Troubleshooting IGMP Joins

As discussed in Chapter 12, "Understanding Protocol Independent Multicast (PIM)," IGMP joins are a line of communication between the multicast receiver (host) and the router. IGMP joins are used by the multicast receiver to inform the multicast router that hosts on the local segment are interested in certain multicast groups; this allows the router to forward multicast packet to the segment. This section discusses issues with IGMP joins. Refer to Figure 13-1 for the troubleshooting flowchart on IGMP join issues.

Figure 13-1. Troubleshooting Flowchart on IGMP Join Problems

Refer to Figure 13-2 for network setup of IGMP join problem.

Figure 13-2. Network Diagram for Case Study on IGMP Join Problem

In Figure 13-2, the multicast host is interested in joining multicast group 225.1.1.1. The multicast host sends the IGMP joins to the multicast address 224.0.0.2, which is the all routers multicast address. The problem is that Router A is not seeing the IGMP joins. To see which multicast groups are joined to the routers through IGMP, use the command show ip igmp group. Example 13-1 shows the show ip igmp group command output for Router A.

Example 13-1 show ip igmp group Command Output for Router A

 RTR_A#  show ip igmp group  IGMP Connected Group MembershipGroup Address    Interface      Uptime     Expires Last Reporter RTR_A# 

The output in Example 13-1 shows that Router A is not seeing the IGMP joins from the multicast host. If Router A is not seeing the IGMP joins from the multicast host, the next logical step in troubleshooting is to question whether the multicast host is sending out IGMP joins and what the router did with the IGMP join packet. To see the IGMP transaction on a router, use the debug ip igmp command, as demonstrated in Example 13-2 for Router A.

Example 13-2 debug ip igmp Command Output on Router A

 RTR_A#  debug ip igmp  IGMP: Received v2 Report from 150.150.2.2 (Ethernet0) for 225.1.1.1 IGMP: Group 225.1.1.1 access denied on Ethernet0 

The debug output shows that the router is receiving the IGMP joins from the host, but they are rejected by the router. Example 13-3 shows Router A's configuration.

Example 13-3 Router A Configuration

 RTR A#  show run  ip multicast-routing interface ethernet 0     ip address 150.150.2.1 255.255.255.0     ip pim dense-mode     ip igmp access-group 1 access-list 1 deny 224.0.0.0 3.255.255.255     access-list 1 permit any 

The configuration in Example 13-3 reveals the reason why the 225.1.1.1 IGMP join is getting rejected ‚ there is an access-group statement configured for IGMP on interface Ethernet 0. The access group is tied to access-list 1, which denies any group joins for the range of 224.0.0.0 to 227.255.255.255. The network administrator wants to deny only groups from 224.0.0.0 to 224.255.255.255, and this is a misconfiguration on the router.

Solution to IGMP Join Problem

The solution for this IGMP join problem is to change access-list 1 to match that shown in Example 13-4.

Example 13-4 Alteration of the Router A Configuration to Permit Joining the Multicast Group of 225.1.1.1

 RTR_A#  access-list 1 deny 224.0.0.0 0.255.255.255   access-list 1 permit any  

With this new configuration, the router no longer will reject the IGMP join for the 225.1.1.1 group. Example 13-5 shows the debug on Router A and the output from the show ip igmp group command after the changes have been made.

Example 13-5 debug ip igmp and show ip igmp group Command Output on Router A

 RTR_A#  debug ip igmp  IGMP: Received v2 Report from 150.150.2.2 (Ethernet0) for 225.1.1.1 RTR_A#  show ip igmp group  Group Address    Interface      Uptime     Expires    Last Reporter 225.1.1.1        Ethernet0     00:00:40  00:02:18  150.150.2.2 

Now Router A shows the 225.1.1.1 group as joined in Ethernet 0 with the IP address of 150.150.2.2 as the IGMP reporter.