load-balancing on Catalyst 3550

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Jun 4th, 2008
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Hi everybody,

I have a Catalyst 3550 with an etherchannel. The load-balancing algorithm is per-destination MAC. Is the switch able to load-balance multicast traffic?

Best regards


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Overall Rating: 3.7 (3 ratings)
Jon Marshall Wed, 06/04/2008 - 10:56
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As far as i know the etherchannel would load-balance a multicast packet in the same way as a unicast packet purely on the mac-address. Obviously with multicast because a groups of multicast addresses share a single mac-address you would not get load-balancing for two multicast groups mapping to the same mac-address but that is it.


s-durando Wed, 06/04/2008 - 23:04
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in the Cisco Doc I found:

"The default port is used to transmit traffic, such as Spanning Tree Protocol (STP), multicasts, and unknown unicasts. The default port can be identified from the output of the command show etherchannel summary by a notation of d."

It seems that all multicasts are sent to the defualt port. Is this valid for every etherchannel or only for etherchannels with LACP? In my network I don't use LACP and I cannot find a port marked with 'd' in the output of show etherchannel summary.



andrew.butterworth Wed, 06/04/2008 - 12:56
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On the 3550 if your EtherChannel is a routed interfaces then the load-balancing is based on the IP Source & Destination when 'src-mac' is used. This is a bit misleading in the CLI as it would suggest that there would be no load-balancing with routed interfaces since the source/destination MAC would be constant. The documentation does however state that when routed interfaces are used the load-balancing is based on Source & Destination IP Addresses:


However in your multicast scenario, unless there are many sources transmitting to the same group then the load-balancing over a Layer-3 EtherChannel will be pretty one-sided....



s-durando Wed, 06/04/2008 - 23:05
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my etherchannel is a layer2 etherchannel



Ryan Carretta Wed, 06/04/2008 - 23:37
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Hi Stefano,

The switch is able to balance this traffic, even though the destination MAC does not change. Many moons ago this wasn't the case, but as of software revision 12.1(19)EA1, load balancing of multicast traffic should work as expected.

Bug ID CSCea68471 tracks this.

s-durando Thu, 06/05/2008 - 00:00
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many thanks the information you provided me.

Is it possible that this bug is still present in my 3550 with 12.2(25)SEB4?

The Catalyst if forwarding a lot of multicast traffic (with many different sources and many different multicast groups). When I use src-mac load-balancing method, I have approximately 35Mbps on the first FastEthernet and 60Mbps on the second one. When I move to dest-mac load-balancing method, all the traffic is forwarded on one FastEthernet only!




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