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Re: Need a good reference for Spanning Tree posted 02/01/2002
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If A is the designated bridge (the root or the one closest to the root) then on B, 1/1 will be the root port and 1/2 will block.  This is controlled by the port ids being received by switch B from Switch A.  To get port 1/2 to be the root port on Switch B, lower the port priority on port 1/2 of Switch A:  set spantree portpri 1/2 20

All else being equal, STP prefers ports with the lowest port ID, which is a 16 bit number made up of port priority and an index number.  Default port priority is 32 on modular Cats.  Index increases left to right across the module.  

-Bob


----- Original Message ----- 
From: "SEAN" <nyboi@xxxxxxxxxx>
To: <ccielab@xxxxxxxxxxxxxx>
Sent: Friday, February 01, 2002 2:16 PM
Subject: Need a good reference for Spanning Tree


> Can anyone recommend a good reference to how spanning tree chooses which
> ports to go up to the root bridge?
> 
> Basically, I have the following set up - two catalyst 5500 switches with
> two trunks between them:
> Switch A port 1/1 to Switch B port 1/1
> Switch A port 2/1 to Switch B port 2/1
> 
> Seems the switch always wants to use 1/1 as the spanning tree root
> (unless it's not available). If 1/1 is disabled, and 2/1 is forwarding
> fine, and I reenable 1/1, spanning tree recalculates, and 1/1 becomes
> forwarding again.
> 
> These are supervisor III's, and I was thinking that it probably wants to
> use the ports with the lowest mac address? But if I do show mod, both
> supervisor modules show the same mac address range.
> 
> Can anyone recommend a good resource on how spanning tree chooses ports?
> 
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