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UniDirectional Link Detection help

I currently have 2 ds3 circuits. One circuit was provisioned by RCN and the other is from Verizon. The circuits connect our office to our datacenter and trunk the necessary VLANs on all trunks ports the circuits where the circuits are connected. STP is allowed across which allows us to assign all the VLANs to both circuits and prevents loops.

We had an incident where one of the bidirectional link was broken on one ds3. This was caused but a provider equipment failure along the path but not within our premises. The port remained up and a loop was created which severely affected our network until the unidirectional link was shutdown.

After doing some research I read about UDLD and thought it would work to prevent this from happening. Unfortunately this will not work on my RCN link. The circuit is built on RCN's SONET platform and the edge devices will not pass this protocol. My question is what can I use as on alternative for UDLD on circuits that cannot support this Cisco-proprietary protocol?

I was given the option to move to RCN's Ethernet platform which will allow us to pass this protocol but that might require an additional charge. Thanks for your help in advance.

ps - both circuits are terminated into cat6509 switches on either side of the circuit.


Re: UniDirectional Link Detection help

Have you considered using the loop-guard feature?

Unidirectional link failures may cause a root port or alternate port to become designated as root if BPDUs are absent. Some software failures may introduce temporary loops in the network. Loop guard checks if a root port or an alternate root port receives BPDUs. If the port is not receiving BPDUs, loop guard puts the port into an inconsistent state until it starts receiving BPDUs again. Loop guard isolates the failure and lets spanning tree converge to a stable topology without the failed link or bridge.

You can enable loop guard on a per-port basis. When you enable loop guard, it is automatically applied to all of the active instances or VLANs to which that port belongs. When you disable loop guard, it is disabled for the specified ports. Disabling loop guard moves all loop-inconsistent ports to the listening state.

If you enable loop guard on a channel and the first link becomes unidirectional, loop guard blocks the entire channel until the affected port is removed from the channel.

Taken from the following document:-



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