Rapid Spanning Tree and Etherchannel

[lcaruso]

Hi,

What configuration is needed to implment Etherchannel between two switches in a rapid spanning tree environment? Any examples/links appreciated.

Is the best load balancing algo dest-ip for this scenario?

One side is root bridge/core switch and other side is access switch both running 12.2 latest IOS.

Thanks.

[Reza Sharifi · Accepted solution]

Hi,

Just add the physical posts to the portchannel.  In this example, I am using LACP, so one side is active and the other side is passive. You can also use PAGP or mode on.  If your access switches are 3560 or 3750 series, the default for load balancing is source-destination MAC. You can just use the default.

example

core switch

inter gi1/1

channel-group 10 mode active

inter gi1/2

channel-group 10 mode active

inter po10

switchport

switchport trunk enca dot

switch port mode trunk

no sh

access switch

inter gi1/1

channel-group 10 mode passive

inter gi1/2

channel-group 10 mode passive

inter po10

switchport

switchport trunk enca dot

switch port mode trunk

no sh

HTH

[lcaruso]

Thanks for your reply. Sorry if I was vague about the example.

My question really was how does an etherchannel configuration inform spanning tree that it is permissible to have multiple active redundant links between the two switches?

My impression was you had to configure that also in addtion to a standard EC setup as you have provided.

Thanks.

[Reza Sharifi]

You don't need to configure spanning tree specifically for the Portchannel.  STP looks at the Portchannel bundle as one logical link.  The STP cost will change as you add more physical link to the Portchannel.  For example: if you have one gig link in the Portchannel the cost is 4, when you add a second link the cost will change to 3, with 3 gig port the cost will be 2 and so on.....

HTH

[lcaruso]

Can UDLD be used with Etherchannel?

[Joseph W. Doherty · Accepted solution]

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Posting

Is the best load balancing algo dest-ip for this scenario?

That depends.  What you want is the best mix for the traffic crossing the link.  If all/most traffic was going to the same host, dest-ip would direct all that traffic to the same physical link.  If your platform supports it, balancing that uses both src and dest generally results in better "randomization" of traffic across the Etherchannel.

[lcaruso]

Thanks. Seems to agree with this post also.

https://supportforums.cisco.com/community/netpro/network-infrastructure/switching/blog/2011/08/09/how-etherchannel-hash-algorithm-works-and-load-distribution-happens