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

[Brett Frankenberger]

On Fri, Jul 04, 2008 at 07:18:05PM +0100, Sam Stickland wrote:
> Sam Stickland wrote:
> >Peter Rathlev wrote:
> >>
> >>Isn't port priority the last thing the Spanning Tree Algorithm looks at?
> >>AFAIK the selection of root port should be, in order: Root Bridge ID,
> >>Port Path Cost, Sending Bridge ID and at last Sending Port ID, which is
> >>Port Priority and Port Index.
> >>
> >>Are the three ports all pointing towards the root bridge?
> >>  
> >Yes, all three ports are pointing towards the root.  You are right, I 
> >had remembered the order wrong. At least something like this causes it 
> >to sink it. (I would test it but I'm having trouble convincing one of 
> >the other routers to change it's bridge identifier on the BVI :| )
> >
> Well I finally convinced the router to change it bridge id (had to 
> change the mac-address on all physical interfaces - even those not in 
> the bridge group!), but I still don't see the result I'd expect:

Bridge Priority is part of the Bridge ID comparison.  Your path costs
are equal, so brige priority gets compared next.  32768 is the
smallest of the three, so that path wins.  You didn't need to convince
the router to change the MAC address it used for the Bridge ID -- just
increasing it's priority setting would have made it less preferred. 
(If you make it higher than 32929, then FA0/1.161 will become the root
port and transition to frowarding.)

Assuming an IOS router and a bridge group:
   bridge X priority 32391

> 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 723
>  Topology change flag not set, detected flag not set
>  Number of topology changes 34 last change occurred 00:02:09 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
> 
> Port 26 (FastEthernet0/1.161) of Bridge group 10 is blocking
>   Port path cost 647, Port priority 128, Port Identifier 128.26.
>   Designated root has priority 10, address 0019.aa7f.3480
>   Designated bridge has priority 32929, address 0019.aab4.f700
>   Designated port id is 112.3, designated path cost 76
>   Timers: message age 6, forward delay 0, hold 0
>   Number of transitions to forwarding state: 6
>   BPDU: sent 870, received 7351
> 
> Port 27 (FastEthernet0/1.162) of Bridge group 10 is blocking
>   Port path cost 647, Port priority 128, Port Identifier 128.27.
>   Designated root has priority 10, address 0019.aa7f.3480
>   Designated bridge has priority 32930, address 0019.aab4.f700
>   Designated port id is 112.3, designated path cost 76
>   Timers: message age 5, forward delay 0, hold 0
>   Number of transitions to forwarding state: 2
>   BPDU: sent 862, received 7330
> 
> Port 28 (Serial0/0/0.10) of Bridge group 10 is forwarding
>   Port path cost 685, Port priority 144, Port Identifier 144.28.
>   Designated root has priority 10, address 0019.aa7f.3480
>   Designated bridge has priority 32768, address 0055.aa7f.3480
>   Designated port id is 128.20, designated path cost 38
>   Timers: message age 4, forward delay 0, hold 0
>   Number of transitions to forwarding state: 6
>   BPDU: sent 552, received 14515

     -- Brett