Enabling IGMP on a Cisco router

Enabling IGMP on a Cisco router

When we enable PIM on a Cisco Router`s Interface, IGMP is automatically enabled.

Example:

In this scenario, we have two routers(R2 and R3) and a client.R1 simulates the client.These three routers are connected through a Layer2 switch.

R1:

interface FastEthernet0/0

 ip address 192.168.123.1 255.255.255.0

R2

interface FastEthernet0/0

 ip address 192.168.123.2 255.255.255.0

 ip pim sparse-mode

R3

interface FastEthernet0/0

 ip address 192.168.123.3 255.255.255.0

 ip pim sparse-mode

We see that IGMP is automatically enabled on R2`s related interface:

R2#show ip igmp interface f0/0

FastEthernet0/0 is up, line protocol is up

  Internet address is 192.168.123.2/24

  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: 1 joins, 0 leaves

  Multicast routing is enabled on interface

  Multicast TTL threshold is 0

  Multicast designated router (DR) is 192.168.123.3 

  IGMP querying router is 192.168.123.2 (this system)

  Multicast groups joined by this system (number of users):

      224.0.1.40(1)

We see that default version is IGMP version2.It can be changed by the interface level  configurations.This output also shows the default timer values of IGMP.

Lets simulate the client:

After we added this command on R1`s F0/0 interface;

ip igmp join-group 239.1.1.1

Client sends IGMP report message to all routers (224.0.0.2)

Its meaning: Client wants the multicast traffic that is destined to 239.1.1.1

R2 and R3 receives this report message:

R2#debug ip igmp

*Mar  1 00:31:27.823: IGMP(0): Received v2 Report on FastEthernet0/0 from 192.168.123.1 for 239.1.1.1

*Mar  1 00:31:27.823: IGMP(0): Received Group record for group 239.1.1.1, mode 2 from 192.168.123.1 for 0 sources

*Mar  1 00:31:27.827: IGMP(0): WAVL Insert group: 239.1.1.1 interface: FastEthernet0/0Successful

*Mar  1 00:31:27.827: IGMP(0): Switching to EXCLUDE mode for 239.1.1.1 on FastEthernet0/0

*Mar  1 00:31:27.827: IGMP(0): Updating EXCLUDE group timer for 239.1.1.1

*Mar  1 00:31:27.831: IGMP(0): MRT Add/Update FastEthernet0/0 for (*,239.1.1.1) by 0

R2#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

 *,239.1.1.1                    192.168.123.1   00:01:01 02:42 2A     Fa0/0

 *,224.0.1.40                   192.168.24.4    00:18:39 02:50 2A     Fa1/0

 *,224.0.1.40                   192.168.123.3   00:27:02 02:35 2LA    Fa0/0

If we remove the “ip igmp join-group 239.1.1.1” command on the client, client sends a leave message:

R2#

*Mar  1 00:35:01.843: IGMP(0): Received Leave from 192.168.123.1 (FastEthernet0/0) for 239.1.1.1

*Mar  1 00:35:01.847: IGMP(0): Received Group record for group 239.1.1.1, mode 3 from 192.168.123.1 for 0 sources

*Mar  1 00:35:01.847: IGMP(0): Lower expiration timer to 2000 msec for 239.1.1.1 on FastEthernet0/0

*Mar  1 00:35:01.847: IGMP(0): Send v2 Query on FastEthernet0/0 for group 239.1.1.1

*Mar  1 00:35:02.851: IGMP(0): Send v2 Query on FastEthernet0/0 for group 239.1.1.1

*Mar  1 00:35:03.851: IGMP(0): Switching to INCLUDE mode for 239.1.1.1 on FastEthernet0/0

Note1:We use IGMP version2.In IGMP version 1, there is no Leave Message.IGMP version1 is not used anymore.

Note2:In this example we have only configured IGMP on the interface.For end-to-end multicast traffic, some other configurations like RP must exists.