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Spanning tree - Root bridge on two switches

scott.mwa
Level 1
Level 1

I am having an issue where a switch I have won't give up root. The switch I want to be root I set at a priority of 4096, and set the priority of the switch that I don't want to be root at 20480. However the switch at 20480 still thinks that it is root on that VLAN and I can't figure out why. They see each other over CDP on that port. They are connected via 1GB Single mode fiber. 

I have set this config for all vlans transported between the switches, yet the switch that is suppose to not be root is root on every single one. I can't find any root guard or bpdu guard settings in the config. 

This config works for 40 other switches connected to the Nexus. 

Primary switch nexus 7010

Secondary is a 2960S

Nexus span

VLAN0152
Spanning tree enabled protocol rstp
Root ID Priority 4248
Address XXXX.XXXX.2bc1
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 4248 (priority 4096 sys-id-ext 152)
Address XXXX.XXXX2bc1
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

2960 Span

 VLAN0152
Spanning tree enabled protocol rstp
Root ID Priority 20632
Address XXXX.XXXX.4800
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 20632 (priority 20480 sys-id-ext 152)
Address XXXX.XXXX.4800
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Thanks for any assistance with helping me find why its stuck as root. 

12 Replies 12

Jon Marshall
Hall of Fame
Hall of Fame

It's not clear whether it just one vlan or all vlans.

This is usually caused by BPDUs not getting through and that usually happens when the vlan has been blocked from passing between the switches on the trunk link.

Jon

I used a specific VLAN as an example, but this is happening on all VLANs. The VLANs are correctly configured on the trunk -- this is live and passing production data. 

Which switch is meant to be secondary for these vlans ?

What values do the switches that are working have ?

Just wondering where the 28480 value has come ie. I know it is a multiple of 4096 so that should be fine.

Jon

The nexus is mean to be primary - which is why it's manually set at 4096.

The 2960 is meant to be secondary - it was manually set at 20632 so it would be lower than the Nexus, but higher than any default switch that might get plugged in (sneaky users~!). 

This is how spanning tree is set for all VLANs on all switches. 

So the 2960 was manually set at 28480 and for vlan 152 that is 28480 + 152 = 20632.

It should work as far as I can see based on your outputs.

It is as though the 2960 is not seeing any BPDUs from the Nexus switch.

How exactly did you set the priority on the 2960 ?

Jon

The 2960 was manually set at 20480 not 28480. I believe that it adds in a random number so it doesn't get the same priority as another vlan. I have all other vlans set at 20480 and they're all slightly different priority. Even when I set it to the lowest prority available it still adds in a random number. 

Priority was set using the spanning-tree priority command. 

PSSW01(config)#spanning-tree vlan 152 priority ?
<0-61440> bridge priority in increments of 4096

I agree it's as though its not seeing any BPDUs but there's no BPDU gaurd enabled on the interface on either switch. 

Detail of the 2960 looks like this:

Port 49 (GigabitEthernet1/0/49) of VLAN0152 is root forwarding
Port path cost 4, Port priority 128, Port Identifier 128.49.
Designated root has priority 4248, address b414.89e3.2bc1
Designated bridge has priority 4248, address b414.89e3.2bc1
Designated port id is 128.1327, designated path cost 0
Timers: message age 16, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 5, received 305

wait -- somehow it's now picking up the nexus as the root bridge. It's been like this for months and now it works??

Sorry typo on my part it was meant to say 20480.

Are you saying it now works ?

Jon

That's correct. It just started working on all VLANS. I mostly just ran show commands, but on vlan 152 only, I ran a "no spanning-tree vlan 152 root primary" command. It still had the 20480 priority though. 

Okay, I had a read of the 2960 configuration guide and that is why I was asking how you set the priority for each vlan.

The guide says to exercise caution when you use the command you did to set the priority but doesn't go into any reasons and I have to say for 6500s I have always used the command you did.

So I am wondering if the command to use would be the one you used in your last post (without the no at the front obviously) but with secondary keyword to tell your switch it is meant to be secondary.

Using that command your switch should work out a lower priority by itself based on the BPDUs it is receiving but it has to receive those BPDUs to be able to work out a priority.

Hope the above makes sense.

Jon

Thanks for that information - It's not meant to be secondary either! I have a separate router that is secondary. I used the secondary tag on that one and set the priority at 8192. 

I still don't know why it wasn't picking up the BPDUs before, but it is now. 

It's working correctly now thanks for your help. 

Just for your info it is not a random number it adds, it adds the vlan ID to the base priority you have set.

Jon

Thank you for that information! 

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