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Old 01-26-2010, 10:59 PM
ashley ashley is offline
 
Join Date: Jan 2010
Posts: 18
Default Spanning-tree Network Unstable

Hi,
I'm using rapid-pvstp in my fiber ring and here is problem: I have to main sites, A and B, and a chain of 30 switches from site A to site B, for the main site A I have the vlan 11 and for the main site B the vlan 12, the
switch closer to site A is configured to be the root of vlan 11 and the switch closer to site B is configured to be the root of vlan 12, the problem that I have is that the network is very unstable, the root bridge is
changing from the right one to other one very often (sometimes every second) and that is causing me that some ports sometimes are in blocked state and a second later in forwarding state and then blocked again.

It looks that the switches that are far away from the root of each vlan are not receiving the BPDU or they receive them with bad values so they start to make them self roots or choosing another root, doing some researching i found this: "When a BPDU propagates from the root bridge toward the leaves of the tree, the age field increments each time the BPDU goes though a bridge. Eventually, the bridge discards the BPDU when the age field goes beyond maximum age. If the root is too far away from some bridges of the network, this issue can occur. This issue affects convergence of the spanning tree."
http://www.cisco.biz/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml#awkwa rd

What I was thinking is to modify the hello, forward and max age timers on the root switches (root for vlan 11 and root for vlan 12) but I'm not sure what value should I use in each timer.

Any other ideas??

Thanks for reading such a long thread.
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Old 01-27-2010, 09:59 PM
m.stone m.stone is offline
 
Join Date: Dec 2009
Posts: 29
Default Re: Spanning-tree Network Unstable

You can consider the max-age time as a sort of TTL for the STP BPDU

max-age / hello frequency gives you how far BPDUs are considered valid

So you may need to increase max-age and to reduce hello interval to achieve an higher ratio so that BPDUs can travel correctly

You need to consider what can happen when a failure changes the ring in a long chain (worst case)

Hope to help
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Old 01-31-2010, 10:21 PM
ashley ashley is offline
 
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Posts: 18
Default Re: Spanning-tree Network Unstable

Quote:
Originally Posted by m.stone View Post
You can consider the max-age time as a sort of TTL for the STP BPDU

max-age / hello frequency gives you how far BPDUs are considered valid

So you may need to increase max-age and to reduce hello interval to achieve an higher ratio so that BPDUs can travel correctly

You need to consider what can happen when a failure changes the ring in a long chain (worst case)

Hope to help
Right know i have the default Hello 2 sec, Max age 20 and Forward 15, do you think this values can do the job:

Hello 2 sec
Max age 40
Forward 15

???? or is there a formula to calculate them???

I mean because of the fact that i have a chain of like 30 switches
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Old 02-02-2010, 10:54 PM
ashley ashley is offline
 
Join Date: Jan 2010
Posts: 18
Default Re: Spanning-tree Network Unstable

I believe this are the formulas:

End-to-end BPDU propagation delay—This value is the amount of time that is necessary for a BPDU to travel from one end of the network to the other end. Assume a diameter of seven hops, three BPDUs that can be lost, and a hello time of 2 sec. In this case, the formula is:
End-to-end_BPDU_propa_delay
= ((lost_msg + 1) x hello) + ((BPDU_Delay x (dia – 1))
= ((3 + 1) x hello) + ((1 x (dia – 1))
= 4 x hello + dia – 1
= 4 x 2 + 6
= 14 sec•Message age overestimate—The purpose of this parameter is to account for the age of the BPDU since origination. Assume that each bridge increases the BPDU message age by 1 sec. The formula is:
Message_age_overestimate
= (dia – 1) x overestimate_per_bridge
= dia – 1
= 6
max_age
= End-to-end_BPDU_propa_delay + Message_age_overestimate
= 14 + 6
= 20 sec

I'm i correct?

This values only have to be changed on the root switches right? For this case on my actual root sw for vlan 11 and my actual root for vlan 12?
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Old 02-02-2010, 11:19 PM
David.C David.C is offline
 
Join Date: May 2009
Posts: 118
Default Re: Spanning-tree Network Unstable

Your calculations are good.

Anyway I would simply use
hello = 1 sec
max-age= 40 sec

This should give margin also for the chain scenario where one link is broken
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Old 02-03-2010, 10:49 PM
ashley ashley is offline
 
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Posts: 18
Default Re: Spanning-tree Network Unstable

This values only have to be changed on the root switches right? For this case on my actual root sw for vlan 11 and my actual root for vlan 12?
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Old 02-20-2010, 09:51 PM
David.C David.C is offline
 
Join Date: May 2009
Posts: 118
Default Re: Spanning-tree Network Unstable

Quote:
Originally Posted by ashley View Post
This values only have to be changed on the root switches right? For this case on my actual root sw for vlan 11 and my actual root for vlan 12?

Yes, sorry for not having noted this.

Of course configure also on secondary root bridge to have consistency in case of failure of primary root bridge.
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Old 02-21-2010, 11:07 PM
ashley ashley is offline
 
Join Date: Jan 2010
Posts: 18
Default Re: Spanning-tree Network Unstable

No problem,

I'll perform this configurations tonight and I'll post how it goes.
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Old 02-21-2010, 11:34 PM
ashley ashley is offline
 
Join Date: Jan 2010
Posts: 18
Default Re: Spanning-tree Network Unstable

Hi all,

I did the changes last night and everything is working as it should be!.

Thanks for your help...
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