Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

[mahesh18]

Hi Everyone.

On Flat Layer 2 Network if distribution switch is 3750  and it is stack of 2 switches.

Currently it has  only 1 uplink to core switch.

If i config another uplink to core switch for redudancy it will be in STP blocking state right as this is default behaviour right?

Also if i config both uplink ports in layer 2 etherchannel bundle and they go to same core switch will  then both ports will be in

forwarding state?

My understanding is that  in etherchannel they do load balance  so if both ports are  in forwarding state will it cause STP issues

with core switch like loop?

Is there any advantage of config  uplink ports as Layer2 etherchannel ?

Thanks

Mahesh

[Reza Sharifi · Accepted solution]

Hi Mahesh,

If i config another uplink to core switch for redudancy it will be in  STP blocking state right as this is default behaviour right?

Correct,

Also if i config both uplink ports in layer 2 etherchannel bundle and  they go to same core switch will  then both ports will be in forwarding state?

Correct

If you add a second link, put them both in an Etherchannel, so there is no blocking and since the 3750 by default utilizes source-destination-mac, it will load balance the traffic from different sources to the links in the portchannel

My understanding is that  in etherchannel they do load balance  so if  both ports are  in forwarding state will it cause STP issues with core switch like loop?

No, there will be no STP since both ports are in an Etherchannel and they are logically one link.

Is there any advantage of config  uplink ports as Layer2 etherchannel ?

Yes, as I mentioned above, load balancing and no STP.

HTH

Reza

[mahesh18]

Hi Reza,

Many thanks again.

Regards

Mahesh

[Joseph W. Doherty · Accepted solution]

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Posting

No, there will be no STP since both ports are in an Etherchannel and they are logically one link.

Just to clarify what Reza described, generally STP is active on Etherchannel too, but as Reza also notes, it's treated at a single link.  I.e. is doesn't block or not the individual Etherchannel links, but it could block or not the whole logical Etherchannel.

For example, the Etherchannel link can block or not if there's another L2 path (which could also be another Etherchannel).

[mahesh18]

Hi Joseph,

Thanks for adding very useful into to the post.

Regards

Mahesh