MAC address table - multiple entries of the same MAC address

Within the MAC address table, there should be only one entry per MAC address. Otherwise, we would see a generally undesirable phenomenon called MAC address flapping.

There is however one exception to this rule, and this involves the use of trunks. Refer to the following simple topology:


The link between SW2 and SW3 is configured as a trunk, and carries VLANs 1, 10, 20, 30, 40, 50, and 60. There are several hosts connected to each switch. Looking at the MAC address table of SW3, we see the following:

SW3#show mac address-table Mac Address Table ------------------------------------------- Vlan Mac Address Type Ports ---- ----------- -------- ----- 1 5254.000b.225e DYNAMIC Gi0/0 1 5254.0010.949f DYNAMIC Gi0/1 1 5254.0010.defe DYNAMIC Gi0/1 1 5254.0011.337a DYNAMIC Gi0/1 1 5254.0012.4282 DYNAMIC Gi0/1 10 5254.0011.337a DYNAMIC Gi0/1 20 5254.0011.337a DYNAMIC Gi0/1 30 5254.0011.337a DYNAMIC Gi0/1 40 5254.0011.337a DYNAMIC Gi0/1 50 5254.0011.337a DYNAMIC Gi0/1 60 5254.0011.337a DYNAMIC Gi0/1 Total Mac Addresses for this criterion: 11 SW3#

Notice that the 5254.0011.337a MAC address appears multiple times on Gi0/1 and it exists within all of the VLANs! What is this MAC address? It is the MAC address that corresponds to the Gi0/1 interface of SW2. Take a look at the following output from the CLI of SW2:

SW2#show inter gi 0/1 GigabitEthernet0/1 is up, line protocol is up (connected) Hardware is iGbE, address is 5254.0011.337a (bia 5254.0011.337a) MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set

Note the Hardware address.

So, multiple MAC address entries may be found within a MAC address table, but only the MAC address of the interface of the connected switch on a trunk port.


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