Multicast Equal Cost Multipathing (ECMP)

Imagine a PIM sparse-mode scenario where multiple senders were sending to two different groups, and you had multiple equal-cost paths to receive the traffic on, but PIM, by default, always picks the neighbor's interface with the highest IP and sends the join up that one.

How can you get some load sharing?

Here's our diagram:

 

R1 is pinging 239.1.1.1 and R2 is pinging 239.1.1.2.  R3 is the RP.  R5 is joined to 239.1.1.1 and 239.1.1.2. 

We have two equal-cost paths between R3 and R4, and we want to use one for 239.1.1.1 and the other for 239.1.1.2, headed towards R5.  Right now Fa0/1, whose neighbor has a higher IP address than Fa0/0's neighbor, is getting all the traffic:

R4(config)#do sh ip mroute 239.1.1.1
<output omitted>

(192.168.13.1, 239.1.1.1), 00:00:19/00:02:47, flags: JT
  Incoming interface: FastEthernet0/1, RPF nbr 192.168.234.3
  Outgoing interface list:
    FastEthernet1/0, Forward/Sparse, 00:00:19/00:02:40

<output omitted>

(192.168.23.2, 239.1.1.2), 00:00:19/00:02:46, flags: JT
  Incoming interface: FastEthernet0/1, RPF nbr 192.168.234.3
  Outgoing interface list:
    FastEthernet1/0, Forward/Sparse, 00:00:19/00:02:40

ip multicast multipath is the answer.

R4(config)#ip multicast multipath
R4(config)#do sh ip mroute
<output omitted>

(192.168.13.1, 239.1.1.1), 00:05:39/00:02:56, flags: JT
  Incoming interface: FastEthernet0/1, RPF nbr 192.168.234.3
  Outgoing interface list:
    FastEthernet1/0, Forward/Sparse, 00:05:39/00:02:13

<output omitted>

(192.168.23.2, 239.1.1.2), 00:05:39/00:02:55, flags: JT
  Incoming interface: FastEthernet0/0, RPF nbr 192.168.34.3
  Outgoing interface list:
    FastEthernet1/0, Forward/Sparse, 00:05:39/00:02:10

Not too difficult.  It uses a hash to achieve this, which I'm frankly not interested enough to look into, because on the IOS version I'm using, you can't change the hash anyway.  It looks like starting in IOS 15 you can make some modifications to it.

Also, one catch here is this only balances by source.  If you want to balance by group, you'll need to do that with your RP assignments, sending some traffic to one RP and other traffic to a different RP.

Enjoy

Jeff Kronlage