Set up Spanning Tree ports to forward or block

Answered Question
Jun 25th, 2007

We have a 3560G switch that we are using on a ONS backbone. We have the last 16 ports running Trunked mode up to a 454 SONET ONS. These ports are currently connected to ML 1000 cards that control the IP portion of the SONET. I need to setup the spanning tree so that only one side of the SONET ring forwards and the other side is in blocking mode. How do I do this on the switch. I have spanning tree PVST enabled. What else do I need?

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Correct Answer by ilya.varlashkin about 9 years 7 months ago

If you only want to prevent loop but don't care at which point it's done, then simply enabling spanning tree will be sufficient. But if you want to have control over your topology and decide yourself which port should be forwarding and which blocking, then you'll need to manually set root bridge and port cost in such way that spanning tree will put desired ports in blocking/forwarding mode.

Search CCO for "Understanding Spanning Tree".

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Overall Rating: 3.5 (4 ratings)
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criss_noh Mon, 06/25/2007 - 23:22

you do not have to tune in any configuration. basically switch mechanism includs spanning tress protocol to prevent from looping.

just connect into approprite ports, you can see block port and forwarding port by show command.

brncopowor Tue, 06/26/2007 - 05:44

Thats my problem that I am trying to figure out. Lets say that I have port 34 going to port 1 on a ML Card and port 35 on port 1 on another ML card. Basically the other side of the ring coming back in. Now when I do a sh spanning-tree inter giga 0/34 it say FWD. I get that same on port 35 (FWD). One of these should say BLK.

criss_noh Tue, 06/26/2007 - 07:07

that's not impossible. i can make 100% sure that there must be blk port and forward port.

you need to see all of switch and see the status of spanning tree by show spanning.

Correct Answer
ilya.varlashkin Tue, 06/26/2007 - 07:04

If you only want to prevent loop but don't care at which point it's done, then simply enabling spanning tree will be sufficient. But if you want to have control over your topology and decide yourself which port should be forwarding and which blocking, then you'll need to manually set root bridge and port cost in such way that spanning tree will put desired ports in blocking/forwarding mode.

Search CCO for "Understanding Spanning Tree".

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