Redundancy design

Answered Question
Aug 21st, 2008

We have designed our Fiber backbone network as a loop.

Sw1->Sw2->Sw3->Sw4->Sw1

Is this design acceptable?

Thanks

I have this problem too.
0 votes
Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Said,

having some blocked ports is normal when using STP.

STP is used to avoid bridging loops.

In trasparent bridging there's no equivalent of IP TTL and the switches are not allowed to modify the frames.

So STP manages the redundancy by building a loop free spanning tree centered on the root bridge and does this by putting in blocking state some ports.

STP in case of a topology change like a link or node failure will react by building a new spanning tree.

So one or more ports that were blocked can move to forwarding state safely.

So don't worry about ports blocked by STP.

Actually the loop is broken by STP by putting a link in blocked state but again this is good news.

Hope to help

Giuseppe

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sachinraja Thu, 08/21/2008 - 21:40

This normally should be avoided. which case is this design for ? Enterprise ? YOu need to have your switches in hierarchical, scalable model, like core/distribution/access etc, to have a good design on your network. Give us more info and we will be able to help you..

Raj

assalihin Fri, 08/22/2008 - 05:41

Raj,

First, thanks for taking the time to read my post and answer to it and also for willing to help.

Yes, the design if for an Enterprise with 3 plants. The link between them is Fiber.

I attached a visio drawing for you.

Thanks again

assalihin Fri, 08/22/2008 - 06:39

Raj,

First, thanks for taking the time to read my post and answer to it and also for willing to help.

Yes, the design if for an Enterprise with 3 plants. The link between them is Fiber.

I attached a visio drawing for you.

Thanks again

Giuseppe Larosa Fri, 08/22/2008 - 01:32

Hello Said,

what happens if Sw3 fails ?

Are SW1 and SW2 able to communicate with SW4 ?

It looks this is possible.

Are these L3 links or L2 links ?

if L2 links you need to consider STP

Hope to help

Giuseppe

cowetacoit Fri, 08/22/2008 - 04:28

I worked for a metro ISP a while back and due to the network being so old and so many customers we couldn't do too much infrastructure redesigning. so we implemented a STP ring. It worked like a charm. We only ran STP on the ring and disabled it on trunks going to "spoke" switches and customer sites. very fast convergence. So yes, if anyone of the switches fails or one of the links is broke then STP will unblock the blocked port and traffic will flow the other way and all switches can talk to each other. This isn't the ideal design at all, but it works. depends on your environment.

assalihin Fri, 08/22/2008 - 05:31

How do I disable it on the spoke switches?

I have Ce 500 series.

Thanks,

assalihin Fri, 08/22/2008 - 05:14

Hello Giuseppe,

Yes, they are able to talk to Sw4 if Sw3 goes down.

The links are L2 links, Sw3 is an L3 switch.

I don't know much about STP, but I noticed on the switches some critical errors saying port XX is blocked by STP.

That's why I am actually posting this topic. I was concerned that my loop was generating these STP errors and consequently blocking some ports.

Thanks for you time.

Correct Answer
Giuseppe Larosa Fri, 08/22/2008 - 05:52

Hello Said,

having some blocked ports is normal when using STP.

STP is used to avoid bridging loops.

In trasparent bridging there's no equivalent of IP TTL and the switches are not allowed to modify the frames.

So STP manages the redundancy by building a loop free spanning tree centered on the root bridge and does this by putting in blocking state some ports.

STP in case of a topology change like a link or node failure will react by building a new spanning tree.

So one or more ports that were blocked can move to forwarding state safely.

So don't worry about ports blocked by STP.

Actually the loop is broken by STP by putting a link in blocked state but again this is good news.

Hope to help

Giuseppe

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