spanning tree

Unanswered Question
Apr 21st, 2008

Hi all, if I plug a port into another port in the same switch, can anyone tell me what spanning tree does to stop the loop and how it identifies it on the same switch ?

I have this problem too.
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VictorAKur Mon, 04/21/2008 - 05:11

It will receive a BPDU with its own bridge ID in it and will block one of the ports (with lowest MAC address I think).

mohammedmahmoud Mon, 04/21/2008 - 05:27

Hi Victor,

The blocked port would be the port with the highest MAC address, and the forwarding port would be the one with the lowest MAC address.


Mohammed Mahmoud.

VictorAKur Mon, 04/21/2008 - 06:28

Hi Mohammed

thank you very much. This particular bit still confues me sometimes.



mohammedmahmoud Mon, 04/21/2008 - 06:45

Hi Victor,

You are very welcomed, as a general rule, switching stuff prefers the lower values, and routing stuff prefers the higher values.


Mohammed Mahmoud.

VictorAKur Mon, 04/21/2008 - 07:03

Aha! :) Now this is the one easy to remember.

Thank you.


Francois Tallet Mon, 04/21/2008 - 07:29

Just for the record, the mac address referred to earlier is in fact the mac address from the bridge. A port is identified by a 16 bit port id on a given bridge. The bridge itself has an 64 bit bridge id. If both ports are on the same bridge, the tie breaker is this 16 bit port id. In the port id, the 4 bits with the higher weight are the port priority (in the latest standard) and allow the user to influence the result.



mohammedmahmoud Mon, 04/21/2008 - 07:41

Hi Francois,

Absolutely agree with you, and to complement the fact, this priority value can be edited per port via the "spanning-tree port-priority x" command where x is the port priority in increments of 16.


Mohammed Mahmoud.

bvsnarayana03 Mon, 04/21/2008 - 21:45

& if the port priorities are left default, then tie breaker would be lowest port id (physical port no.).


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