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spanning tree vlan mac address

From my observation, spanning tree vlan mac address is the device (SUP) base mac address + (vlan_id MOD 256). To avoid duplicate (if I create vlan 506 then create vlan 250, FA is already used by vlan 506), the second lowest byte might change. Is the spanning tree mac address allocation behavior officially documented anywhere?

This is from the test on 6500

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Hall of Fame Cisco Employee

Hi,

Certain 6500 supervisors have a set of 1024 MAC addresses to be used for various purposes, and they may assign consecutive MAC addresses to consecutive per-VLAN STP instances, so that every per-VLAN STP instance uses a unique Bridge ID.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SXF/native/configuration/guide/swcg/spantree.html#wp1096536

Nowadays, a feature called the MAC Address Reduction is used so that instead of unique MAC addresses to create a unique Bridge ID, the VLAN ID is embedded into the lowmost 12 bits of the (formerly) configurable priority field of the Bridge ID, again yielding unique Bridge ID for every per-VLAN STP instance even with a single MAC address.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/spantree.html#wp1174398

http://www.cisco.com/c/en/us/td/docs/ios/lanswitch/command/reference/lsw_book/lsw_s2.html#wp1110581

You may also want to read another thread where this feature was previously discussed:

https://supportforums.cisco.com/discussion/11884116/why-did-we-really-need-spanning-tree-extend-system-id-command

Best regards,
Peter

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Thanks Peter. `show catalyst6000 chassis-mac-address` is exactly what I was looking for.

I was asking because I came across people deliberately disabled system-id extension to make the switch stp root instead of adjusting priority. Is there any side effect from this kind of practice?

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Hall of Fame Cisco Employee

Hi,

I came across people deliberately disabled system-id extension to make the switch stp root instead of adjusting priority.

Really? That's a very objectionable practice.

Is there any side effect from this kind of practice?

I cannot talk from a first-hand experience as I am not in charge of any 6500 Catalysts with this feature. However, these are issues I can think of:

  • 1024 is the upper limit of available MAC addresses, and if you happen to have more VLANs, there will be no MAC addresses left to generate unique STP BIDs from.
  • Although BPDUs do not indicate whether Extended System ID is in use, network management software may assume that it is the case, as it has been the common practice for years. For example, Wireshark now displays the BID as Priority / VLAN / MAC. If the Extended System ID is disabled, Wireshark (and other network monitoring or management software) will be misled, as they will interpret the lowmost 12 bits of the Priority field as the VLAN ID.
  • If nothing else, I am a staunch opponent of using inappropriate means to achieve a specific goal just because it happens to be a desirable side effect.

Best regards,
Peter

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