ONVL), which is based on Canonical's Ubuntu Linux
distribution, is the name of the network switch operating system that Dell offers from
Pluribus Networks. Pluribus’ software-centric solution is called Virtualization-Centric
Fabric or VCF. The fabric is typically a Clos architecture whose switches are
coalesced into a single management domain (“fabric”) that can be managed through
CLI or a C, RESTful API, as well as by DevOps tools, such as Ansible. Each switch runs
an instance of a proprietary distributed database clustering software to create the
management domain and it is used for configuration and state management across
the physical network.
A VCF network does not leverage a centralized controller like BCF and BMF, nor is it
an overlay solution, like NSX. Instead, it employs a distributed control plane across
the fabric. All L2 and L3 control plane protocols, data plane forwarding mechanisms,
multipathing and loop mitigation considerations are the same as they are in so-called
legacy networks. The inter-switch links between leaf and spine may be L2 trunks or
L3 interfaces. VCF has come a long way in its support for table stakes technologies
and it continues to travel down that path, with support for VRF and other network
virtualization technologies on the roadmap.
VCF’s value comes from its ability to provide deep analytics and visibility (Insight
Analytics) to existing and archived traffic flows across the network without having to
deploy a separate tool farm, as one does with a typical packet broker-based solution.
Analyzed production traffic flows “inline” and the analytics engines run on the
switches themselves, with no need to purchase separate appliances. Traffic analysis
can be done via a GUI or through CLI.
Specifically, the fabric analytics engine provides the following visibility:
 Telemetry – inspects every individual TCP connection and client-server
aggregated connection fabric statistics.
 vFlow - filtering fabric-wide data center switching traffic on a granular flow
level and applying security/QoS actions or forwarding decisions on each
defined flow.
 vPort - tracking endpoints/VMs on a global, fabric-wide endpoint table.
This paper aimed to provide the necessary background, technical information and
historical context to understand and appreciate SDN’s foundational concepts and
their relationship to Open Networking. While other vendors focus on a particular
solution set and approach to delivering programmatic networks, Dell Networking
offers engineers and architects the ability to choose a path that best suits their needs
and meets their requirements through its championing of the disaggregated
hardware/software model.
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