Spanning-tree "port id"

[rob.hicks1]

Hi,

I am experimenting with spanning-tree on a home lab. when using for example a "show spanning-tree int fa0/1" command the response refers to port fa0/1 as "port 13" which seems strange? Can someone explain the reason for this, and also tell me if there is a way of listing the port id's in IOS.

Many Thanks

[Rajat Chauhan]

Please paste the output and describe briefly the topology.

Thanks

[rob.hicks1]

Hi,

The topology is simply 3 2950 switches - all interconnected (i.e each has two connections),I have just upgraded the IOS to a newer version on switch 2 - and the port id now appears in a different format which seems to make more senses (e,g 128.1 - the priority and interface number) although im not sure how this would translate when looking at ports on a modular switch?

The output below is from the two 2950 switches:

Switch-1#sh spanning-tree int fa0/1

Interface Fa0/1 (port 13) in Spanning tree 1 is FORWARDING

Port path cost 19, Port priority 128

Designated root has priority 8192, address 0009.7cfa.3b81

Designated bridge has priority 8192, address 0009.7cfa.3b81

Designated port is 1, path cost 0

Timers: message age 2, forward delay 0, hold 0

BPDU: sent 3025, received 2266

Switch-2#sh spanning-tree int fa0/1

Port 1 (FastEthernet0/1) of VLAN1 is forwarding

Port path cost 19, Port priority 128, Port Identifier 128.1.

Designated root has priority 8192, address 0009.7cfa.3b81

Designated bridge has priority 8192, address 0009.7cfa.3b81

Designated port id is 128.1, designated path cost 0

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

BPDU: sent 2294, received 1763

[Francois Tallet]

On your first switch, the port id would have been 128.13 also. It's just that the display is less explicit.

It seems that the numbering of the port has also changed between the two IOS releases. Even if it's not really nice, it's not something "illegal" either. In theory, should should not expect any scheme in the way the port number are attributed. It is however desirable that the port number stays the same (after a shut/no shut for instance) so that the network topology stays consistent. For instance, on the cat6k, a port number will be based on slot number + interface number on the line card. I guess that on a stack of switches, the switch number is going to enter into the port number calculation...

In any case, don't try to guess this formula;-)

Regards,

Francois

[rob.hicks1]

Thanks for the info ... thats cleared that up anyway :0)