Two root bridge in same network

[abhisar patil]

Dear Team,

As I checked, there are two root bridge in the same LAN.

We have 6500 which is manually configured as root bridge and this is showing root for all the vlans in the network. Once switch connected to 6500 through 4500 is showing root for the vlans that not assigned to any of the port. Please help to clear it.

Setup

Cisco 6500 -- Cisco 4500 -- Cisco3560 -- Cisco 3560

Cisco 6500

CORE_SW#show spanning-tree root detail
VLAN0001
  Root ID    Priority    24577
             Address     0025.84d9.ac80
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
VLAN0002
  Root ID    Priority    24578
             Address     0025.84d9.ac80
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Cisco 3560 Second

Access#show spanning-tree root de
VLAN0001
  Root ID    Priority    24577
             Address     0025.84d9.ac80
             Cost        16
             Port        28 (GigabitEthernet0/4)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
VLAN0002
  Root ID    Priority    32770
             Address     000a.b8ff.be00
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Here, I have not assigned any port in vlan 2 and this is showing root bridge for vlan 2. In which cases such thing can happen?

Thank You,

Abhisar.

[ghostinthenet · Accepted solution]

By default, Cisco switches run one spanning tree instance per VLAN and negotiate the topology with other connected switches. If your 3560 believes it is the root for VLAN 2 and there are no ports using VLAN 2, it will consider itself the to be the root because it hasn't been able to negotiate a topology for this VLAN with any other devices. This is normal. Once ports are connected to VLAN 2 and the 3560 can talk to the other switches, the spanning tree will be renegotiated and should behave as you expect.

If you want to have a single spanning tree topology for all VLANs and avoid this behaviour, consider moving to a single-instance MSTP configuration.

[abhisar patil]

Dear Jody,

Thank you for comment. Cleared now.

So once I add the ports in vlan 2, the 6500 will become root as it is configured manually to be as root.

Thank You,

Abhisar.