7206VXR Port-Channel default load balance method

Unanswered Question
Mar 19th, 2009


I am wondering if anyone can tell me what method of load balancing is used by default on a port-channel interface (2 gig) on a 7206VXR chassis, running an NPE-G2.

The other end of the port-channel would go to 7609s, layer 2 trunks.

Of course I can specify the load-balance method on the 7600s, with port-channel load-balance command, to be src-des ip or mac etc, but I do not know a way to change it on the 7206.

Reason I am concerned is if I am running VoIP over the port-channel, I want to make sure a certain flow stays on one port of the port channel.


I have this problem too.
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Paolo Bevilacqua Thu, 03/19/2009 - 13:18

A flow will always stay on a single port.

Be advised that even with NPE-G2, with significant bidirectional traffic, the router will not be able to keep up with 2 fully loaded giga links, thus negating the need for ethernchannel.

Jasonch518_2 Thu, 03/19/2009 - 13:22

Thanks for the quick response.

I do understand that the router itself would never be able to support 2 saturated gig ports, but the real reason for the port channel was for redundancy, so that the layer 2 path to the 7206 could be connected to 2 seperate switches.

I could not seem to think of a better way to get a redundant layer 2 path to a router that needs to use DOT1Q tagging on a trunk port, but if you have any other design ideas, I am open to suggestions.

Thanks again.

Paolo Bevilacqua Thu, 03/19/2009 - 13:29

The alternative would be to put both interfaces (or subinterface) in bridge-group, and with irb, you would have the L3 configuration under a BVI interface. One circuit will stay blocked due to STP.

This is quite a common configuration.

I didn't even know that it was possible to span etherchannel across switches.

Jasonch518_2 Thu, 03/19/2009 - 14:29

Well, on the switch side, there is no "etherchannel" so it is not spanning across switches, but no matter which port the router decides to use on the etherchanel, the downstream switch (7609) has a layer 2 path to the end host.

Things have been working fine, I just was not sure how it would act once VoIP traffic is going across the port-channel on the router, but if it will always keep a single flow on a single interface, it should be fine.

Going further on your BVI scenario, how would you do VLAN tagging on the BVI interface if I had multiple VLAN's going to the 7206?

Jasonch518_2 Thu, 03/19/2009 - 14:55

Could we do something along the lines of:


encapsulation dot1q 10

bridge-group 1


encapsulation dot1q 20

bridge-group 2


encapsulation dot1q 10

bridge-group 1


encapsulation dot1q 20

bridge-group 2

bridge irb

bridge 1 protocol ieee

bridge 1 route ip

bridge 2 protocol ieee

bridge 2 route ip

interface bvi 1

ip address x.x.x.x x.x.x.x

interface bvi 2

ip address x.x.x.x x.x.x.x

I would think that would accomplish the VLAN tagging.

I assume that the router can only do the standard IEEE PVST?

I have not had a chance to fully lab this, but you said that I would have a forwarding and blocking port, because of STP.

Would the blocking port be on the switch side? I am running MST on my 7609s and downstream switches, for faster convergence, sub-second, but would I loose that fast convergence time in this scenario? At least at the 7206.

Paolo Bevilacqua Thu, 03/19/2009 - 15:44

Yes, that is the config. Sorry, I don't know about the convergence times for the router. Fortunately, a switch failure is an extremely rare occurrence.

Jasonch518_2 Thu, 03/19/2009 - 15:51

Thanks again for the reply.

I will do some lab testing to determine the convergence time.

I think this solution will work better for when using VoIP, since there would be no load balancing, just a primary and protect path, since I have no need for anywhere near a gig of throughput in most cases, 300Mbps max.


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