1.1.a (i) Managing MAC address table

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Starting at the bottom with this one… Well not all the way, but close. Mac Addresses are some of the most basic addresses that you will encounter and therefore need to understand when working towards a CCIE. Each and every network device has a mac address. This doesn’t matter if they are wired or wireless, computer or router- they all have one. They have one and they are all unique! So now that a very brief re-visit to our CCNA days is complete, how does this help us now?

You will need to know how to navigate around the switches you are working on when you are working with a mac address. The first and most common command you will use is “show mac address-table”. This is useful to get an idea of the complete form of the mac address-table. You can also use “show mac address-table address XXXX.XXXX.XXXX” to filter your results if you are looking for one specific entry. It might look a little bit like this:

show mac address table

If you specified a specific address, you might see an output like this:

show mac address table address

That command and output has come in handy for me many a time. Here’s a scenario to demonstrate. You get a syslog error that there is a device on your network sending invalid packets or something else of that manner that is causing a problem. You have a mac address, otherwise you have no idea where that device is. By using the previous command, you can track down a device to an exact port so you can isolate the issue. What happens if it is on another switch you might ask. The port listed will be the uplink to another switch such as your core device or another access switch. You can follow the chain along until you find the access port in question!

The last command to make mention of is show mac address-table aging-time. The default is set at 300 (seconds). This command will display the global settings as well as any specific settings that might be in place for a specific VLAN.

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