20G interface spanning tree design issue

Answered Question

Hi,

We have two 6500 with two 20-G uplink (using etherchannel to combine two 10-G interface) to two 3750E which are stacked together. Currently one of the 6500 is the root bridge. We are testing failure scenario when one of the 10-G interface in the port-channel on the 6500 or 3750E (in the forward state)go down, which make the etherchannel to become 10-G only, we would like the spanning tree to block this 10-G link and make the 20-G link (the other etherchannel) forwarding.

When we test shutting down one 10-G link in the forwarding etherchannel, the path cost go from 1 to 2, but the port is still in forwarding.

Any suggestion that we can make the 20-G etherchannel to become forwarding while the 10-G will be blocked automatically?

Thanks.

Ben

Correct Answer by davy.timmermans about 7 years 6 months ago

I would advise to use spanning-tree path cost long mode

see:

http://books.google.be/books?id=3LQDyLjMUYkC&pg=PA177&lpg=PA177&dq=spanning-tree+path+cost+mode&source=bl&ots=lQS6gNctD0&sig=I7bfd1vBfjayoFjtGbv8X8vhGng&hl=nl&ei=SXp4SsOZI6WsjAfIgaW9Bw&sa=X&oi=book_result&ct=result&resnum=1#v=onepage&q=spanning-tree%20path%20cost%20mode&f=false

As Roman said, in case of failure of a link in the etherchannel, the root will remain root.

> Election root bridge

root Bridge ID

pathcost

bridge id

...

If the link between the 2 x 6509 is also 20Gig and 1 uplink fails in the uplink between root and 3750, then the port of the 3750 will be blocked towards the root. This means that the traffic will go via backup root over the crosslink towards the root

Correct Answer by Roman Rodichev about 7 years 6 months ago

I assume your link between two 6500's is cost of 1. When one of your uplink's port cost changes from 1 to 2, you now have two equal cost paths:

1. Stack - 10G (cost 2) - Root 6509 = Path Cost 2

2. Stack - 20G (cost 1) - Backup Root 6509 - 20G (cost 1) = Path Cost 2

In an equal cost path scenario, the bridge (3750 stack) will make a decision based on the lowest bridge ID of the neighbor. In your case, Root bridge 6509 has lower BID than Backup Root 6509, therefore it will continue to forward on the 10G uplink. One way you could solve this is manually increase port cost of all 10G interfaces on the stack from 2 to a higher value.

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Correct Answer
Roman Rodichev Tue, 08/04/2009 - 07:41

I assume your link between two 6500's is cost of 1. When one of your uplink's port cost changes from 1 to 2, you now have two equal cost paths:

1. Stack - 10G (cost 2) - Root 6509 = Path Cost 2

2. Stack - 20G (cost 1) - Backup Root 6509 - 20G (cost 1) = Path Cost 2

In an equal cost path scenario, the bridge (3750 stack) will make a decision based on the lowest bridge ID of the neighbor. In your case, Root bridge 6509 has lower BID than Backup Root 6509, therefore it will continue to forward on the 10G uplink. One way you could solve this is manually increase port cost of all 10G interfaces on the stack from 2 to a higher value.

Correct Answer
davy.timmermans Tue, 08/04/2009 - 10:28

I would advise to use spanning-tree path cost long mode

see:

http://books.google.be/books?id=3LQDyLjMUYkC&pg=PA177&lpg=PA177&dq=spanning-tree+path+cost+mode&source=bl&ots=lQS6gNctD0&sig=I7bfd1vBfjayoFjtGbv8X8vhGng&hl=nl&ei=SXp4SsOZI6WsjAfIgaW9Bw&sa=X&oi=book_result&ct=result&resnum=1#v=onepage&q=spanning-tree%20path%20cost%20mode&f=false

As Roman said, in case of failure of a link in the etherchannel, the root will remain root.

> Election root bridge

root Bridge ID

pathcost

bridge id

...

If the link between the 2 x 6509 is also 20Gig and 1 uplink fails in the uplink between root and 3750, then the port of the 3750 will be blocked towards the root. This means that the traffic will go via backup root over the crosslink towards the root

Thanks for your suggestion.

But with the 'long' method, it is still the same problem, the pathcost goes from 1000 to 2000 when the etherchannel go from 20G to 10G, but it is the same problem as the short method, the 2000 pathcost is the same as the alternate path (1000+1000) and because of the primary path is connected to the root bridge, it is still 'forwarded'.

The only way is if i can set the '10G' path cost to something higher than the default '2000' pathcost, that it will fix the issue, but I can't find any command to do that.

Any suggestion?

Thanks!

Ben

Thanks for your reply. What you said was right.

Is there a way to set a 10-G to pathcost 3 specifically?

The problem is, if we increase the path cost of the 10G to something higher than 2, when the 10G link is backup, the primary path will still have cost higher than the backukp path... then it will not use the primary path anymore.

Thanks.

Ben

Roman Rodichev Tue, 08/04/2009 - 13:23

It sounds that you've figured it out from your other post. Do you still need any info?

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