On a Cisco Catalyst switch you issue a show mac address-table command

In the topology above, someone connected a cheap (unmanaged) switch that they brought from home to the FastEthernet 0/1 interface of our Cisco switch. Sometimes people like to bring an extra switch from home to the office. As a result, our Cisco switch will learn the MAC address of H1 and H2 on its FastEthernet 0/1 interface.

Of course, we don’t want people to bring their own switches and connect them to our network, so we want to prevent this from happening. This is how we can do it:

Switch(config)#interface fa0/1
Switch(config-if)#switchport port-security
Switch(config-if)#switchport port-security maximum 1

Use the switchport port-security command to enable port security. I have configured port security, so only one MAC address is allowed. Once the switch sees another MAC address on the interface, it will be in violation, and something will happen. I’ll show you what happens in a bit…

Besides setting a maximum on the number of MAC addresses, we can also use port security to filter MAC addresses. You can use this to only allow specific MAC addresses. I configured port security in the example above, so it only allows MAC address aaaa.bbbb.cccc. This is not the MAC address of my computer, so it’s perfect for demonstrating a violation.

Switch(config)#interface fa0/1
Switch(config-if)#switchport port-security mac-address aaaa.bbbb.cccc

Use the switchport port-security mac-address command to define the MAC address that you want to allow. Now we’ll generate some traffic to cause a violation:

 C:\Documents and Settings\H1>ping 1.2.3.4

I’m pinging to some bogus IP address…there is nothing with IP address 1.2.3.4; I just want to generate some traffic. Here’s what you will see:

 SwitchA#
%PM-4-ERR_DISABLE: psecure-violation error detected on Fa0/1, putting Fa0/1 in err-disable state
%PORT_SECURITY-2-PSECURE_VIOLATION: Security violation occurred, caused by MAC address 0090.cc0e.5023 on port FastEthernet0/1.
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
%LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to down

We have a security violation, and as a result, the port goes in err-disable state. As you can see, it is now down. Let’s take a closer look at port security:

 Switch#show port-security interface fa0/1
Port Security              : Enabled
Port Status                : Secure-shutdown
Violation Mode             : Shutdown
Aging Time                 : 0 mins
Aging Type                 : Absolute
SecureStatic Address Aging : Disabled
Maximum MAC Addresses      : 1
Total MAC Addresses        : 1
Configured MAC Addresses   : 1
Sticky MAC Addresses       : 0
Last Source Address:Vlan   : 0090.cc0e.5023:1
Security Violation Count   : 1

Here is a useful command to check your port security configuration. Use show port-security interface to see the port security details per interface. You can see the violation mode is shutdown and that the last violation was caused by MAC address 0090.cc0e.5023 (H1).

Switch#show interfaces fa0/1
FastEthernet0/1 is down, line protocol is down (err-disabled)

Shutting the interface after a security violation is a good idea (security-wise), but the problem is that the interface will stay in err-disable state. This probably means another call to the helpdesk and you bringing the interface back to the land of the living! Let’s activate it again:

Switch(config)#interface fa0/1
Switch(config-if)#shutdown
Switch(config-if)#no shutdown

To get the interface out of the err-disable state, you need to type “shutdown” followed by “no shutdown.” Only typing “no shutdown” is not enough!

It might be easier if the interface could recover itself after a certain time. You can enable this with the following command:

Step 1.
Process functions on the incoming interface, if the interface is currently in an up/up (connected) state, as follows:
A. If configured, apply port security logic to filter the frame as appropriate.
B. If the port is an access port,determine the interface’s accessVLAN.
C. If the port is a trunk, determine the frame’s tagged VLAN.

Step 2.
Make a forwarding decision. Look for the frame’s destination MAC address in the MAC address table, but only for entries in the VLAN identified in Step 1. If the destination MAC is...
A. Found (unicast), forward the frame out the only interface listed in the matched address table entry.
B. Not found(unicast),flood the frame out all other access ports(except the incoming port) in that same VLAN, plus out trunks that have not restricted the VLAN from that trunk (as discussed in Chapter 11, “Implementing Ethernet Virtual LANs,” as related to the show interfaces trunk command).
C. Broadcast, flood the frame, with the same rules as the previous step.

What command will show the mac

Which of the following command list the mac

What is the command to check the mac

What command will show you the MAC table entries associated with ports using port security?