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Finding Entries in the MAC Address Table

Finding Entries in the MAC Address Table
With a single switch and only four hosts connected to them, you can just read the details
of the MAC address table and find the information you want to see. However, in real networks,
with lots of interconnected hosts and switches, just reading the output to find one
MAC address can be hard to do. You might have hundreds of entries—page after page of
output—with each MAC address looking like a random string of hex characters. (The MAC
addresses used in the examples in this book are configured to make it easier to learn.)

Thankfully, Cisco IOS supplies several more options on the show mac address-table command
to make it easier to find individual entries. First, if you know the MAC address, you
can search for it—just type in the MAC address at the end of the command, as shown in
Example 7-4. All you have to do is include the address keyword, followed by the actual
MAC address. If the address exists, the output lists the address. Note that the output lists
the exact same information in the exact same format, but it lists only the line for the matching
MAC address.

Example 7-4 show mac address-table dynamic with the address Keyword
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Example 7-4 show mac address-table dynamic with the address Keyword
Free CISCO CCNA Routing and Switching ICND1 Study Guide

While useful, often times the engineer troubleshooting a problem does not know the MAC
addresses of the devices connected to the network. Instead, the engineer has a topology
diagram, knowing which switch ports connect to other switches and which connect to endpoint
devices.

Sometimes you might be troubleshooting while looking at a network topology diagram,
and want to look at all the MAC addresses learned off a particular port. IOS supplies that
option with the show mac address-table dynamic interface command. Example 7-5 shows
one example, for switch SW1’s F0/1 interface.

Example 7-5 show mac address-table dynamic with the interface Keyword
Free CISCO CCNA Routing and Switching ICND1 Study Guide

Finally, you may also want to find the MAC address table entries for one VLAN. You
guessed it—you can add the vlan parameter, followed by the VLAN number. Example 7-6
shows two such examples from the same switch SW1 from Figure 7-7—one for VLAN 1,
where all four devices reside, and one for a non-existent VLAN 2.

Example 7-6 The show mac address-table vlan command
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Free CISCO CCNA Routing and Switching ICND1 Study Guide

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