Switch Interface MACs in mac-address table

[John White]

Hello,

I am curious about MAC learning between trunked switches on a per-vlan basis.

In a production environment, I have two switches trunked over a single link. Switch A has 7 VLANs configured with the trunk link to switch B allowed in each. When I look at the mac-address tables for each VLAN, they all contain several MACs, but the MAC address of the trunk interface of switch B shows up in only one of them.

What is the default behavior between trunked links when learning MAC addresses, and what configuration choices would change that?

Specifically, why am I learning the switch interface mac of switch B on one VLAN but not the others, and does this add any specific considerations as traffic for that VLAN traverses the wider set of switches?

[paolo bevilacqua]

Unless you are running PVST, MAC address of other switches will be in VLAN 1 only. That is normal and expected.

[John White]

In this case, I beleive I am running PVST and the neighboring switch trunk interface MAC is showing in VLAN 2553, but not in 601, 856, or many other VLANs.

I received another response not shown here in the thread that mentioned MAC addresses will show up in the table from spanning-tree BPDUs. Would this imply that VLAN 2553 is running spanning-tree, but my other VLANs are not? I will be looking later to investigate the hunch.

My real question is what kind of traffic and configuration will cause a switch to learn a neighboring switche's MAC address. Thanks again.

[John White]

Here are some sample outputs. I have truncated data for disclosure/sensitive information purposes.

Out of 14 Vlans between Switch 1 and 2. Both switches are using Fa1/0/6 as the trunk interface. Switch 2 MAC ends in

xxxx.xx16.2408.

show mac-address-table vlan 420 | i DYN

420    xxxx.xx03.bfb5    DYNAMIC     Fa1/0/6

420    xxxx.xx5e.3c3e    DYNAMIC     Gi1/1/1

show mac-address-table vlan 603 | i DYN

603    xxxx.xx5e.3c3e    DYNAMIC     Gi1/1/1

603    xxxx.xx01.5924    DYNAMIC     Fa1/0/6

show mac-address-table vlan 3537 | i DYN

3537    xxxx.xxd4.6d21    DYNAMIC     Fa1/0/6

3537    xxxx.xxe2.99e1    DYNAMIC     Fa1/0/17

3537    xxxx.xx1d.9271    DYNAMIC     Fa1/0/6

3537    xxxx.xx13.fe2d    DYNAMIC     Fa1/0/6

3537    xxxx.xxb3.d419    DYNAMIC     Fa1/0/6

3537    xxxx.xx66.228b    DYNAMIC     Fa1/0/6

3537    xxxx.xx24.4017    DYNAMIC     Fa1/0/6

3537    xxxx.xx9c.9360    DYNAMIC     Fa1/0/6

3537    xxxx.xx16.2408    DYNAMIC     Fa1/0/6

3537    xxxx.xx61.c1e7    DYNAMIC     Fa1/0/6

3537    xxxx.xx61.c2f7    DYNAMIC     Fa1/0/6

3537    xxxx.xx66.0de1    DYNAMIC     Fa1/0/6

3537    xxxx.xx66.0ead    DYNAMIC     Fa1/0/6

3537    xxxx.xx8f.7839    DYNAMIC     Fa1/0/6

3537    xxxx.xx34.1051    DYNAMIC     Fa1/0/6

3537    xxxx.xx32.9821    DYNAMIC     Fa1/0/6

3537    xxxx.xx52.5df1    DYNAMIC     Fa1/0/6

spanning-tree mode pvst

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

ME-C3750-24TE