Tricking Spanning Tree

Unanswered Question
Feb 26th, 2009

I need to connect two 3750G together. Right now I have one FastEthernet connected to another FastEthernet on the ohter 3750 and one GigE to a GigE on the other 3750. Spanning-tree is doing its job and blocking an interface. Both interfaces are in the same vlan. I have turned on errdisable recovery cause all with interval of 30 but all that does it bring up the errdisable interface after spanning tree shuts it down. This happens every 30 seconds. I have to use this design so I need to such trick spanning tree. How do I do it?

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Yudong Wu Thu, 02/26/2009 - 12:42

Can you confirm the port is in err-disable state or just blocked by spanning tree? That's two different causes. If it was in err-disable state, you need figure out why it was err-diabled.

mlenco Thu, 02/26/2009 - 12:58

the log shows the port in errdisable due to spanning-tree blocking.

Giuseppe Larosa Thu, 02/26/2009 - 13:01

Hello Matthew,

the only STP related event that can cause an errdisable is

spanning-tree bpuguard

check the configuration of the involved ports

Hope to help


Leo Laohoo Thu, 02/26/2009 - 14:27

Agree with Giuseppe. Make sure that BPDUguard configured on the port that goes to error-disable.

Francois Tallet Thu, 02/26/2009 - 13:02

blocking is a logical state: the port remains up but no data traffic is going through.

err-disable is the result of a feature bringing down the link.

There are optional STP feature (like bpduguard) that can err-disable a link. But without any additional configuration, STP can only block a port.



mlenco Thu, 02/26/2009 - 17:21

Yeah, I forgot about the bpduguard that is configured. I will remove it. I assume this will block my second link. In the event the gig link fails this link will come out of blocking state (pvst+) and traffic will flow over it. Similar to a second link in a port channel, correct?

Francois Tallet Thu, 02/26/2009 - 12:59

You have not said exactly what you want to achieve. Do you want both links forwarding at the same time in the same vlan?

That's what every one here is supposing. You cannot form a channel because those are different interface types: one link *must* be blocked. Now, I can show you how to have both of them forwarding, but you would certainly regret it;-)



mlenco Thu, 02/26/2009 - 17:22

The second link is a gig interface forced down to 100 on both 3750. I want would like to load balance over the two if possible instead of just have all copies sent out but interfaces.

Francois Tallet Fri, 02/27/2009 - 06:09

You won't be able to use both your interfaces at the same time with STP. The only solution would be to bundle both interfaces into a channel, so that STP only sees one logical link made of the two physical ones. Unfortunately, this is impossible because you can only aggregate ports with the same bandwidth.

So you won't be able to do load-balancing within a single vlan. If you have several vlans, you can send some vlans on the fast ethernet link and some others on the gig link, that's about all you can do to use your redundant connection.

STP does not need BPDUguard to protect against the loop (that's basically the job of STP to prevent loops, without any additional configuration). STP will block the link with the lowest bandwidth by default.



junfengwu Thu, 02/26/2009 - 14:29

try again, If it is mode on, you have to finish the configuration in 30seconds in both side. If it is mode PAGP/LACP you can try shut down the interface in both side then finish the configuration, after that no shut the interface.


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