PAgP and STP

joshgibson82

When issuing the show pagp neighbor command....is there anything in the output that will tell which interfaces are sending STP updates?

I have a practice question with this scenario and it is stumping me how they came to the answer.

F0/1 and F0/2 in a channel using pagp. The Partner Device IDs are identical, Flags are Identical (SC) but the age is less on F0/1.

The answer is : only F0/1 sends STP packets out.

HELP!

dtlokee

I think I remember reading somewhere that it uses the lowest numbered active interface for sending your L2 protocols such as STP. I looked for it but I couldn find a link.

cisco_trooper

Catalyst 2950 Desktop Switch Software Configuration Guide, 12.1(9)EA1 - Configuring EtherChannels [Cisco Catalyst 2950 Series Switches] - Cisco Systems

PAgP Interaction with Other Features

The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and receive packets over the physical interfaces in the EtherChannel. Trunk ports send and receive PAgP protocol data units (PDUs) on the lowest numbered VLAN.

STP sends packets over a single physical interface in the EtherChannel. Spanning tree regards the EtherChannel as one port.

PAgP sends and receives PAgP PDUs only from interfaces that are up and have PAgP enabled for auto or desirable modes.

accely

Thanks for posting this! I ran across the same question and just assumed it was the lowest port but I wasn't sure either.

cisco_trooper

No problem man. I don't remember what I was looking for but it seemed like useful info and someone else obviously was looking for it at some point.

vigil4u

my 2 cents:

STP uses one of the physical ports for BPDU.
By algorithm, STP uses lowest PORT ID (i.e Port priority + Port number) and hence lower port number.

Hth.