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Managing the MAC Address Table (Aging, Clearing)

This chapter closes with a few comments about how switches manage their MAC address
tables. Switches do learn MAC addresses, but those MAC addresses do not remain in the
table indefinitely. The switch will remove the entries due to age, due to the table filling, and
you can remove entries using a command.

First, for aging out MAC table entries, switches remove entries that have not been used
for a defined number of seconds (default of 300 seconds on many switches). To do that,
switches look at every incoming frame, every source MAC address, and does something
related to learning. If it is a new MAC address, the switch adds the correct entry to the table
of course. However, if that entry already exists, the switch still does something: it resets the
inactivity timer back to 0 for that entry. Each entry’s timer counts upward over time to measure
how long the entry has been in the table. The switch times out (removes) any entries
whose timer reaches the defined aging time.

Example 7-7 shows the aging timer setting for the entire switch. The aging time can be configured
to a different time, globally and per-vlan.

Example 7-7 The MAC Address Default Aging Timer Displayed
Free CISCO CCNA Routing and Switching ICND1 Study Guide

Each switch also removes the oldest table entries, even if they are younger than the aging
time setting, if the table fills. The MAC address table uses content-addressable memory
(CAM), a physical memory that has great table lookup capabilities. However, the size of the
table depends on the size of the CAM in a particular model of switch. When a switch tries
to add a new table entry, and finds the table full, the switch times out (removes) the oldest
table entry to make space. For perspective, the end of Example 7-7 lists the size of a Cisco
Catalyst switch’s MAC table at about 8000 entries—the same four existing entries from the
earlier examples, with space for 7299 more.

Finally, you can remove the dynamic entries from the MAC address table with the clear
mac address-table dynamic command. Note that the show commands in this chapter can
be executed from user and enable mode, but the clear command happens to be a privileged
mode command.

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