Two ways to force an IGMP join

In some situations it is desirable to manually configure multicast group membership on a router interface, particularly during testing and in the absence of IGMP-capable recipients. IOS provides two commands to statically designate group membership on an interface: ip igmp join-group and ip igmp static-group. So what's the difference? The join-group command makes the router behave as a member of the specified group, whereas the static-group command allows for fast-switching of group traffic out the designated interface(s).

Assume we wish to forward multicast traffic for the 239.10.10.10 group to the 192.168.10.0/24 subnet. To do this, we apply ip igmp join-group 239.10.10.10 to R2's F0/1 interface:

R2(config)# interface f0/1
R2(config-if)# ip igmp join-group 239.10.10.10

We can verify that R1 and R2 now have a multicast route for this group:

R2# show ip mroute 239.10.10.10
...

(*, 239.10.10.10), 00:01:58/00:02:26, RP 1.1.1.1, flags: SJCL
  Incoming interface: FastEthernet0/0, RPF nbr 10.0.0.1
  Outgoing interface list:
FastEthernet0/1, Forward/Sparse, 00:01:58/00:02:26

A ping to the group from R1 receives a response from R2 (note that two pings are actually sent from R1, thus two replies):

R1# ping 239.10.10.10

Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.10.10.10, timeout is 2 seconds:

Reply to request 0 from 10.0.0.2, 16 ms
Reply to request 0 from 10.0.0.2, 24 ms

Now we'll replace the join-group command with static-group. After removing join-group we clear the multicast routing table on both routers and apply ip igmp static-group 239.10.10.10 to R2's F0/1 interface:

R2(config)# interface f0/1
R2(config-if)# no ip igmp join-group 239.10.10.10
R2(config-if)# do clear ip mroute *
R2(config-if)# ip igmp static-group 239.10.10.10

Examining the multicast routing table verifies that we again have and entry for the 239.10.10.10 group on both routers:

R2# show ip mroute 239.10.10.10
...

(*, 239.10.10.10), 00:03:32/stopped, RP 1.1.1.1, flags: SJC
  Incoming interface: FastEthernet0/0, RPF nbr 10.0.0.1
  Outgoing interface list:
FastEthernet0/1, Forward/Sparse, 00:03:32/00:02:28

However, a ping to the group from R1 fails this time:

R1# ping 239.10.10.10 repeat 5

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 239.10.10.10, timeout is 2 seconds:
.....

The ping fails because R2, configured with the static-group command, no longer acts a group member itself; instead, it merely fast-switches multicast traffic out F0/1.