Portchannels and IP Multicast

Unanswered Question
Jul 16th, 2009

I had a colleague tell me that when you form a Portchannel with Ethernet interfaces on IOS that IP multicast is not equally balanced across the links of that group. Is that true, and can someone point me in the direction of docs that support that?

I have this problem too.
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Depending on the switch you are using you will have to setup what type of load balancing is done on the etherchannel link. Here is the global command:

port-channel load-balance {src-mac | dst-mac | src-dst-mac | src-ip | dst-ip | src-dst-ip | src-port | dst-port | src-dst-port | mpls}

Of course if you are load balancing based on only src-ip/dst-ip (if a single multicast server) then multicast will not be load balanced.

Reference: http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml

rwilsonmetrocast Fri, 07/17/2009 - 05:16

So he was correct. Does anyone know of a good way to balance the load with multicast traffic?

Giuseppe Larosa Fri, 07/17/2009 - 05:54

Hello Robbie,

actually modern switches can use different algorithms with different type of traffic

see the example above taken from a C6500

sh etherchannel load-balance

EtherChannel Load-Balancing Configuration:


mpls label-ip

EtherChannel Load-Balancing Addresses Used Per-Protocol:

Non-IP: Source XOR Destination MAC address

IPv4: Source XOR Destination IP address

IPv6: Source XOR Destination IP address

MPLS: Label or IP

multicast is still IPv4 packets so a source exor destination should be enough

You can see options with

port-channel load-balance ?

dst-ip Dst IP Addr

dst-mac Dst Mac Addr

dst-port Dst TCP/UDP Port

mpls Load Balancing for MPLS packets

src-dst-ip Src XOR Dst IP Addr

src-dst-mac Src XOR Dst Mac Addr

src-dst-port Src XOR Dst TCP/UDP Port

src-ip Src IP Addr

src-mac Src Mac Addr

src-port Src TCP/UDP Port

Hope to help


rwilsonmetrocast Fri, 07/17/2009 - 07:23

So what could I do when I have a heavy flow between a set of hosts? Since both the source/dest macs, IPS, and ports are the same, is there anyway to get the router to split it across the second link in the bundle?

Giuseppe Larosa Fri, 07/17/2009 - 10:39

Hello Robbie,

the operation is an exor of last significant digits of source IP address and destination IP address.

In case of multicast flows the destination is the multicast group address and the source is the source address.

So yes a given (S,G) multicast flow is forwarded out a single link of the bundle.

if you have multiple sources of G groups with a wise choice you can get some load balancing.

If you have routers or L3 switches you can consider to use the links as independent L3 links.

And then you can use ip multicast multipath command in global config






this is still flow based so no real advantage over a L2 etherchannel bundle.

Hope to help



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