10-05-2021 11:09 PM
Hello,
having a plain L2 network with almost 60 (sometimes older) switches. On all switches rapid pvst is configured, VTP is also configured. No comments please
The "VTP-Server" is an 3750g Stack with 12.2(55)SE1. There are 128 Vlan's configured. Now when adding one more Vlan (for e.g. 224) the switch throws a message that he could not start a spanning-tree instance for vlan 224 because the maximum of spanning-tree instances is already reached. At the same time the switch writes "no spanning-tree vlan 224" to it's running-config AND to all running-configs of all switches in the same VTP-Domain... and a loop occourred - So far so bad.
So, after deleting some "never used" vlan's (now there are 124 left) I decided to give vlan 224 a second try.
On the VTP-Server switch I typed in "spanning-tree vlan 224"... an the network "stops". The cpu went to almost 100% and the syslog fills up with messages of flapping mac and ports. It feels like that spanning-tree starts an entire recalculation...
I never tought that (on "per vlan spanning-tree") the command "spanning-tree vlan 224" (which just starts a spanning-tree instance just for vlan224) has such impact...
Any thoughts or ideas are very appreciated!
Kind regards,
Andreas
Solved! Go to Solution.
10-06-2021 04:56 AM - edited 10-06-2021 04:58 AM
Hello
It’s not just the stp loop, it what it effects, broadcast storms increase exponentially multiple process not only large amount of stp transitions, interface, CPU/memory utilization , switch cam and router arp tables will also get flooded from consistently being updated, IGP peering will flap,all this will have a drastic effect on your network, until that loop is broken and stp convergence is completed.
10-05-2021 11:43 PM
Hello,
what is the root switch for the new Vlan ?
show spanning-tree vlan 224 root
10-06-2021 12:04 AM
Hello,
the switch itself is the root for that vlan :
switch#sh spanning-tree vlan 224 root
Vlan Root ID Cost Time Age Dly Root Port ---------------- -------------------- --------- ----- --- --- ------------ VLAN0224 32992 001d.e63e.2980 0 2 20 15
switch#sh spanning-tree vlan 224 VLAN0224 Spanning tree enabled protocol rstp Root ID Priority 32992 Address 001d.e63e.2980 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32992 (priority 32768 sys-id-ext 224) Address 001d.e63e.2980 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Gi1/0/25 Desg BKN*4 128.25 P2p *LOOP_Inc Gi1/0/26 Desg BKN*4 128.26 P2p *LOOP_Inc Gi1/0/27 Desg BKN*4 128.27 P2p *LOOP_Inc Po1 Desg FWD 3 128.488 P2p St1 Desg FWD 4 128.872 P2p Gi4/0/25 Desg FWD 4 128.187 P2p Gi4/0/26 Desg BKN*4 128.188 P2p *LOOP_Inc Gi4/0/27 Desg BKN*4 128.189 P2p *LOOP_Inc
Kind regards,
Andreas
10-06-2021 12:15 AM
Hello,
what if you delete and recreate the Vlan ?
no vlan 224
vlan 224
10-06-2021 12:33 AM
Hi,
sorry but this can't be done, because it's in use already.
I did not having problems creating vlans. The thing is, that the command "spanning-tree vlan 224" caused that the entire network went down. It feels like that STP was recalculating for all (124)vlan's. And this brings the switch to 100% and then the switch seems to flood all ports, resulting in loops, mac- and port flaps...
Kind regards
Andreas
10-06-2021 12:49 AM
Hello
If you have exceeded the stp limitation < sh stp summary total), things like broadcast storms can occur, as stp isn’t able to be cope with the extra stp instances.
Manually pruning vlans off the trunks could cut down on the virtual port totasl but long term it a matter of upgrading your switches or migrating on the MST
10-06-2021 01:01 AM
Hello Paul,
yes, I know about the limitation. For long term this network will be migrated to a L3 topology with NO spanning-tree anymore...
But for now I need to figure out the root cause of the network outage. I never imagined that (with 124 configured vlans - so 4 left until the living hell) bringing another stp instance (for vlan 224) up, would cause in a total network "meltdown"...
Could it be the lack of the old 3750g to handle the per vlan spanning-tree?
Kind regards,
Andreas
10-06-2021 01:47 AM - edited 10-06-2021 01:49 AM
Hello
@Andreas Schneider wrote:
But for now I need to figure out the root cause of the network outage. I never imagined that (with 124 configured vlans - so 4 left until the living hell) bringing another stp instance (for vlan 224) up, would cause in a total network "meltdown"...
It was probably down to a stp loop, as stp couldnt support that additional vlan as such its basically the same a turning stp off for that vlan and allowing it to propagate through your network without it getting blocked
10-06-2021 02:27 AM
Hi,
yes it looks like an stp loop, but I don't understand why this happens for all vlans and not just for the vlan 224...
Kind regards,
Andreas
10-06-2021 04:56 AM - edited 10-06-2021 04:58 AM
Hello
It’s not just the stp loop, it what it effects, broadcast storms increase exponentially multiple process not only large amount of stp transitions, interface, CPU/memory utilization , switch cam and router arp tables will also get flooded from consistently being updated, IGP peering will flap,all this will have a drastic effect on your network, until that loop is broken and stp convergence is completed.
10-06-2021 10:41 PM
Hello Paul, sorry for the delay.
ok, I understand the CPU/mem, cam-table etc. But I would expect (especially if running pvst) that behavior just for the vlan 224, not for all vlans...
Kind regards,
Andreas
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