Cisco Environment – Using Channel-Group Mode On

ciscocisco-2960cisco-3750etherchannelieee-802.1ax

this is my first question over here.

I have a question about using channel-group mode on or channel-group mode active.

I have a core 3750x with 2 stacked SFPs nodes. Each pair of ports of a node with the same port number is an etherchannel:

  • Gi1/0/1 + Gi2/0/1 = Po1
  • Gi1/0/2 + Gi2/0/2 = Po2
  • and so on

These etherchannel connects to the 2960 switches, where they have a SFP module, using 2 SFPs ports to connect to the 3750x. Those two ports are also an etherchannel.

All the Po's interfaces are in channel-group mode on on both sides.
The Po interfaces are ok, all UP and not giving me errors.

But I'd like to know:

  • Is this a good pratice?
  • Is it better to use LACP? Why?
  • Any of you guys ever had any problem using mode on?

Thank you very much!

Obs.:
I read the article Preventing STP loops in etherchannels configurations

But in my case I know and I manage both sides of the connection

Best Answer

LACP gives you a significant advantage in that it verifies connectivity in both directions over each link. You can think of it as also combining the features of UDLD with link aggregation. For example, imagine you have the links configured in "mode on". Now lets say one of the links from the 3750 to the 2960 fails but in only one direction, say the direction toward the 2960 (lets say the fiber got kinked and only one of the two strands has broken). The 3750 will still see link on both ports (because the direction from the 2960 to the 3750 is fine) and it will still try to use the failed path because it does not know that the 2960 has lost link.

If you configure "mode active" the LACP packets must flow over the link in both directions between the 3750 and the 2960. If the link fails in one direction, LACP fails to work and the 3750 stops forwarding over the failed link even though it still has light from the 2960.

So -- using LACP gives you protection against unidirectional link failure. It also gives you protection against plugging the link into the wrong port. If you accidentally plug the link into the wrong port, LACP will fail, the 3750 will not bring that link up, and you have protected yourself against a problem that is difficult to troubleshoot.

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