Troubleshooting Multicast Routing

By Edward Tetz from Cisco Networking All-in-One For Dummies

When troubleshooting for information about the multicast routing protocol, first use the show ip pim command, which requires an additional option before providing information. The following shows the major options available, from interfaces to RPs to neighbors:

Router2>enable
Password:
Router2#show ip pim ?
  autorp      Global AutoRP information
  bsr-router  Bootstrap router (v2)
  interface   PIM interface information
  mdt         Multicast tunnel information
  neighbor    PIM neighbor information
  rp          PIM Rendezvous Point (RP) information
  rp-hash     RP to be chosen based on group selected
  vrf         Select VPN Routing/Forwarding instance

Sparse mode PIM has several requirements for RPs on the network. So, if you are in Sparse mode, you want to take a close look at the several RP-based options. In addition, reviewing neighbors is always important, because if you do not see any neighbors, you probably have significant issues with the protocol implementation. Output about neighbor and rp look like the following; notice particularly the version and rp information:

Router2>enable
Password:
Router2#show ip pim neighbor
PIM Neighbor Table
Neighbor          Interface                Uptime/Expires    Ver   DR
Address                                                            Prio/Mode
192.168.1.2       FastEthernet0/0          00:02:16/00:01:26 v2    1 / S
Router2#show ip pim rp
Group: 224.1.0.1, RP: 192.168.1.2, v2, uptime 00:06:13, expires never
Group: 224.0.1.39, RP: 192.168.1.2, v2, uptime 00:05:58, expires never
Group: 224.0.1.40, RP: 192.168.1.2, v2, uptime 00:06:15, expires never
Group: 239.192.152.143, RP: 192.168.1.240, v2, v1, next RP-reachable in 00:00:31
Group: 239.255.255.250, RP: 192.168.1.240, v2, v1, next RP-reachable in 00:00:31

Also, much of the multicast routing involves requests from client devices via IGMP requests from the specific receiving devices that are connected to your network switches. To see that data, use one of the following commands:

Router2>enable
Password:
Router2#show ip igmp ?
  groups       IGMP group membership information
  interface    IGMP interface information
  membership   IGMP membership information for forwarding
  snooping     Snooping info on Catalyst Vlans
  ssm-mapping  Display IGMP SSM mapping info
  tracking     IGMP Explicit Tracking information
  udlr         IGMP undirectional link multicast routing information
  vrf          Select VPN Routing/Forwarding instance

Within the IGMP data are groups, interfaces, and membership, and although you may find that these options are the most important ones, it is recommended that you test all the options to see exactly what is going on.

In the following output, the groups option shows you the addresses for which the router is routing, as well as when the membership expires (because this is Sparse mode data), interface shows statistical information for the interface, and membership shows addresses or groups, who is subscribing to those addresses, and the interface on which they are working.

Router2>enable
Password:
Router2#show ip igmp groups
IGMP Connected Group Membership
Group Address    Interface                Uptime    Expires   Last Reporter
224.1.0.1        FastEthernet0/0          00:08:22  00:02:07  192.168.1.103
224.0.1.39       FastEthernet0/0          00:06:22  00:02:07  192.168.1.2
224.0.1.40       FastEthernet0/0          00:08:24  00:02:10  192.168.1.240
239.192.152.143  FastEthernet0/0          00:08:21  00:02:13  192.168.1.10
239.255.255.250  FastEthernet0/0          00:08:23  00:02:09  192.168.1.10
Router2#show ip igmp interface
FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.1.240/24
  IGMP is enabled on interface
  Current IGMP host version is 2
  Current IGMP router version is 2
  IGMP query interval is 60 seconds
  IGMP querier timeout is 120 seconds
  IGMP max query response time is 10 seconds
  Last member query count is 2
  Last member query response interval is 1000 ms
  Inbound IGMP access group is not set
  IGMP activity: 5 joins, 0 leaves
  Multicast routing is enabled on interface
  Multicast TTL threshold is 0
  Multicast designated router (DR) is 192.168.1.240 (this system)
  IGMP querying router is 192.168.1.2
  Multicast groups joined by this system (number of users):
      224.0.1.40(1)
Router2#show ip igmp membership
Flags: A  - aggregate, T - tracked
       L  - Local, S - static, V - virtual, R - Reported through v3
       I - v3lite, U - Urd, M - SSM (S,G) channel
       1,2,3 - The version of IGMP the group is in
Channel/Group-Flags:
       / - Filtering entry (Exclude mode (S,G), Include mode (*,G))
Reporter:
       <mac-or-ip-address> - last reporter if group is not explicitly tracked
       <n>/<m>      - <n> reporter in include mode, <m> reporter in exclude
 Channel/Group                  Reporter        Uptime   Exp.  Flags  Interface
 *,224.1.0.1                    192.168.1.103   00:09:40 02:52 1A     Fa0/0
 *,224.0.1.39                   192.168.1.2     00:07:41 02:53 2A     Fa0/0
 *,224.0.1.40                   192.168.1.103   00:09:42 02:54 1LA    Fa0/0
 *,239.192.152.143              192.168.1.10    00:09:39 02:57 2A     Fa0/0
 *,239.255.255.250              192.168.1.31    00:09:41 02:49 2A     Fa0/0