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Frequent Contributor

Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

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

2 ACCEPTED SOLUTIONS

Accepted Solutions
VIP Expert

Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

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

View solution in original post

VIP Expert

Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

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).

View solution in original post

4 REPLIES 4
VIP Expert

Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

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

View solution in original post

Frequent Contributor

Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

Hi Reza,

Many thanks again.

Regards

Mahesh

VIP Expert

Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

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).

View solution in original post

Frequent Contributor

Spanning Tree , Etherchannel Layer 2,and Stack Switch Behaviour

Hi Joseph,

Thanks for adding very useful into to the post.

Regards

Mahesh

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