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New Member

spanning tree

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 ?

9 REPLIES
New Member

Re: spanning tree

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).

Re: spanning tree

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.

BR,

Mohammed Mahmoud.

New Member

Re: spanning tree

Hi Mohammed

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

Regards,

Victor.

Re: spanning tree

Hi Victor,

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

BR,

Mohammed Mahmoud.

New Member

Re: spanning tree

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

Thank you.

Regards,

Re: spanning tree

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.

Regards,

Francois

Re: spanning tree

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.

BR,

Mohammed Mahmoud.

Re: spanning tree

Victor,

Totally Agree :)

BR,

Mohammed Mahmoud.

Re: spanning tree

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

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