ECMP Multicast Load Splitting

This is a pretty simple concept. By default when two paths to the RP exist, the router sends a join to the one with the highest IP address. When you enable multicast multipath, the router will send joins up multiple paths depending on Source address (this hash is modifiable in some IOS)

Here is the topology:


R4 has joined group 239.0.0.1. R5, R6 and R7 are all sending pings to this address. Before enabling multipath, this is what R1's mroute table looks like (it's actually bigger I am omitting output for the sake of brevity):

R1#show ip mroute | be \(

(*, 239.0.0.1), 00:00:09/stopped, RP 2.2.2.2, flags: SJC
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:09/00:02:50

(6.6.6.6, 239.0.0.1), 00:00:07/00:02:58, flags: JT
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:07/00:02:52

(150.100.56.5, 239.0.0.1), 00:00:05/00:02:58, flags: JT
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:05/00:02:54

(150.100.56.6, 239.0.0.1), 00:00:07/00:02:58, flags: JT
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:07/00:02:52

(150.100.56.7, 239.0.0.1), 00:00:10/00:02:57, flags: JT
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:10/00:02:49

Notice that it has sent joins only on Serial 1/3. Thus, R2 only sends multicast traffic for 239.0.0.1 out of this interface. R2 OIL looks like this:

R2#show ip mroute 239.0.0.1 | sec Outgoing

Outgoing interface list:
Serial1/3, Forward/Sparse, 00:34:58/00:02:44

Outgoing interface list:
Serial1/3, Forward/Sparse, 00:13:39/00:02:44

Outgoing interface list:
Serial1/3, Forward/Sparse, 00:13:39/00:02:46

Outgoing interface list:
Serial1/3, Forward/Sparse, 00:13:39/00:02:45

Let's enable multicast multipath on R1:

R1(config)#ip multicast multipath

Now we can see Joins have been sent out of both interfaces:

R1#show ip mroute | be \(
(*, 239.0.0.1), 00:00:01/stopped, RP 2.2.2.2, flags: SJC
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:01/00:02:58

(6.6.6.6, 239.0.0.1), 00:00:01/00:02:58, flags: JT
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:01/00:02:58

(150.100.56.5, 239.0.0.1), 00:00:01/00:02:58, flags: J
Incoming interface: Serial1/2, RPF nbr 150.100.12.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:01/00:02:58

(150.100.56.6, 239.0.0.1), 00:00:01/00:02:58, flags: JT
Incoming interface: Serial1/3, RPF nbr 150.100.21.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:01/00:02:58

(150.100.56.7, 239.0.0.1), 00:00:00/00:02:59, flags: J
Incoming interface: Serial1/2, RPF nbr 150.100.12.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:00/00:02:59

R2's OIL now looks like this:

R2#show ip mroute 239.0.0.1 | section Outg

Outgoing interface list:
Serial1/3, Forward/Sparse, 00:03:03/00:03:26

Outgoing interface list:
Serial1/3, Forward/Sparse, 00:01:31/00:03:26

Outgoing interface list:
Serial1/2, Forward/Sparse, 00:01:02/00:03:25

Outgoing interface list:
Serial1/3, Forward/Sparse, 00:01:31/00:03:26

At first I wasn't sure if hashing is done on source or source/group, but I found out by sending to different groups from the same address to see if it splits up. From what I can tell it uses the source to hash, so one source sending to multiple groups will not get split.

R1#show ip mroute | be \(

(150.100.100.5, 238.0.0.1), 00:00:04/00:02:55, flags: JT
Incoming interface: Serial1/2, RPF nbr 150.100.12.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:04/00:02:55

(150.100.100.5, 239.0.0.2), 00:00:49/00:02:17, flags: JT
Incoming interface: Serial1/2, RPF nbr 150.100.12.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:49/00:02:56

(150.100.100.5, 239.0.0.3), 00:00:45/00:02:17, flags: JT
Incoming interface: Serial1/2, RPF nbr 150.100.12.2
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:00:50/00:02:50

There is another train of IOS where you can select what to hash on, but my IOS doesn't have it.

Key thing to remember:

-Enabling multipath causes Joins to get sent towards the RP on more than one interface. This is what causes the load-splitting. Careful not to confuse this with the downstream sending of traffic, by default the router will send it out all interfaces (in the OIL) anyway!