04-09-2012 08:33 AM - edited 03-07-2019 06:01 AM
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
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04-09-2012 10:06 AM
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.
04-09-2012 10:06 AM
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.
04-09-2012 11:52 PM
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 ?
04-10-2012 03:10 AM
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
04-10-2012 03:25 AM
Thank you Adam !!!
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