[c-nsp] STP root switch election MAC address

[Clinton Work]

The chassis or supervisor (depends upon platform) will have a range of 
reserved MACs assigned to it. MACs will be taken from the reserved range to 
use as the STP bridge ID.

How the MACs are assigned depends if extended system ID (MAC reduction) is 
enabled or not. If extended system ID is enabled, one MAC or Bridge ID will 
be used for all STP instances (aka VLANs). If extended system ID is disabled 
then a unique MAC will be used for each STP instance (aka VLAN). Some 
switches don't allow you to disable extended system ID (ex 2950).

7609# show spanning-tree summary  | inc Ext
Extended system ID           is enabled

The exec command to look at the reserved MAC range varies by platform:

7609# show catalyst6000 chassis-mac-addresses
   chassis MAC addresses: 64 addresses from 0011.20d8.7700 to 0011.20d8.773f

4500# show idprom chassis  | inc MAC
  MAC Base = 000d.65ab.aac0
  MAC Count = 64

You can always check each STP instance to grab the bridge ID of the local 
switch.

7609> show spanning vlan 1000
VLAN1000
   Spanning tree enabled protocol ieee
   Root ID    Priority    5096
              Address     0011.20d8.7700
              This bridge is the root
              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

   Bridge ID  Priority    5096   (priority 4096 sys-id-ext 1000)
              Address     0011.20d8.7700   <-- Switch bridge ID
              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
              Aging Time 300

....

List the local bridge ID for all configured VLAN STP instances.

7609> show spanning bridge
                                                    Hello  Max  Fwd
Vlan                         Bridge ID              Time  Age  Dly  Protocol
---------------- --------------------------------- -----  ---  ---  --------
VLAN1500           5596 (4096,1500) 0011.20d8.7700    2    20   15  ieee
VLAN1501           5597 (4096,1501) 0011.20d8.7700    2    20   15  ieee
VLAN1502           5598 (4096,1502) 0011.20d8.7700    2    20   15  ieee
VLAN1504           5600 (4096,1504) 0011.20d8.7700    2    20   15  ieee
VLAN1505           5601 (4096,1505) 0011.20d8.7700    2    20   15  ieee
...



Amol Sapkal wrote:
> Hi,
> 
> I was going through my network's STP setup and suddenly I realized that I
> wasn't aware of any command that would let me know the switch MAC that takes
> part in the root bridge selection procedure.
> 
> Have sifted through a lot of Cisco pages, but unable to hit the correct one.
> Any ideas?
> 
> 
> 

-- 
===================================================
Clinton Work	clinton at scripty.com
Calgary, AB