Loop avoidance

Unanswered Question
Jul 18th, 2009

Locations A & B are linked by wireless bridges. Similarly B & C are linked by wireless bridges. And location A& C are linked by wireless bridges. This is a traingler network. At A 3560 L3 switch is there. At B & C Cisco linksys SRW224G4. At each locations VLANs are there. Now we want to avoid the loop between B & C. It should come up only when A & C or A & B is down. Pls. suggest how this can be achieved ?

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Roman Rodichev Sat, 07/18/2009 - 22:19

the easiest way to achieve this is to configure 3560 in location A with the lowest bridge priority for all VLANs to ensure that it's always the root bridge in your spanning-tree domain. Make sure that your triangle is symmetrical, meaning that you have the same number of hops and interface speeds on both paths:


3560 - BRIDGE-A1 - BRIDGE-B1 - LINKSYS - BRIDGE-B2


3560 - BRIDGE-A2 - BRIDGE-C1 - LINKSYS - BRIDGE-C2


this will ensure that BRIDGE-B2 - BRIDGE-C2 link is in spanning-tree blocking state, and will come up only if some other link fails


if your triangle is not symmetrical and/or if you have a different setup, then you might also need to modify spanning-tree port costs on some of the links. For example, you need to make your B-C bridge link with a large path cost (on each vlan)



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