[c-nsp] Quick spanning-tree and bridge-group question

[Peter Rathlev]

On Fri, 2008-07-04 at 17:52 +0100, Sam Stickland wrote:
> Guys,
> 
> Maybe I'm going a little code-blind here.
> 
> Ports Fa0/1.161, Fa0/1.162 and Se0/0/0.10 are all members of same IEEE 
> bridge-group.
> 
> The port path cost on all three interfaces is the same, but I've set the 
> priority of the Serial interface port to be 144.
> 
> How come the Se0/0/0.10 is still forwarding? What am I missing?
>
> R1#sh spanning-tree
> 
>  Bridge group 10 is executing the ieee compatible Spanning Tree protocol
>   Bridge Identifier has priority 32768, address 0013.8050.b191
>   Configured hello time 2, max age 20, forward delay 15
>   Current root has priority 10, address 0019.aa7f.3480
>   Root port is 28 (Serial0/0/0.10), cost of root path is 685
>   Topology change flag not set, detected flag not set
>   Number of topology changes 21 last change occurred 00:22:10 ago
>           from FastEthernet0/1.161
>   Times:  hold 1, topology change 35, notification 2
>           hello 2, max age 20, forward delay 15
>   Timers: hello 0, topology change 0, notification 0, aging 300
> 
<cut>

According to the output above Se0/0/0.10 is the root port, which will
never be blocking. You don't state what's beyond these three ports, but
if Se0/0/0.10 is the only path towards the root, it will never block, no
matter how you set cost or priority.

Since the other ports are blocking, they are probably all paths towards
the root. The port cost is 38 for Se0/0/0.10 and 76 for Fa0/1.161 and
Fa0/1.162, so even if all ports went to the same device, Se0/0/0.10
would be preferred because of a lower cost.

Regards,
Peter