2 spanning-tree roots

[Amafsha1]

I obviously didn't do a good job reading.  Can someone remind me why there would be cases of 2 spanning-tree roots in a network like mine below?

It seems that vlan 20 has 2 different roots

VLAN20
Spanning tree enabled protocol rstp
Root ID Priority 33545
Address 220f.73ee.bd00
Cost 6
Port 4096 (port-channel1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 33545 (priority 32768 sys-id-ext 20)
Address 0023.04ee.be01
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1 Root FWD 1 128.4096 (vPC peer-link) Network P2p
Po22 Desg FWD 1 128.4097 (vPC) P2p
Po3 Desg FWD 1 128.4098 (vPC) P2p
Po34 Root FWD 1 128.4100 (vPC) P2p
Po66 Desg FWD 1 128.4101 (vPC) P2p
Po67 Desg FWD 1 128.4103 (vPC) P2p Peer(STP)
Po11 Desg FWD 1 128.4104 (vPC) P2p Peer(STP)
Po8 Desg FWD 1 128.4121 (vPC) Edge P2p
Eth1/22 Desg FWD 4 128.278 P2p Peer(STP)
Eth1/27 Desg FWD 4 128.283 Edge P2p
Eth1/28 Desg FWD 4 128.284 Edge P2p

[mlund · Accepted solution]

Hi

This is the normal state in a vpc environment when you are looking in the vpc secondary switch.

The root is present behind Po34, and the vpc primary is on Po1.

/Mikael

[Amafsha1]

Ok, I forgot that, thank you.  But it's still odd because this is what I see on the 2 cores for that VLAN 20. If I follow po34 it ends up 2 hops away to some random edge switch at the edge of the network that is the root, where as all the other vlans are normal for being the root on both cores.  Could this cause high cpu issues?

Core1#

VLAN0401 4497 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0500 4596 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0501 33269 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0502 33270 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0503 33271 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0504 33272 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0600 4696 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0601 33369 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0631 33399 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0632 4728 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0664 4760 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0686 33454 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0696 33464 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN020 33545 000f.2423.bd00 5 2 20 15 port-channel34 (I follow this to some random access switch at the edge of the network)

Core2#

VLAN0401 4497 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0500 4596 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0501 33269 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0502 33270 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0503 33271 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0504 33272 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0600 4696 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0601 33369 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0602 33370 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0631 33399 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0632 4728 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0636 33404 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0664 4760 0023.04ee.be01 0 2 20 15 This bridge is root
VLAN0696 33464 0023.04ee.be01 0 2 20 15 This bridge is root

VLAN020   33545 000f.2423.bd00       6    2   20  15     port-channel1 (to Core 1)

[mlund]

Hi

I can't see why that should cause high cpu. But if that switch is not suppose to be root for vlan 20, I would definitely make the core switch the root instead.

Be aware though, that it can be traffic disruptive to change root.

/Mikael

[Amafsha1]

Ok, thanks.  how would be the easiest way to do so?

[mlund]

There are 2 ways to do it. Either login to the switch that is currently the root bridge, and there you configure a higher value than existing  with "spanning-tree vlan 20 priority <value>". Or login to the core switches and configure a lower value for example 28672 with command "spanning-tree vlan 20 priority 28672"

The values must be in multiples of 4096, and the switch that have the lowest value will be the root bridge.

/Mikael