Spanning Tree Issue

[bitflowcisco]

Hello

We have a very simple network and I'm seeing some strange STP behaviour wherein setting the bridge priority of the switch does not take effect.   I must be issuing the command incorrectly  The command: spanning-tree vlan 1-4094 priority 4096 does not seem to take effect on any of the switches that I run it on.  For example, after running the above command and then doing a sh span I see that the switch priority is still 32768.

Details:

-There is one core switch (Catalyst 6504-E pair running in VSS); and two access switches (Catalyst 4948-10GE).

-There are no redundant/blocking ports anywhere (i.e. no loops). Access switches are etherchanneled into the core.

-Running MST with only 1 instance (0).  MST Digest of all switches match.

-Here is the STP config on the core:

spanning-tree mode mst
spanning-tree extend system-id
spanning-tree pathcost method long
!
spanning-tree mst configuration
name region1
revision 1
!
spanning-tree vlan 1-4094 priority 4096

Here is the output of sh span on the core:

MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address 0008.e3ff.fc04
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address 0008.e3ff.fc04
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Po10 Desg FWD 1000 128.5763 P2p
Po12 Desg FWD 1000 128.5765 P2p

Why wouldn't the Root ID priority of the core be 4096 as per the running config? I also tried running that line manually from the terminal with no effect!?

Here is the config on one of the access switches:

spanning-tree mode mst
spanning-tree extend system-id
spanning-tree pathcost method long
!
spanning-tree mst configuration
name region1
revision 1
!
spanning-tree vlan 1-4094 priority 28672

Here is the output of sh span on the same access switch:

MST0
Spanning tree enabled protocol mstp
Root ID Priority 32768
Address 0008.e3ff.fc04
Cost 0
Port 1292 (Port-channel12)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address 1cdf.0f87.0840
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Po12 Root FWD 1000 128.1292 P2p

Again, why is its priority 32768 when I set it to 28672?

Thanks in advance, I'm sure I'm issuing the command incorrectly!?

Cheers

[Paul Chapman · Accepted solution]

Hi -

You need to specify the bridge priority in your MST configuration.  The configuration shown above applies to PVST+ and RPVST+.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_52_se/configuration/guide/3560scg/swmstp.html#wp1038772

For the common spanning-tree (MST0) you could issue the command:

spanning-tree mst 0 priority 4096

The key to MST is that each MST instance, which includes 1 OR MORE VLANs, is a STP topology.  This means that when you change the priority for a topology, you are changing the priority for all VLANs that are members of that topology.  If you don't specify secondary topologies (MST1, MST2, etc...) then all VLANs are automatically members of MST0.

Good Luck!

PSC

[bitflowcisco]

THANK-YOU! I knew it was something simple.  Can't believe I overlooked that!  Much appreciated.

Cheers