MAC aging removes a dynamically learned MAC address from a switch’s MAC address table after five minutes without receiving a frame from that address. It prevents inactive entries from remaining indefinitely while switches continue learning from new source frames. section 6.3.1 section 6.6
Follow the table entry through its normal lifecycle:
- Receives a frame and records the source MAC address with the receiving port; this creates a dynamic entry. section 6.3.1
- Uses the destination MAC address to forward known unicast frames through the associated port. section 6.6
- Removes the dynamic entry after five minutes without a frame from that MAC address. section 6.3.1
- Shows current entries with show mac address-table; clears dynamic entries with clear mac address-table dynamic or more specific variants. section 6.3.3
Two-switch learning scenario
As PCs communicate through two connected switches, each switch learns source MAC addresses and associates them with local or inter-switch ports. If a learned device sends no frames for five minutes, its dynamic entry ages out. A later frame causes the switch to learn the source address again. section 6.3.1
SW1# show mac address-table #1
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 5254.0017.7cd2 DYNAMIC Gi0/0 #2
1 d8bb.c1cc.ff01 DYNAMIC Gi0/1 #2
1 d8bb.c1cc.ff02 DYNAMIC Gi0/2 #2
1 d8bb.c1cc.ff03 DYNAMIC Gi0/0 #2
1 d8bb.c1cc.ff04 DYNAMIC Gi0/0 #2
#1 Views SW1’s MAC address table
#2 A list of MAC addresses and the port each was learned on
Use the Cisco IOS listing to inspect VLAN, MAC address, entry type, and learned port; entries marked DYNAMIC represent automatically learned addresses. section 6.3.3
