Chapter 3 (Part B)
CSC465 – Computer Networks
Spring 2004
Underlying Technologies
-Continued-
Dr. J. Harrison
These slides were produced from material by Behrouz Forouzan for the text
“TCP/IP Protocol Suite (2nd Edition)” and from Kurose & Ross,
“Computer Networking; A Top-down Approach Featuring the Internet”
Hubs, Bridges and Switches
CONNECTING
DEVICES
Internet
•
•
•
•
• Used for extending LANs in terms of
geographical coverage, number of nodes,
administration capabilities, etc.
• Differ in regards to:
– collision domain isolation
– layer at which they operate
Connecting devices
Internet is neither single LAN or WAN
Interconnected WANs and LANs
Interconnection achieved via connecting devices
Repeaters & Hubs operate in 1st TCP/IP Layer
– same as 1st ISO/OSI Layer
• Bridges operate in the first two layers
• Routers operate in the first three layers
• Switches
– Sophisticated bridge
– Sophisticated router
1
Repeater
Repeaters
• Signals attenuate over longer distances
• Before attenuation causes the signal to become
too weak or become corrupted, repeater
regenerates and transmits the original bit pattern
• Operates only at physical layer
• Simply retransmits bit with more energy
– no CSMS/CD performed
– No address filtering capability
• Partitions network into segments
• All segments in same collision domain.
Hubs
• Overloaded term but means: multiport repeater
• Used to create connections in a physical star
configuration
• All nodes must use same technology
Hubs
Multi-tier Hub Design
Backbone Hub with
Point-to-Point Connection
LAN Segment
LAN Segment
– Example: 10BASE-T Ethernet LAN
• Hubs can be used to build hierarchies to
overcome attenuation limitations
Network still one LAN with logical bus topology
Every packet sent is still received by all stations
Hierarchy removes length limitations of 10BASE-T
Hubs
• Physical Layer devices: essentially repeaters operating at
bit levels: repeat received bits on one interface to all other
interfaces
• Hubs can be arranged in a hierarchy (or multi-tier
design), with a backbone hub at its top
• Each connected LAN is referred to as a LAN segment
• Hubs do not isolate collision domains: a node may
collide with any node residing at any segment in the LAN
Hubs (Cont.)
• Hub Advantages:
+ Simple, inexpensive device
+ Multi-tier provides graceful degradation: portions of the LAN
continue to operate if one of the hubs malfunction
+Extends maximum distance between node pairs (100m per Hub)
• Hub Limitations:
- Single collision domain results in no increase in max throughput;
the multi-tier throughput same as the the single segment
throughput
- Individual LAN restrictions pose limits on the number of nodes
in the same collision domain (thus, per Hub); and on the total
allowed geographical coverage
- Cannot connect different Ethernet types (e.g., 10BaseT and
100baseT)
2
Bridges
• Link Layer devices: they operate on Ethernet frames,
examining the frame header and selectively forwarding a
frame based on its destination
• Bridge isolates collision domains since it buffers frames
• When a frame is to be forwarded on a segment, the
bridge uses CSMA/CD to access the segment and
transmit
• Bridge advantages:
Segmenting Ethernet
Bridge
Bridge
• Network Segmentation
• Increased Bandwidth
• Protocol Independent
Hub
+ Isolates collision domains resulting in higher total max
throughput, and does not limit the number of nodes nor
geographical coverage
+ Can connect different type Ethernet since it is a store and
forward device
+ Transparent: no need for any change to hosts LAN adapters
Segment 1
Bridges
•
•
•
•
Segment 2
Bridge Filtering
Operates in physical and data link layers
Physical layer: regenerates signal it receives
Data Link Layer: filtering and forwarding
–
–
–
Hub
• Bridges learn which hosts can be reached through
which interfaces and maintain filtering tables
• A filtering table entry:
Examines the physical addresses in the packet
Decides if packet should be forwarded
If forwarded, decides which interface and places
frame in that interface’s buffer
(Node LAN Address, Bridge Interface, Time Stamp)
• Filtering procedure:
if destination is on LAN on which frame was received
then drop the frame
else { lookup filtering table
if entry found for destination
Decision based on table that map addresses to
interfaces
then forward the frame on interface indicated;
else flood; /* forward on all but the interface on which the
frame arrived*/
}
Bridge Learning
• When a frame is received, the bridge “learns”
from the source address and updates its filtering
table (Node LAN Address, Bridge Interface,
Time Stamp)
• Stale entries in the Filtering Table are dropped
– TTL could be 60 minutes
Bridge Table
Contains tuples:
1. LAN address of node (62-AE-F7-23-45-B4)
2. Bridge interface of node
3. Timestamp of table entry
•
•
•
•
Bridge will “learn” physical addresses
Examines traffic and builds bridge table
Simplifies the work of the network admin
“Self-learning” bridge also called “transparent”
bridge
3
Primary Functions of Bridge
Primary Functions of a Bridge
Bridge
Address Port
A
1
B
1
C
1
D
2
E
2
F
2
G
2
learn
fwd/filter
MAC frame
IP Datagram
CRC
DATA
MAC frame
SA
H
DA
A
E
Hub
BRIDGE examines these fields:
learns SA for future decisions
forwards or filters based on DA
Bridge checks and
recalculates CRC
Hub
Hub
A
D
C
B
E
F
Segment 1
G
Segment 2
Single Subnet
Total Capacity = 20Mb/s
Further Segmenting: Multiport Bridges
Typical Network topology with Bridges
Bridge
MAC frame
Router
A
E
Hub
A
C
B
Segment 1
D
Hub
B3
B1
Hub
B2
Hub
Hub
E
F
Segment 2
G
B5
B4
Segment 3
Single Subnet
Total Bandwidth = 30Mb/s
Self-learning Bridge
•
•
•
•
•
•
Bridge table initially empty
If a frame arrives and the destination is not in
the table, the frame is copied to output buffers
of all interfaces
Each interfaces sends frame using CSMA/CD
If every node eventually sends a frame, every
node will eventually get recording in the table
Bridge table removes entries after the aging
time has elapsed (like e-mail)
If PC gets new adapter, physical address of old
adapter will eventually be purged from table
Bridges
• Unlike a hub, bridge runs the CSMA/CD
algorithm
– Refrains from sending if it hears traffic and employs
exponential backoff
• Like a NIC except there are no addresses
– Does not insert a physical address into the packet
• Can be used to combine Ethernet segments that
use different technologies (10BASE-T,10BASE2)
4
Bridge
Backbone Bridge
• LAN segments should be connected with a
backbone
• Backbone: network that has direct connection to
LAN segments
• Each pair of LAN segments can communicate
without passing through a third LAN segment
Interconnection Without
Backbone Bridge
Interconnection with
Backbone Bridge
• Two disadvantages:
- Single point of failure at Computer Science hub
- All traffic between EE and SE must path over CS segment
Bridges Spanning Tree
• For increased reliability, it is desirable to
have redundant, alternate paths from a
source to a destination
• With multiple simultaneous paths however,
cycles result on which bridges may multiply
and forward a frame forever
• Solution is organizing the set of bridges in a
spanning tree by disabling a subset of the
interfaces in the bridges
Bridges Spanning Tree
Disabled
5
Preventing Loops:
Spanning Tree Protocol
Preventing Loops
Spanning Tree Protocol
Root
B1
LAN1
B1
B4
B3
LAN2
B5
LAN4
LAN5
B8
B1
B2
B2
LAN2
B3
Root
LAN1
LAN3
B6
B3
B4
B7
LAN5
LAN7
B8
B9
LAN6
B5
LAN3
LAN4
B6
B7
B2
B8
B6
B7
B9
LAN7
B9
LAN6
B5
Bridges exchange “Bridge Protocol Data Units”
(i.e. special packets) to build spanning tree.
B1 has smallest address and is selected as Root
B3 is selected to connect LAN5 to LAN2
Switches
A bridge connects segments of a LAN together.
All segments connected to bridge are still part of
the one LAN
Bridge does not change the physical addresses
contained in packet
Two (network-related) meanings
• Two-layer switch
–
–
–
–
Bridge with many interfaces
Each interface to a new segment with one station
No competing traffic on segments (no collisions)
Like bridges, vulnerable to broadcast storms
• Three-layer switch
– Application specific ICs for better performance
– Customized for specific network layer protocols (IP)
– In some cases, learn routing tables from routers
Ethernet Switches
Switched Ethernet
• A switch is a device that incorporates bridge functions as
well as point-to-point ‘dedicated connections’
• A host attached to a switch via a dedicated point-to-point
connection will sense the medium as idle
• Ethernet Switches provide a combinations of
shared/dedicated, 10/100/1000 Mbps connections
• Some Ethernet switches support cut-through switching:
frame forwarded immediately to destination without
awaiting for assembly of the entire frame in the switch
buffer; slight reduction in latency
• Ethernet switches vary in size, with the largest ones
incorporating a high bandwidth interconnection network
An example of a switch
A
Address
A
B
C
D
1
Step 1: Node D sends
a packet to Node C
D
Step 3: Switch looks up
Port based on DA
C
4
2
Port
1
2
3
4
B
3
Step 2: Switch reads
Destination Address
at Source Port
Step 4: Switch sends
packet to Port 3
C
C
6
Switched Ethernet
Switched Ethernet
Multiple transactions in a switch
A1
A2
A3
A
B1
B2
B3
1
D
Address
Port
A1
1
A2
1
A3
1
B1
3
B2
3
B3
3
.
.
.
Else
Broadcast
Hub
1
2
4
B
Hub
3
2
3
Hub
C
Increased throughput needs
increased forwarding capability
C1
Shared
C3
Multiport Bridge
Bandwidth shared among all
nodes on each segment
Routers
Ethernet Switches (Cont)
Dedicated
C2
•
•
•
•
3-layer device (physical, data link & network)
Physical: regenerates signal it receives
Data link: examines physical addresses in packet
Network layer: checks the network layer
addresses (IP)
• Router is an internetworking device:
– Can connect 2 LANs together
– Can connect 2 WANs together
– Can connect a LAN and WAN together
Routing example
A repeater or a bridge connects segments
of a LAN.
A router connects independent LANs or
WANs to create an internetwork
(internet).
7
Bridges Vs. Routers
A router changes the physical addresses in
a packet.
Bridges Pros and Cons:
+ Bridge operation is simpler requiring less
processing bandwidth, i.e., faster
- Topologies are restricted with bridges: a spanning
tree must be built to avoid cycles
- Bridges do not offer protection from “broadcast
storms” (endless broadcasting by a host will be
forwarded by a bridge)
• Both are store-and-forward devices, but Routers are
Network Layer devices (examine network layer headers)
and Bridges are Link Layer devices
• Routers maintain routing tables and implement routing
algorithms, bridges maintain filtering tables and
implement filtering, learning and spanning tree algorithms
Routers Pros and Cons:
+ Arbitrary topologies can be supported, cycling is
limited by TTL counters
+ Can provide firewall protection against broadcast
storms
- Often requires IP address configuration
- not plug and play; DHCP is exception
- Require higher processing bandwidth
• Bridges do well in small (few hundred hosts)
while routers are required in large networks
(thousands of hosts)
Routers vs. Repeaters/Bridges (con’t)
Routers vs. Repeaters/Bridges (con’t)
• A router has both a physical and logical (IP)
address for each of its interfaces
• A router acts only on those packets in which the
destination address matches the address of the
interface at which the packet arrives
• A router separates collision domains
– True for unicast, multicast and broadcast
• A router changes the physical address of the
packet (both source and destination) when
forwarding the packet
– Can improve performance
– But 3-layer processing more time consuming than 2layer processing
• Transparent bridges are “plug-and-play”
• Routers require configuration (more admin)
• Bridges implement spanning tree over nodes to
eliminate cycles
– Implemented by virtually disconnecting nodes
– Results in self-imposed traffic increases
– Frames concentrated on spanning tree links when it
could be spread on all links in original topology
8
Routers vs. Repeaters/Bridges
• If failure in a link, new spanning tree is created
automatically
• Bridges offer no protection from layer-2
broadcast storms
• Bridges increase traffic flow and require no
manual IP configuration
• Routers provide better isolation of traffic, control
broadcast storms and can use more intelligent
routes among hosts
Switches
Two (network-related) meanings
• Two-layer switch
–
–
–
–
Bridge with many interfaces
Each interface to a new segment with one station
No competing traffic on segments (no collisions)
Like bridges, vulnerable to broadcast storms
• Three-layer switch
– Application specific ICs for better performance
– Customized for specific network layer protocols (IP)
– In some cases, learn routing tables from routers
9
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