MAC Address Tables with Multiple Switches

[cisco_path]

Hi guys,

on CCNA 200-301: Official Cert Guide Volume 1 by Mr. Odom I saw something confusing.

Example 5-8 The MAC Address Table on Two Switches

Why doesn't SW1 list SW2 in his mac address-table? Also SW2 doesn't list SW1 in his table.

Shouldn't the total mac addresses be 5 instead of 4, right?

Would appreciate any reply.

[Joseph W. Doherty]

Without "seeing" the underlying "example", cannot say for sure, but MACs are a) only recorded when switch "sees" a frame, ingress one of its ports, with a MAC in the source MAC address field and, b), also assuming the MAC table entry has not aged out and been flushed from the MAC table.

Also consider, a "dumb" switch doesn't even have a MAC address, so a "smart" switch, which generally supports a management interface and/or some "special" management traffic, still needs to send a frame to the other switch for the management interface's MAC to be recorded.  I.e. even a "smart" switch might be configured that it does not send frames, sourced by itself, to another switch.

[cisco_path]

An example below:

Host A communicates with Host B.

Both switches in the topology learn the mac addresses.

In below screenshot, SW1 shows also the SW2-port mac address on Gig0/1, but if I understand your explanation correctly, from Gig0/1 comes only one frame (from Host B) with only one source MAC address, so the table should not contain two Gig0/1 mac addresses. 

For me this looks correct, but in the book of Mr. Odom, the switch-port mac address is missing. it only lists the Hosts MAC address. 

 

 

[Joseph W. Doherty · Accepted solution]

For the topology shown, both switches, in theory, only need have the two PC hosts' MACs, this assuming they are the only devices exchanging data.  (Likely this is what Odom's book is "teaching".)

Physically, switch interfaces might have MACs and/or a MAC for a virtual interface, but, to what are they, the switches, sending or receiving frames? So for "real" or simulated Cisco "smart" switches, again, by default, those switches are likely transmitting "special" L2 only frames, like CDP, DTP and/or BPDU, etc., which can explain adjoining switch MACs in a L2 neighboring switch.