PVST+: root mac address increases by 1 (as VLAN number does too) in logs

[student100]

Hello:

I have a network running PVST+. Catalyst switch A (CatOS) is the root for the entire network and VLANs, as I can see in the "sh spantree" output. Switch A's MAC address is 01-0b-48-f7-80-00.

We made a couple changes in the network and while the switch was recalculating STP, we got the following logs (in swich A):

%SPANTREE-5-ROOTCHANGE:Root changed for Vlan 1: New root port 14/7. New Root mac address is 01-0b-48-f7-80-00.

%SPANTREE-5-ROOTCHANGE:Root changed for Vlan 2: New root port 14/7. New Root mac address is 01-0b-48-f7-80-01.

%SPANTREE-5-ROOTCHANGE:Root changed for Vlan 3: New root port 14/7. New Root mac address is 01-0b-48-f7-80-02.

%SPANTREE-5-ROOTCHANGE:Root changed for Vlan 4: New root port 14/7. New Root mac address is 01-0b-48-f7-80-03

%SPANTREE-5-ROOTCHANGE:Root changed for Vlan 5: New root port 14/7. New Root mac address is 01-0b-48-f7-80-04.

...and so on for each VLAN.

My question is: why does the log show the root mac address increased by one (1), starting from the real root brigde MAC (01-0b-48-f7-80-00) as VLAN number increases?

Thanks in advance.

[Peter Paluch · Accepted solution]

Hello David,

This switch is currently not using the so-called MAC address reduction which is common on all recent switches, and instead assigns a unique MAC address (in fact, a unique Bridge ID) to each STP instance as run in different VLANs. The STP standard mandates that each bridge should have a unique Bridge ID. Now because your switch behaves as multiple bridges, one for each VLAN, every VLAN gets its own Bridge ID derived from the base MAC address and the number of the VLAN added to the MAC address.

Newer switches do not assign Bridge IDs this way. Instead, they reduce the priority field in STP Bridge ID from 16 bits to just 4 topmost bits, and the freed 12 bits from the priority field are populated by the VLAN number. This way, the Bridge ID is unique without a need to change MAC addresses. This is called Extended System ID (or MAC address reduction) and was originally described in IEEE 802.1t, now integrated into 802.1D.

You may be interested in reading more here:

http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/7.5/configuration/guide/spantree.html#wp1174385

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/6.x/configuration/guide/cfg_6_3.pdf

Best regards,

Peter

[student100]

Thanks a lot Peter. It's clear for me now. So i guess the log should say "New Root bridge ID is xx-xx-xx-xx-xx-xx" instead of "New Root mac address is xx-xx-xx-xx-xx-xx" (it would be less confusing, more accurate).

[Peter Paluch]

Hi David,

Yes, I agree, such an output would be more understandable. One small detail, though - a Bridge ID is an 8B number composed of Priority (2B) and MAC address (6B). If the switch wanted to print out the BID, it should also indicate the priority, not just the MAC address. MAC address is not a BID, rather, it is a part of a BID.

Anyway - it was quite refreshing for me to see a switch that still uses unique MACs for BIDs in VLANs

Best regards,

Peter