topic Re: RSTP + Path Costs in Switches, Hubs, and Modems

RSTP + Path Costs

RICHARDM001 — Sat, 15 Aug 2009 07:39:55 GMT

Hi. How can I confirm that my RSTP and allocated path costs are actually following the path with the least cost. I have 2 core switches, one priorit 0 and the backup 1. I have a trunk between them with a cost of 200 and priority 64, from the root I have a fiber to the first edge switch (edge01), that fiber has a cost of 2000 and priority 64. From the second core switch I have a fiber to edge02, that fiber has a cost of 40000 and priority of 128. From edge02 there is another fiber to edge03, the fiber has a cost of 40000 and pririty of 128. edge03 and edge01 are interlinked with copper gigabit. I am finding that the path for traffic always seems to go via edge02 and not via edge01, I figured this because if I continual icmp to root and boot edge02 we drop a few packets. The path via root to edge01 is a direct fiber link to get to aux data centre from main data center, the path via edge02 needs to go via that switch to get to our aux data centre? i tried setting switch priorities, but made no diff.

Sincerely

Re: RSTP + Path Costs

Richard Brodie_1 — Mon, 17 Aug 2009 09:04:05 GMT

log onto edge03, and "show span"

If everything is as you expect you should see the correct root MAC address, root path cost, and root port. Also the backup link should show as blocking.

Re: RSTP + Path Costs

RICHARDM001 — Mon, 17 Aug 2009 09:20:16 GMT

that is the problem, no ports show as blocked. Attched is output of sh span...

Re: RSTP + Path Costs

Richard Brodie_1 — Mon, 17 Aug 2009 09:50:36 GMT

The blocked link must be on one of the other switches, I suppose.

If you do a "show span" and "show lldp info remote" round the set, that should be enough to reconstruct the full topology.