root bridge issues

[hector.grajales1]

Hello All

I have an issue releated to root bridge. I have one core switch and 3 access switch, It is supposed that the core switch be the root bridge for all the vlans anyway my access switches are marked as rood bridge for some VLANs. My understandig is that the switch with the lowest number has the highest priority and in this way you can select wich switch is the root. I already did this but the access switch are still marked as root bridge,  can you please assist me on this issue,  see below some outputs:

core switch

xxxxx0001#show spanning-tree vlan 13

VLAN0013
Spanning tree enabled protocol rstp
Root ID Priority 20493
Address c47d.4f81.3500
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 20493 (priority 20480 sys-id-ext 13)
Address c47d.4f81.3500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Access switch

xxxxx1002#show spanning-tree vlan 13

VLAN0013
Spanning tree enabled protocol rstp
Root ID Priority 32781
Address 68bd.ab17.a800
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32781 (priority 32768 sys-id-ext 13)
Address 68bd.ab17.a800
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 4 128.1 P2p
Fa1/0/44 Desg FWD 19 128.48 P2p

[Giuseppe Larosa · Accepted solution]

Hello Hector,

your core swich is a Cisco router running Rapid PVST C4:7D:4F Cisco Systems, Inc.

Rapid PVST BPDUs travel within the corresponding vlan-id over trunk links between switches.

It looks like that vlan 13 is not permitted over the 802.1Q trunk between the core switch and access switch.

If this happens and the access switch has at least an access port in vlan 13 it will claim to be root bridge of vlan 13.

So you have two root bridge for vlan 13 because you have broken the L2 connectivity in vlan 13 broadcast domain.

You need to fix the list of allowed vlans on the link between core and access switch to solve your issue.

the command to be used carefully in interface mode is:

interface type x/y

switchport trunk allowed vlan add 13

SPECIAL CARE if you omit the add keyword the list of permitted vlans becomes only vlan 13 !

you can use CDP or LLDP to find the ports that connect core switch to access switch.

After the change above made on both ends of the link a single root bridge for vlan 13 will be elected

Hope to help

Giuseppe

[hector.grajales1]

Hello Guissepe

Thanks a lot for your quick answer and explanation, you are right, I added the missing VLANs in the core switch and that fixed the issue.

[kousikdutta]

Hi,

Helpful info.

Thanks,