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Re: spanning tree recalculating

 
stieven struyf
Frequent Advisor

spanning tree recalculating

We have (rapid) spanning tree running on a part of our network. Every few hours it recalculates.
Is this normal?
The resulting spanning tree topo is always the same, in the logs i can't find any reason(like plugged/unplugged ports).
I already upgraded to the latest firmware on all switches.
This network part has 2 3400cl's as core and multiple 2848 at the edge.
20 REPLIES 20
Mohieddin Kharnoub
Honored Contributor

Re: spanning tree recalculating

Hi

Topology changes make RSTP algorithm to restart again.

Usually in RSTP you can use many features to avoid that like assign edge ports where its connected and its by default OFF.

Stop BPDU in some places on the network.

If that was not helpful, then i would ask you to attach the config of the root at least.

Good Luck !!!

Science for Everyone
stieven struyf
Frequent Advisor

Re: spanning tree recalculating

all ports are set to edge ports, only connections between switches(gig. ethernet) have spanning tree enabled.
Problem also exists when i remove all redundant links,but keep rstp running.
We already had a consultant looking at it without result.
I also don't see any problem on the uplinks when spanning tree recalculates.
Mohieddin Kharnoub
Honored Contributor

Re: spanning tree recalculating

Hi

Usually Spanning-Tree won't recalculate unless you have some topology changes happened

Like if one of your blocked links between 2800 and the core changed its path cost (or speed), that will affect.

I would like to ask you if you can attach the 3400 or the Root show tech, and little explain about your network map.

Good Luck !!!
Science for Everyone
Matt Hobbs
Honored Contributor

Re: spanning tree recalculating

If it's only every few hours I wouldn't lose much sleep over it. If you want to try and track it down though, I would configure syslogging on each switch and see if you can correlate the last topology change to any events that happened at the time.

Otherwise in the 3400 release notes it provides some good information on how to read the output of 'show span detail'

You may also want to look into the new BPDU filtering and detection options available.
stieven struyf
Frequent Advisor

Re: spanning tree recalculating

i didn't knew the command. Is there a full explanation what every parameter means(and how i can use this to track the problem).
It happens to often to just leave it.
i attached the show tech of the root switch.
I will add a map of the switches in the next reply.
stieven struyf
Frequent Advisor

Re: spanning tree recalculating

network map
stieven struyf
Frequent Advisor

Re: spanning tree recalculating

I found some increasing parameters(cfg bpdu) on a port, i am quite sure that this is a hub. Is it possible that a hub causes this behaviour?

I also can't filter bpdu's, i have done this in the past. Does it only works for some stp versions?
On this port i also see that operedgeport is no, while i forced this port to edge.

BESW56KE(config)# show spanning-tree 36 detail

Status and Counters - RSTP Port(s) Detailed Information

Port : 36
Status : Up
BPDU Filtering : No
Role : Designated
State : Forwarding
Priority : 128
Path Cost : 200000
Root Path Cost : 4
Root Bridge ID : 16384:001279-034c00
Designated Bridge ID : 40960:001560-002680
Designated Port ID : 128:36
AdminEdgePort : Yes
OperEdgePort : No
AdminPointToPointMAC : Force-False
OperPointToPointMAC : No
Aged BPDUs Count : 0
Loop-back BPDUs Count : 0
TC Detected : 6
TC Flag Transmitted : 0 TC ACK Flag Transmitted : 22
TC Flag Received : 0 TC ACK Flag Received : 0

RSTP RSTP CFG CFG TCN TCN
BPDUs Tx BPDUs Rx BPDUs Tx BPDUs Rx BPDUs Tx BPDUs Rx
---------- ---------- ---------- ---------- ---------- ----------
868 0 83418 9 0 19

BESW56KE(config)#
Matt Hobbs
Honored Contributor

Re: spanning tree recalculating

With the show span detail you really need to capture it off each switch before and after a topology change, and then study the differences. The ones that are most helpful are TC Flag Received, and TC Detected. From the root keep following the port that increases on TC Flag Received until you get to a switch that has a TC Detected.

In your config I noticed that you had single port trunks configured which is not necessary. It shouldn't cause a problem though.
stieven struyf
Frequent Advisor

Re: spanning tree recalculating

thanks for the tip, but i already did that.
now i have very strange errors.
first i found a port with a lot of tc detected. i disabled it and another port on another switch had the same issue.
this morning i checked again, again another switch and another port.
this are always ports from a switch directly to one of the cores.
This morning it was happening on one of the cores towards a user switch.
what i find strange is that only one side of the link sees a topology change. If there was really a problem i would think both sides would notice.
btw. all switches are connected through cat5e cables on gigabit. Can cable lenghts cause such a problem?