07-26-2012 07:34 PM - edited 03-07-2019 08:00 AM
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
Solved! Go to Solution.
07-27-2012 01:08 AM
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
07-27-2012 01:08 AM
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
07-27-2012 01:17 PM
Hi Giuseppe,
Many thanks again.
Regards
Mahesh
07-04-2022 10:33 AM - edited 07-04-2022 10:33 AM
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
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