Etherchannel PAgP frames behaviour

[mahesh18]

Hi everyone,

Need to know few things on etherchannel

DTP and CDP send and receive packets over all the  physical interfaces  in the EtherChannel, while PAgP sends and receives PAgP Protocol Data   Units only from interfaces  that are up and have PAgP enabled for auto  or  desirable modes.

When an EtherChannel bundle is configured as a   trunk port, the trunk  sends and receives PAgP frames on the lowest  numbered  VLAN. STP always chooses the first operational port in  an  EtherChannel bundle.

Need to understand ---  the trunk  sends and receives PAgP frames on the lowest  numbered   VLAN. STP always chooses the first operational port in  an   EtherChannel bundle?????????

Is this by design or there is some reason for above statement???

Thanks

MAhesh

[Giuseppe Larosa · Accepted solution]

Hello Mahesh,

for STP the whole bundle is a single logical link. However, when it comes to sending and receiving STP BPDUs STP does not send and receive on all member links, but it choices the member link that was the first to join the bundle. This is the meaning of STP uses the first operational port in the bundle.

This is done by design  and it explains why we should use LACP or PAGP in order to check all member links of a bundle because STP does not perform this job.

Another note is that when you build an etherchannel that uses PAGP all member links should be configured for PAGP you cannot mix member links with different channel-group mode for example desirable and on or desirable and active.

Hope to help

Giuseppe

[mahesh18]

Hi Giuseppe,

Many thanks again.

Regards

Mahesh

[pgasparovic]

Appreciated even 10 years later, today.

Had a riddle of mind, why in my CML lab between two IOSvL2 switches, one SW-01 already on 2G channel while its peer SW-02 just 1G (unfinished config), the latter one saw its channel as root port (SW-01 root bridge) on higher portID! than unassigned G0/3 of lower ID. (both peer on same ports, G0/2 vs G0/2, same for G0/3... G0/2 of lower portID than G0/3)...

And this is the answer. As SW-01 moved to speaking on its 1st member port G0/2, dropping talk over G0/3, the peer easily picked this and made his G0/3 a designated port. Without channels, G0/2 was root port and G0/3 alternate from SW-02 perspective.

BR

Peter