Need help -- Root bridge is showing as an access layer switch

[vipinrajrc]

Hi Experts,

Today i have checked root bridge of a network topology of our client's. It is showing root bridge as an access layer switch.

That topology is like below

access --> Distribution --> core

From distribution there is reduntant connection to two core switches.

Core 1 is the VTP server, all others are client. Nothing is configured as transparent.

Also Core1 is the active HSRP router.

So root bridge should be the core switch, right?? if it so i want to make core switch as root bridge. Also i need to enable uplink fast.

Is there any way to make core1 as the root bridge. Also one more thing there is almost 30 VLAN.

Please suggest a proper way to handle this.

Thanks

Vipin

[Edison Ortiz]

Please refer to the spanning-tree vlan command in the documentation:

http://www.cisco.com/en/US/docs/ios/lanswitch/command/reference/lsw_s2.html#wp1113230

You can use spanning-tree vlan priority root and a macro will automatically calculate the priority value in your environment and apply it to the switch.

You can also manually apply the value yourself w/o the macro.

Regards,

Edison

[vipinrajrc]

Hi ,

thanks for your reply.

But my doubt is like is it necessary  core switch shoud be the root bridge???

Thanks

Vipin

[Jon Marshall · Accepted solution]

Vipin

Your network will work whether the root bridge is a core switch or an access-layer switch but as a general answer, yes you should make the core switch your root bridge with the commands Edison has given you.

There are a couple of main reasons -

1) core switches are generally more powerful and therefore can handle the BPDU processing needed for STP. The last thing you want is an overloaded access-layer switch being the root and dropping or missing BPDUs

2) optimal paths in the network eg.

2 distribution switches connected by a L2 trunk sw1 and sw2

one access-layer switch connected via L2 trunks to sw1 and sw2

HSRP is running on the distribution switches and they are doing inter-vlan routing

if the access-layer switch becomes root for all vlans then both uplinks from the access-layer are forwarding. This means the trunk link between sw1 and sw2 must block. sw1 and sw2 need to exchange HSRP messages but they can't send them directly to each other because that trunk link is blocked so they have to send them via the access-layer switch.

This would apply to any information the sw1 and sw2 needed to exchange including routing  peerings etc. so it would place even more load onto the access-layer switch.

Jon