stp and port id

[sarahr202]

Hi everybody

I observed STP starts assigning port number from 42 as shown below:

R1#show spanning-tree int f1/1

   Port path cost 19, Port priority 128, Port Identifier 128.42.

R1#show spanning-tree int f1/2

   Port path cost 19, Port priority 128, Port Identifier 128.43.

R1#show spanning-tree int f1/4

   Port path cost 19, Port priority 128, Port Identifier 128.45.

Does spanning tree start assigning port number from 42?

What about a switch with multiple modules i.e  f0/1, f1/1,f2/1?

What will be the default stp port id for these above ports?

thanks and have a great weekend

[Aileron88 · Accepted solution]

I believe this is platform dependant. It depends on the slot the line card is in & the port number etc. So for example if you have a 4506 with 2 x 48 port copper line cards in you may find that the port numbers start from 1,2,3 but when they hit the second line card they just continue through 49,50,51 and so on. This is just an example though, it would be best to look on the platform itself to see what this would be.

[Dumitru Otel]

by David Hucaby, the port_ID is formed from 16 bits, 8 bits for port priority and 8 bits for port number ..., the default priority is 128 and can be changed by incrementing or decrementing of 16 value number. The 8 bits of  port number allow a value from 0 - 255, this value is dependent on the physical slot, and can't be changed ...

I was surprised when on Catalyst 6513, the  port number was greater than 8 bits, with a value of 11 bits...

Mst Instance        Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

MST0                Desg FWD 20000    128.1198 P2p Edge

Hwo can help me ?

[Aileron88]

8 bits for the port priority and 8 bits for the port number are derived from the 16 bit Port Identifier field within a BPDU. Having 8 bits for each is how it used to work, before the definition of the 802.1t standard. The 802.1t standard changes the allocation of the16 bits so that 4 bits are used for the port priority and 12 bits are used for the port number (ID). This means that you have a much larger range for port numbers. The priority can range from 0 to 240, in increments of 16.

Thanks,

Adam

[Dumitru Otel]

Thank you Adam !!!