From: Bob Sinclair (bsin@cox.net)
Date: Sat Feb 28 2004 - 23:29:39 GMT-3
Sam,
Port ID is a 16 bit number that is composed of port priority and port
number, or interface index. It is used as the final tie-breaker when (and
only when) the sender bridge IDs in the bpdus being compared are equal.
This will only be true when the compared bpdus are sourced from the same
switch. The spanning-tree algorithm compares the following numbers in the
bpdus in this order until a tie is broken:
1. root BID
2. root path cost
3. sender BID
4. Port ID
You can think of the data in the bpdu as a single, compound number:
rootbid.rootpathcost.senderbid.portid.
Comparing the numbers left to right, as soon as one bit is zero and the
other is one, the comparison ends and lower number wins.
HTH,
Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net
----- Original Message -----
From: "Hossam" <sam6626@yahoo.com>
To: <ccielab@groupstudy.com>
Sent: Saturday, February 28, 2004 5:24 PM
Subject: Root Port election process in SPT!
> Group,
> I understand that SPT while electing the root port for a switch goes in
the following sequence:
> 1- Port Cost
> 2- Port Priorty
> 3- Port ID
> In my lab setup this goes only when the two candidate root ports are
connected to ONE switch
>
> CATA CATB CATC--- ROOT
> |--FA 0/1--FA 0/1---| |
> | |-FA 0/3-----FA 0/3 ----|
> |--FA 0/2--FA 0/2---| |
>
>
> In the above diagram everything goes as expected. CATA-FA0/1 is FWD while
CATA-FA0/2 ius BLK.
> When i change port-priorty on CAT B-FA0/2 to 64 the roles are reversed.
CATA-FA0/1 is BLK and CATA-FA0/2 is FWD. SO far so good.
> My problem is when i replace CAT B with two separte Switches.
>
>
> CATA CATB1 CATC--- ROOT
> |--FA 0/1--FA 0/1---|--- FA0/3---FA0/3 ----|
> | | |
> | |
> | |
> | |--- FA 0/3--FA 0/3 ----|
> |--FA 0/2--FA 0/2---| |
> CATB2
>
> In this setup CATA-FA 0/1 stays FWD even when i change the CATB2- FA0/2
port Priorty to 64?
> I can only change the ROot port via the Port cost command in the second
scenario?
> For me it seems that port priorty does't envolve in the root port election
process if the two candidate ports are connected to the same switch!!
> Is that regular? or is there something that i am missing?
>
> Thanks
> SAM
>
>
>
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