Lab - Securing Layer 2 Switches

Lab - Securing Layer 2 Switches
CCNA Security
Lab - Securing Layer 2 Switches
Topology
Note: ISR G1 devices use FastEthernet interfaces instead of GigabitEthernet interfaces.
IP Addressing Table
Device
Interface
IP Address
Subnet Mask
Default Gateway
Switch Port
R1
G0/1
192.168.1.1
255.255.255.0
N/A
S1 F0/5
S1
VLAN 1
192.168.1.2
255.255.255.0
N/A
N/A
S2
VLAN 1
192.168.1.3
255.255.255.0
N/A
N/A
PC-A
NIC
192.168.1.10
255.255.255.0
192.168.1.1
S1 F0/6
PC-B
NIC
192.168.1.11
255.255.255.0
192.168.1.1
S2 F0/18
Objectives
Part 1: Configure Basic Switch Settings

Build the topology.

Configure the hostname, IP address, and access passwords.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 1 of 23
Lab – Securing Layer 2 Switches
Part 2: Configure SSH Access to the Switches

Configure SSH version 2 access on the switch.

Configure an SSH client to access the switch.

Verify the configuration.
Part 3: Configure Secure Trunks and Access Ports

Configure trunk port mode.

Change the native VLAN for trunk ports.

Verify trunk configuration.

Enable storm control for broadcasts.

Configure access ports.

Enable PortFast and BPDU guard.

Verify BPDU guard.

Enable root guard.

Enable loop guard.

Configure and verify port security.

Disable unused ports.

Move ports from default VLAN 1 to alternate VLAN.

Configure the PVLAN Edge feature on a port.
Part 4: Configure IP DHCP Snooping

Configure DHCP on R1.

Configure Inter-VLAN communication on R1.

Configure S1 interface F0/5 as a trunk.

Verify DHCP operation on PC- A and B.

Enable DHCP Snooping.

Verify DHCP Snooping.
Background / Scenario
The Layer 2 infrastructure consists mainly of interconnected Ethernet switches. Most end-user devices, such
as computers, printers, IP phones, and other hosts, connect to the network via Layer 2 access switches. As a
result, switches can present a network security risk. Similar to routers, switches are subject to attack from
malicious internal users. The switch Cisco IOS software provides many security features that are specific to
switch functions and protocols.
In this lab, you will configure SSH access and Layer 2 security for S1 and S2. You will also configure various
switch protection measures, including access port security and Spanning Tree Protocol (STP) features, such
as BPDU guard and root guard.
Note: The router commands and output in this lab are from a Cisco 1941 router using Cisco IOS software,
release 15.4(3)M2 (with a Security Technology Package license). The switch commands and output are from
Cisco WS-C2960-24TT-L switches with Cisco IOS Release 15.0(2)SE4 (C2960-LANBASEK9-M image).
Other routers, switches, and Cisco IOS versions can be used. See the Router Interface Summary Table at the
end of the lab to determine which interface identifiers to use based on the equipment in the lab. The
commands available to the user and the output produced may vary depending on which router, switch, and
Cisco IOS version is used.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 2 of 23
Lab – Securing Layer 2 Switches
Note: Make sure that the routers and switches have been erased and have no startup configurations.
Required Resources

1 Router (Cisco 1941 with Cisco IOS Release 15.4(3)M2 image with a Security Technology Package
license)

2 Switches (Cisco 2960 with cryptography IOS image for SSH support – Release 15.0(2)SE7 or
comparable)

2 PCs (Windows 7 or Windows 8 with SSH client software

Ethernet cables as shown in the topology

Console cables to configure Cisco networking devices
Part 1: Configure Basic Switch Settings
In Part 1, you will set up the network topology and configure basic settings, such as the hostnames, IP
addresses, and device access passwords.
Step 1: Cable the network as shown in the topology.
Attach the devices, as shown in the topology diagram, and cable as necessary.
Step 2: Configure basic settings for the router and each switch.
Perform all tasks on R1, S1, and S2. The procedure for S1 is shown here as an example.
a. Configure hostnames, as shown in the topology.
b. Configure interface IP addresses, as shown in the IP Addressing Table. The following configuration
displays the VLAN 1 management interface on S1:
S1(config)# interface vlan 1
S1(config-if)# ip address 192.168.1.2 255.255.255.0
S1(config-if)# no shutdown
c.
Prevent the router or switch from attempting to translate incorrectly entered commands by disabling DNS
lookup. S1 is shown here as an example.
S1(config)# no ip domain-lookup
d. HTTP access to the switch is enabled by default. Prevent HTTP access by disabling the HTTP server and
HTTP secure server.
S1(config)# no ip http server
S1(config)# no ip http secure-server
Note: The switch must have a cryptography IOS image to support the ip http secure-server command.
HTTP access to the router is disabled by default.
e. Configure the enable secret password.
S1(config)# enable algorithm-type scrypt secret cisco12345
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 3 of 23
Lab – Securing Layer 2 Switches
f.
Configure console password.
S1(config)# line
S1(config-line)#
S1(config-line)#
S1(config-line)#
S1(config-line)#
console 0
password ciscoconpass
exec-timeout 5 0
login
logging synchronous
Step 3: Configure PC host IP settings.
Configure a static IP address, subnet mask, and default gateway for PC-A and PC-B, as shown in the IP
Addressing Table.
Step 4: Verify basic network connectivity.
a. Ping from PC-A and PC-B to the R1 F0/1 interface at IP address 192.168.1.1.
If the pings are unsuccessful, troubleshoot the basic device configurations before continuing.
b. Ping from PC-A to PC-B.
If the pings are unsuccessful, troubleshoot the basic device configurations before continuing.
Step 5: Save the basic configurations for the router and both switches.
Save the running configuration to the startup configuration from the privileged EXEC mode prompt.
S1# copy running-config startup-config
Part 2: Configure SSH Access to the Switches
In Part 2, you will configure S1 and S2 to support SSH connections and install SSH client software on the
PCs.
Note: A switch IOS image that supports encryption is required to configure SSH. If this version of image is not
used you cannot specify SSH as an input protocol for the vty lines and the crypto commands are unavailable.
Task 1: Configure the SSH Server on S1 and S2 Using the CLI.
In this task, use the CLI to configure the switch to be managed securely using SSH instead of Telnet. SSH is
a network protocol that establishes a secure terminal emulation connection to a switch or other networking
device. SSH encrypts all information that passes over the network link and provides authentication of the
remote computer. SSH is rapidly replacing Telnet as the preferred remote login tool for network professionals.
It is strongly recommended that SSH be used in place of Telnet on production networks.
Note: A switch must be configured with local authentication or AAA in order to support SSH.
Step 1: Configure a domain name.
Enter global configuration mode and set the domain name.
S1# conf t
S1(config)# ip domain-name ccnasecurity.com
Step 2: Configure a privileged user for login from the SSH client.
Use the username command to create the user ID with the highest possible privilege level and a secret
password.
S1(config)# username admin privilege 15 algorithm-type scrypt secret
cisco12345
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 4 of 23
Lab – Securing Layer 2 Switches
Step 3: Generate the RSA encryption key pair for the router.
The switch uses the RSA key pair for authentication and encryption of transmitted SSH data.
Configure the RSA keys with 1024 modulus bits. The default number of modulus bits is 512, and the range is
from 360 to 2,048.
S1(config)# crypto key generate rsa general-keys modulus 1024
The name for the keys will be: S1.ccnasecurity.com
% The key modulus size is 1024 bits
% Generating 1024 bit RSA keys, keys will be non-exportable...[OK]
S1(config)#
00:15:36: %SSH-5-ENABLED: SSH 1.99 has been enabled
Step 4: Configure SSH version 2
S1(config)# ip ssh version 2
Step 5: Verify the SSH configuration.
a. Use the show ip ssh command to see the current settings.
S1# show ip ssh
b. Fill in the following information based on the output of the show ip ssh command:
SSH version enabled:
______________________________________________
Authentication timeout: ______________________________________________
Authentication retries:
______________________________________________
Step 6: Configure SSH timeouts and authentication parameters.
The default SSH timeouts and authentication parameters can be altered to be more restrictive using the
following commands.
S1(config)# ip ssh time-out 90
S1(config)# ip ssh authentication-retries 2
Step 7: Configure the incoming vty lines.
a. Configure vty access on lines 0 to 4. Specify a privilege level of 15. This will ensure that a user with the
highest privilege level (15) will default to privileged EXEC mode when accessing the vty lines. Other users
will default to user EXEC mode. Specify the use of local user accounts for mandatory login and validation
and accept only SSH connections.
S1(config)# line
S1(config-line)#
S1(config-line)#
S1(config-line)#
S1(config-line)#
S1(config-line)#
vty 0 4
privilege level 15
exec-timeout 5 0
login local
transport input ssh
exit
b. Disable login for switch vty lines 5 to 15 by allowing no transport input.
S1(config)# line vty 5 15
S1(config-line)# transport input none
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 5 of 23
Lab – Securing Layer 2 Switches
Step 8: Save the running configuration to the startup configuration.
S1# copy running-config startup-config
Task 2: Configure the SSH Client
PuTTy and Tera Term are two terminal emulation programs that can support SSHv2 client connections. This
lab uses PuTTY.
Step 1: (Optional) Download and install an SSH client on PC-A and PC-B.
If the SSH client is not already installed, download PuTTY from the following link:
http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html
Note: The procedure described here is for PuTTY and pertains to PC-A.
Step 2: Verify SSH connectivity to S1 from PC-A.
a. Launch PuTTY by double-clicking the putty.exe icon (and clicking Run if prompted).
b. Input the S1 IP address 192.168.1.2 in the Host Name (or IP address) field.
c.
Verify that the SSH radio button is selected. PuTTY defaults to SSH version 2.
d. Click Open.
Note: Upon first connection, the user is prompted with a PuTTY Security Alert stating that the server’s
host key is not cached in the registry.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 6 of 23
Lab – Securing Layer 2 Switches
e. In the PuTTY Security Alert window, click Yes to cache the server’s host key.
f.
In the PuTTY window, enter admin as the username and cisco12345 as the password.
g. At the S1 privileged EXEC mode prompt, enter the show users command.
S1# show users
Which users are connected to S1 at this time?
____________________________________________________________________________________
____________________________________________________________________________________
h. Close the PuTTy SSH session window with the exit or quit command.
Try to open a Telnet session to S1 from PC-A. Were you able to open the Telnet session? Explain.
____________________________________________________________________________________
____________________________________________________________________________________
Step 3: Save the configuration.
Save the running configuration to the startup configuration from the privileged EXEC mode prompt.
S1# copy running-config startup-config
Part 3: Configure Secure Trunks and Access Ports
In Part 3, you will configure trunk ports, change the native VLAN for trunk ports, and verify trunk configuration.
Securing trunk ports can help stop VLAN hopping attacks. The best way to prevent a basic VLAN hopping
attack is to explicitly disable trunking on all ports except the ports that specifically require trunking. On the
required trunking ports, disable DTP (auto trunking) negotiations and manually enable trunking. If no trunking
is required on an interface, configure the port as an access port. This disables trunking on the interface.
Note: Tasks should be performed on S1 or S2, as indicated.
Task 1: Secure Trunk Ports
Step 1: Configure S1 as the root switch.
For the purposes of this lab, S2 is currently the root bridge. You will configure S1 as the root bridge by
changing the bridge ID priority level.
a. From the console on S1, enter global configuration mode.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 7 of 23
Lab – Securing Layer 2 Switches
b. The default priority for S1 and S2 is 32769 (32768 + 1 with System ID Extension). Set S1 priority to 0 so
that it becomes the root switch.
S1(config)# spanning-tree vlan 1 priority 0
S1(config)# exit
Note: You can also use the spanning-tree vlan 1 root primary command to make S1 the root switch for
VLAN 1.
c.
Issue the show spanning-tree command to verify that S1 is the root bridge, to see the ports in use, and
to see their status.
S1# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID
Priority
1
Address
001d.4635.0c80
This bridge is the root
Hello Time
2 sec Max Age 20 sec
Bridge ID
Forward Delay 15 sec
Priority
1
(priority 0 sys-id-ext 1)
Address
001d.4635.0c80
Hello Time
2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface
---------------Fa0/1
Fa0/5
Fa0/6
Role
---Desg
Desg
Desg
Sts
--FWD
FWD
FWD
Cost
--------19
19
19
Prio.Nbr
-------128.1
128.5
128.6
Type
-------------------------------P2p
P2p
P2p
d. What is the S1 priority?
____________________________________________________________________________________
Which ports are in use and what is their status?
____________________________________________________________________________________
____________________________________________________________________________________
Step 2: Configure trunk ports on S1 and S2.
a. Configure port F0/1 on S1 as a trunk port.
S1(config)# interface f0/1
S1(config-if)# switchport mode trunk
Note: If performing this lab with a 3560 switch, the user must first enter the switchport trunk
encapsulation dot1q command.
b. Configure port F0/1 on S2 as a trunk port.
S2(config)# interface f0/1
S2(config-if)# switchport mode trunk
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 8 of 23
Lab – Securing Layer 2 Switches
c.
Verify that S1 port F0/1 is in trunking mode with the show interfaces trunk command.
S1# show interfaces trunk
Port
Fa0/1
Mode
on
Encapsulation
802.1q
Status
trunking
Native vlan
1
Port
Fa0/1
Vlans allowed on trunk
1-4094
Port
Fa0/1
Vlans allowed and active in management domain
1
Port
Fa0/1
Vlans in spanning tree forwarding state and not pruned
1
Step 3: Change the native VLAN for the trunk ports on S1 and S2.
a. Changing the native VLAN for trunk ports to an unused VLAN helps prevent VLAN hopping attacks.
From the output of the show interfaces trunk command in the previous step, what is the current native
VLAN for the S1 F0/1 trunk interface?
____________________________________________________________________________________
b. Set the native VLAN on the S1 F0/1 trunk interface to an unused VLAN 99.
S1(config)# interface f0/1
S1(config-if)# switchport trunk native vlan 99
S1(config-if)# end
c.
The following message should display after a brief period of time:
02:16:28: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on
FastEthernet0/1 (99), with S2 FastEthernet0/1 (1).
What does the message mean?
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
d. Set the native VLAN on the S2 F0/1 trunk interface to VLAN 99.
S2(config)# interface f0/1
S2(config-if)# switchport trunk native vlan 99
S2(config-if)# end
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 9 of 23
Lab – Securing Layer 2 Switches
Step 4: Prevent the use of DTP on S1 and S2.
Setting the trunk port to nonegotiate also helps to mitigate VLAN hopping by turning off the generation of
DTP frames.
S1(config)# interface f0/1
S1(config-if)# switchport nonegotiate
S2(config)# interface f0/1
S2(config-if)# switchport nonegotiate
Step 5: Verify the trunking configuration on port F0/1.
S1# show interfaces f0/1 trunk
Port
Fa0/1
Mode
on
Encapsulation
802.1q
Status
trunking
Native vlan
99
Port
Fa0/1
Vlans allowed on trunk
1-4094
Port
Fa0/1
Vlans allowed and active in management domain
1
Port
Fa0/1
Vlans in spanning tree forwarding state and not pruned
1
S1# show interfaces f0/1 switchport
Name: Fa0/1
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Off
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 99 (Inactive)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 10 of 23
Lab – Securing Layer 2 Switches
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
Step 6: Verify the configuration with the show run command.
Use the show run command to display the running configuration, beginning with the first line that has the text
string “0/1” in it.
S1# show run | begin 0/1
interface FastEthernet0/1
switchport trunk native vlan 99
switchport mode trunk
switchport nonegotiate
<output omitted>
Task 2: Secure Access Ports
Network attackers hope to spoof their system, or a rogue switch that they add to the network, as the root
bridge in the topology by manipulating the STP root bridge parameters.. If a port that is configured with
PortFast receives a BPDU, STP can put the port into the blocking state by using a feature called BPDU
guard.
Step 1: Disable trunking on S1 access ports.
a. On S1, configure Fa0/5, the port to which R1 is connected, as access mode only.
S1(config)# interface f0/5
S1(config-if)# switchport mode access
b. On S1, configure Fa0/6, the port to which PC-A is connected, as access mode only.
S1(config)# interface f0/6
S1(config-if)# switchport mode access
Step 2: Disable trunking on S2 access ports.
On S2, configure Fa0/18, the port to which PC-B is connected, as access mode only.
S2(config)# interface f0/18
S2(config-if)# switchport mode access
Task 3: Protect Against STP Attacks
The topology has only two switches and no redundant paths, but STP is still active. In this step, you will
enable switch security features that can help reduce the possibility of an attacker manipulating switches via
STP-related methods.
Step 1: Enable PortFast on S1 and S2 access ports.
PortFast is configured on access ports that connect to a single workstation or server, which enables them to
become active more quickly.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 11 of 23
Lab – Securing Layer 2 Switches
a. Enable PortFast on the S1 Fa0/5 access port.
S1(config)# interface f0/5
S1(config-if)# spanning-tree portfast
%Warning: portfast should only be enabled on ports connected to a single host.
Connecting hubs, concentrators, switches, bridges, etc... to this interface when
portfast is enabled, can cause temporary bridging loops. Use with CAUTION
%Portfast has been configured on FastEthernet0/5 but will only
have effect when the interface is in a non-trunking mode.
b. Enable PortFast on the S1 Fa0/6 access port.
S1(config)# interface f0/6
S1(config-if)# spanning-tree portfast
c.
Enable PortFast on the S2 Fa0/18 access ports.
S2(config)# interface f0/18
S2(config-if)# spanning-tree portfast
Step 2: Enable BPDU guard on the S1 and S2 access ports.
BPDU guard is a feature that can help prevent rogue switches and spoofing on access ports.
a. Enable BPDU guard on the switch port F0/6.
S1(config)# interface f0/6
S1(config-if)# spanning-tree bpduguard enable
S2(config)# interface f0/18
S2(config-if)# spanning-tree bpduguard enable
Note: PortFast and BPDU guard can also be enabled globally with the spanning-tree portfast default
and spanning-tree portfast bpduguard commands in global configuration mode.
Note: BPDU guard can be enabled on all access ports that have PortFast enabled. These ports should
never receive a BPDU. BPDU guard is best deployed on user-facing ports to prevent rogue switch
network extensions by an attacker. If a port is enabled with BPDU guard and receives a BPDU, it is
disabled and must be manually re-enabled. An err-disable timeout can be configured on the port so that
it can recover automatically after a specified time period.
b. Verify that BPDU guard is configured by using the show spanning-tree interface f0/6 detail command
on S1.
S1# show spanning-tree interface f0/6 detail
Port 6 (FastEthernet0/6) of VLAN0001 is designated forwarding
Port path cost 19, Port priority 128, Port Identifier 128.6.
Designated root has priority 1, address 001d.4635.0c80
Designated bridge has priority 1, address 001d.4635.0c80
Designated port id is 128.6, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
The port is in the portfast mode
Link type is point-to-point by default
Bpdu guard is enabled
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 12 of 23
Lab – Securing Layer 2 Switches
BPDU: sent 3349, received 0
Step 3: Enable root guard.
Root guard is another option to help prevent rogue switches and spoofing. Root guard can be enabled on all
ports on a switch that are not root ports. It is normally enabled only on ports connecting to edge switches
where a superior BPDU should never be received. Each switch should have only one root port, which is the
best path to the root switch.
a. The following command configures root guard on S2 interface Gi0/1. Normally, this is done if another
switch is attached to this port. Root guard is best deployed on ports that connect to switches that should
not be the root bridge. In the lab topology, S1 F0/1 would be the most logical candidate for root guard.
However, S2 Gi0/1 is shown here as an example, as Gigabit ports are more commonly used for interswitch connections.
S2(config)# interface g0/1
S2(config-if)# spanning-tree guard root
b. Issue the show run | begin Gig command to verify that root guard is configured.
S2# show run | begin Gig
interface GigabitEthernet0/1
spanning-tree guard root
Note: The S2 Gi0/1 port is not currently up, so it is not participating in STP. Otherwise, you could use the
show spanning-tree interface Gi0/1 detail command.
Note: The expression in the command show run | begin is case-sensitive.
c.
If a port that is enabled with BPDU guard receives a superior BPDU, it enters a root-inconsistent state.
Use the show spanning-tree inconsistentports command to determine if there are any ports currently
receiving superior BPDUs that should not be.
S2# show spanning-tree inconsistentports
Name
Interface
Inconsistency
-------------------- ---------------------- -----------------Number of inconsistent ports (segments) in the system : 0
Note: Root guard allows a connected switch to participate in STP as long as the device does not try to
become the root. If root guard blocks the port, subsequent recovery is automatic. The port returns to the
forwarding state if the superior BPDUs stop.
Step 4: Enable Loop Guard
The STP loop guard feature provides additional protection against Layer 2 forwarding loops (STP loops).
An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the
forwarding state. This usually happens because one of the ports of a physically redundant topology (not
necessarily the STP blocking port) no longer receives STP BPDUs. Having all ports in forwarding state
will result in forwarding loops. If a port enabled with loopguard stops hearing BPDUs from the designated
port on the segment, it goes into the loop inconsistent state instead of transitioning into forwarding state.
Loop inconsistent is basically blocking, and no traffic is forwarded. When the port detects BPDUs again it
automatically recovers by moving back into blocking state.
a. Loop guard should be applied to non-designated ports. Therefore, the global command can be configured
on non-root switches.
S2(config)# spanning-tree loopguard default
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 13 of 23
Lab – Securing Layer 2 Switches
b. Verify Loopguard configuration
S2# show spanning-tree summary
Switch is in pvst mode
Extended system ID
is
Portfast Default
is
PortFast BPDU Guard Default is
Portfast BPDU Filter Default is
Loopguard Default
is
EtherChannel misconfig guard is
UplinkFast
is
BackboneFast
is
Configured Pathcost method used
enabled
disabled
disabled
disabled
enabled
enabled
disabled
disabled
is short
Name
Blocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ---------VLAN0001
0
0
0
3
3
---------------------- -------- --------- -------- ---------- ---------
Task 4: Configure Port Security and Disable Unused Ports
Switches can be subject to a CAM table, also known as a MAC address table, overflow, MAC spoofing
attacks, and unauthorized connections to switch ports. In this task, you will configure port security to limit the
number of MAC addresses that can be learned on a switch port and disable the port if that number is
exceeded.
Step 1: Record the R1 Fa0/0 MAC address.
From the R1 CLI, use the show interface command and record the MAC address of the interface.
R1# show interfaces g0/1
GigabitEthernet0/1 is up, line protocol is up
Hardware is CN Gigabit Ethernet, address is fc99.4775.c3e1 (bia fc99.4775.c3e1)
Internet address is 192.168.1.1/24
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full Duplex, 100Mbps, media type is RJ45
<Output Omitted>
What is the MAC address of the R1 G0/1 interface?
____________________________________________________________________________________
____________________________________________________________________________________
Step 2: Configure basic port security.
This procedure should be performed on all access ports that are in use. S1 port Fa0/5 is shown here as an
example.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 14 of 23
Lab – Securing Layer 2 Switches
a. From the S1 CLI, enter interface configuration mode for the port that connects to the router (Fast Ethernet
0/5).
S1(config)# interface f0/5
b. Shut down the switch port.
S1(config-if)# shutdown
c.
Enable port security on the port.
S1(config-if)# switchport port-security
Note: A switch port must be configured as an access port to enable port security.
Note: Entering just the switchport port-security command sets the maximum MAC addresses to 1 and
the violation action to shutdown. The switchport port-security maximum and switchport portsecurity violation commands can be used to change the default behavior.
d. Configure a static entry for the MAC address of R1 Fa0/1/ interface recorded in Step 1.
S1(config-if)# switchport port-security mac-address xxxx.xxxx.xxxx
Note: xxxx.xxxx.xxxx is the actual MAC address of the router G0/1 interface.
Note: You can also use the switchport port-security mac-address sticky command to add all the
secure MAC addresses that are dynamically learned on a port (up to the maximum set) to the switch
running configuration.
e. Enable the switch port.
S1(config-if)# no shutdown
Step 3: Verify port security on S1 Fa0/5.
a. On S1, issue the show port-security command to verify that port security has been configured on S1
F0/5.
S1# show port-security interface f0/5
Port Security
Port Status
Violation Mode
Aging Time
Aging Type
SecureStatic Address Aging
Maximum MAC Addresses
Total MAC Addresses
Configured MAC Addresses
Sticky MAC Addresses
Last Source Address:Vlan
Security Violation Count
:
:
:
:
:
:
:
:
:
:
:
:
Enabled
Secure-up
Shutdown
0 mins
Absolute
Disabled
1
1
1
0
0000.0000.0000:0
0
What is the Security Violation Count? ________
What is the status of the F0/5 port?
____________________________________________________________________________________
____________________________________________________________________________________
What is the Last Source Address and VLAN?
____________________________________________________________________________________
____________________________________________________________________________________
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 15 of 23
Lab – Securing Layer 2 Switches
b. From the R1 CLI, ping PC-A to verify connectivity. This also ensures that the R1 Fa0/1 MAC address is
learned by the switch.
R1# ping 192.168.1.10
c.
Now, violate security by changing the MAC address on the router interface. Enter interface configuration
mode for the Fast Ethernet 0/1. Configure a MAC address for the interface on the interface, using
aaaa.bbbb.cccc as the address.
R1(config)# interface G0/1
R1(config-if)# mac-address aaaa.bbbb.cccc
R1(config-if)# end
Note: You can also change the PC MAC address attached to S1 F0/6 and achieve similar results to those
shown here.
d. From the R1 CLI, ping PC-A. Was the ping successful? Explain.
____________________________________________________________________________________
____________________________________________________________________________________
e. On S1 console, observe the messages when port F0/5 detects the violating MAC address.
*Jan 14 01:34:39.750: %PM-4-ERR_DISABLE: psecure-violation error detected on Fa0/5,
putting Fa0/5 in err-disable state
*Jan 14 01:34:39.750: %PORT_SECURITY-2-PSECURE_VIOLATION: Security violation occurred,
caused by MAC address aaaa.bbbb.cccc on port FastEthernet0/5.
*Jan 14 01:34:40.756: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/5,
changed state to down
*Jan 14 01:34:41.755: %LINK-3-UPDOWN: Interface FastEthernet0/5, changed state to down
f.
On the switch, use the show port-security commands to verify that port security has been violated.
S1# show port-security
Secure Port MaxSecureAddr CurrentAddr SecurityViolation Security Action
(Count)
(Count)
(Count)
-------------------------------------------------------------------Fa0/5
1
1
1
Shutdown
---------------------------------------------------------------------Total Addresses in System (excluding one mac per port)
: 0
Max Addresses limit in System (excluding one mac per port) : 8192
S1# show port-security interface f0/5
Port Security
Port Status
Violation Mode
Aging Time
Aging Type
SecureStatic Address Aging
Maximum MAC Addresses
Total MAC Addresses
Configured MAC Addresses
Sticky MAC Addresses
Last Source Address:Vlan
Security Violation Count
:
:
:
:
:
:
:
:
:
:
:
:
Enabled
Secure-shutdown
Shutdown
0 mins
Absolute
Disabled
1
1
1
0
aaaa.bbbb.cccc:1
1
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 16 of 23
Lab – Securing Layer 2 Switches
S1# show port-security address
Secure Mac Address Table
----------------------------------------------------------------------------Vlan
Mac Address
Type
Ports
Remaining Age
(mins)
--------------------------------1
fc99.4775.c3e1
SecureConfigured
Fa0/5
----------------------------------------------------------------------------Total Addresses in System (excluding one mac per port)
: 0
Max Addresses limit in System (excluding one mac per port) : 8192
g. Remove the hard-coded MAC address from the router and re-enable the Fast Ethernet 0/1 interface.
R1(config)# interface g0/1
R1(config-if)# no mac-address aaaa.bbbb.cccc
Note: This will restore the original FastEthernet interface MAC address.
From R1, try to ping the PC-A again at 192.168.1.10. Was the ping successful? Why or why not?
____________________________________________________________________________________
____________________________________________________________________________________
Step 4: Clear the S1 Fa0/5 error disabled status.
a. From the S1 console, clear the error and re-enable the port using the commands shown in the example.
This will change the port status from Secure-shutdown to Secure-up.
S1(config)# interface f0/5
S1(config-if)# shutdown
S1(config-if)# no shutdown
Note: This assumes the device/interface with the violating MAC address has been removed and replaced
with the original device/interface configuration.
b. From R1, ping PC-A again. You should be successful this time.
R1# ping 192.168.1.10
Step 5: Remove basic port security on S1 F0/5.
From the S1 console, remove port security on Fa0/5. This procedure can also be used to re-enable the port,
but port security commands must be reconfigured.
S1(config)# interface f0/5
S1(config-if)# no switchport port-security
S1(config-if)# no switchport port-security mac-address fc99.4775.c3e1
You can also use the following commands to reset the interface to its default settings:
S1(config)# default interface f0/5
S1(config)# interface f0/5
Note: This default interface command also requires that you reconfigure the port as an access port to
re-enable the security commands.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 17 of 23
Lab – Securing Layer 2 Switches
Step 6: (Optional) Configure port security for VoIP.
This example shows a typical port security configuration for a voice port. Three MAC addresses are allowed
and should be learned dynamically. One MAC address is for the IP phone, one is for the switch, and one is for
the PC connected to the IP phone. Violations of this policy result in the port being shut down. The aging
timeout for the learned MAC addresses is set to two hours.
The following example displays S2 port F0/18:
S2(config)# interface f0/18
S2(config-if)# switchport mode access
S2(config-if)# switchport port-security
S2(config-if)# switchport port-security maximum 3
S2(config-if)# switchport port-security violation shutdown
S2(config-if)# switchport port-security aging time 120
Step 7: Disable unused ports on S1 and S2.
As a further security measure, disable ports that are not being used on the switch.
a. Ports F0/1, F0/5, and F0/6 are used on S1. The remaining Fast Ethernet ports and the two Gigabit
Ethernet ports will be shut down.
S1(config)# interface range f0/2 - 4
S1(config-if-range)# shutdown
S1(config-if-range)# interface range f0/7 - 24
S1(config-if-range)# shutdown
S1(config-if-range)# interface range g0/1 - 2
S1(config-if-range)# shutdown
b. Ports Fa0/1 and Fa0/18 are used on S2. The remaining Fast Ethernet ports and the Gigabit Ethernet
ports will be shut down.
S2(config)# interface range f0/2 – 17 , f0/19 – 24 , g0/1 - 2
S2(config-if-range)# shutdown
Step 8: Move active ports to a VLAN other than the default VLAN 1.
As a further security measure, you can move all active end-user ports and router ports to a VLAN other than
the default VLAN 1 on both switches.
a. Configure a new VLAN for users on each switch using the following commands:
S1(config)# vlan 20
S1(config-vlan)# name Users
S2(config)# vlan 20
S2(config-vlan)# name Users
b. Add the current active access (non-trunk) ports to the new VLAN.
S1(config)# interface f0/6
S1(config-if-range)# switchport access vlan 20
S2(config)# interface f0/18
S2(config-if)# switchport access vlan 20
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 18 of 23
Lab – Securing Layer 2 Switches
Note: This will prevent communication between end-user hosts and the management VLAN IP address of
the switch, which is currently VLAN 1. The switch can still be accessed and configured using the console
connection.
Note: To provide SSH access to the switch, a specific port can be designated as the management port
and added to VLAN 1 with a specific management workstation attached. A more elaborate solution is to
create a new VLAN for switch management (or use the existing native trunk VLAN 99), and configure a
separate subnet for the management and user VLANs. In Part 4 you will enable trunking with
subinterfaces on R1 to provide communication between the management and user VLAN subnets.
Step 9: Configure a port with the PVLAN Edge feature.
Some applications require that no traffic be forwarded at Layer 2 between ports on the same switch so that
one neighbor does not see the traffic generated by another neighbor. In such an environment, the use of the
Private VLAN (PVLAN) Edge feature, also known as protected ports, ensures that there is no exchange of
unicast, broadcast, or multicast traffic between these ports on the switch. The PVLAN Edge feature can only
be implemented for ports on the same switch and is locally significant.
For example, to prevent traffic between host PC-A on S1 (port Fa0/6) and a host on another S1 port (e.g. port
Fa0/7, which was previously shut down), you could use the switchport protected command to activate the
PVLAN Edge feature on these two ports. Use the no switchport protected interface configuration command
to disable protected port.
a. Configure the PVLAN Edge feature in interface configuration mode using the following commands:
S1(config)# interface f0/6
S1(config-if)# switchport protected
S1(config-if)# interface f0/7
S1(config-if)# switchport protected
S1(config-if)# no shut
S1(config-if)# end
b. Verify that the PVLAN Edge Feature (protected port) is enabled on Fa0/6.
S1# show interfaces fa0/6 switchport
Name: Fa0/6
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 20 (Users)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 19 of 23
Lab – Securing Layer 2 Switches
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: true
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
c.
Deactivate protected port on interfaces Fa0/6 and Fa0/7 using the following commands:
S1(config)# interface range f0/6 - 7
S1(config-if-range)# no switchport protected
Part 4: Configure DHCP Snooping
DHCP snooping is a Cisco Catalyst feature that determines which switch ports can respond to DHCP requests. It
enables only authorized DHCP servers to respond to DHCP requests and distribute network information to clients.
Task 1: Set Up DHCP
Step 1: Set up DHCP on R1 for VLAN 1.
R1(config)# ip dhcp pool CCNAS
R1(dhcp-config)# network 192.168.1.0 255.255.255.0
R1(dhcp-config)# default-router 192.168.1.1
R1(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.4
Step 2: Set up DHCP on R1 for VLAN 20.
R1(config)# ip dhcp pool 20Users
R1(dhcp-config)# network 192.168.20.0 255.255.255.0
R1(dhcp-config)# default-router 192.168.20.1
R1(config)# ip dhcp excluded-address 192.168.20.1
Task 2: Configure Inter-VLAN Communication
Step 1: Configure subinterfaces on R1.
R1(config)# interface g0/1
R1(config-if)# shutdown
R1(config-if)# no ip address 192.168.1.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# int g0/1.1
R1(config-if)# encapsulation dot1q 1
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# int g0/1.20
R1(config-if)# encapsulation dot1q 20
R1(config-if)# ip address 192.168.20.1 255.255.255.0
R1(config-if)# int g0/1.99
R1(config-if)# encapsulation dot1q 99
R1(config-if)# ip address 192.168.99.1 255.255.255.0
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 20 of 23
Lab – Securing Layer 2 Switches
Step 2: Configure S1 interface f0/5 as a trunk port.
S1(config)# int f0/5
S1(config-if)# switchport mode trunk
S1(config-if)# switchport trunk native vlan 99
Step 3: Configure PC-A and PC-B to obtain an IP Address using DHCP.
Change network settings on PC-A and PC-B to obtain an IP Address automatically.
Step 4: Verify DHCP operation.
Use ipconfig at the command prompt of PC-A and PC-B.
Task 3: Configure DHCP Snooping
Step 1: Enable DHCP snooping globally.
S1(config)# ip dhcp snooping
S1(config)# ip dhcp snooping information option
Step 2: Enable DHCP snooping for VLAN 1 and 20.
S1(config)# ip dhcp snooping vlan 1,20
Step 3: Limit the number of DHCP requests on an interface.
S1(config)# interface f0/6
S1(config-if)# ip dhcp snooping limit rate 10
S1(config-if)# exit
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 21 of 23
Lab – Securing Layer 2 Switches
Step 4: Identify the trusted interface(s). DHCP responses are only permitted through trusted
ports.
S1(config)# interface f0/5
S1(config-if)# description connects to DHCP server
S1(config-if)# ip dhcp snooping trust
Step 5: Verify DHCP snooping configuration.
S1# show ip dhcp snooping
DHCP snooping is configured on following VLANs:
1,20
DHCP snooping is operational on following VLANs:
1,20
DHCP snooping is configured on the following L3 Interfaces:
Insertion of option 82 is enabled
circuit-id default format: vlan-mod-port
remote-id: 0022.568a.3a80 (MAC)
Option 82 on untrusted port is not allowed
Verification of hwaddr field is enabled
Verification of giaddr field is enabled
DHCP snooping trust/rate is configured on the following Interfaces:
Interface
Trusted
Allow option
Rate limit (pps)
------------------------------------------------------FastEthernet0/5
yes
yes
unlimited
FastEthernet0/6
no
no
10
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 22 of 23
Lab – Securing Layer 2 Switches
Router Interface Summary Table
Router Interface Summary
Router Model
Ethernet Interface #1
Ethernet Interface #2
Serial Interface #1
Serial Interface #2
1800
Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/0/0 (S0/0/0)
Serial 0/0/1 (S0/0/1)
1900
Gigabit Ethernet 0/0
(G0/0)
Gigabit Ethernet 0/1
(G0/1)
Serial 0/0/0 (S0/0/0)
Serial 0/0/1 (S0/0/1)
2801
Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/1/0 (S0/1/0)
Serial 0/1/1 (S0/1/1)
2811
Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/0/0 (S0/0/0)
Serial 0/0/1 (S0/0/1)
2900
Gigabit Ethernet 0/0
(G0/0)
Gigabit Ethernet 0/1
(G0/1)
Serial 0/0/0 (S0/0/0)
Serial 0/0/1 (S0/0/1)
Note: Determine how the router is configured by identifying the type of router and the number of interfaces the
router has. There is no way to effectively list all the combinations of configurations for each router class. This
table includes identifiers for the possible combinations of Ethernet and Serial interfaces in the device. The table
does not include any other type of interface, even though a specific router may contain one. For example, an
ISDN BRI interface. The string in parenthesis is the legal abbreviation that can be used in Cisco IOS commands
to represent the interface.
© 2016 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public.
Page 23 of 23
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement