IGMP Version 2

IGMP version 2 is the “enhanced” version of IGMP version 1. One of the major reasons for a new version was to improve the “leave” mechanism. In IGMP version 1, hosts just stop listening to the multicast group address, but they never report this to the router. Here are the new features:

• Leave group messages: when a host no longer wants to listen to a multicast group address then it will report to the router that it has stopped listening.
• Group-specific membership query: the router can now send a membership query for a specific group address. When the router receives a leave group message, it will use this query to check if there are still any hosts interested in receiving the multicast traffic.
• MRT (Maximum Response Time) field: a new field in query messages. It specifies how much time hosts have to respond to the query. I will explain later why we use this with an example.
• Querier election process: when there are two routers in the same subnet, then only one of them should send query messages. The election ensures only one router becomes the active querier. The router with the lowest IP address becomes the active querier.

To demonstrate these new features, I’ll use the following topology:

Above, we have one multicast-enabled router and two hosts.

Let’s start with R1:

R1(config)#ip multicast-routing
R1(config)#interface GigabitEthernet 0/1
R1(config-if)#ip pim sparse-mode 

First, we have to enable multicast routing and PIM on the interface otherwise, the router won’t process IGMP traffic. We can verify that it’s running:

R1#show ip igmp interface GigabitEthernet 0/1
GigabitEthernet0/1 is up, line protocol is up
  Internet address is 192.168.1.1/24
  IGMP is enabled on interface
  Current IGMP host version is 2
  Current IGMP router version is 2
  IGMP query interval is 60 seconds
  IGMP configured query interval is 60 seconds
  IGMP querier timeout is 120 seconds
  IGMP configured 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: 6 joins, 5 leaves
  Multicast routing is enabled on interface
  Multicast TTL threshold is 0
  Multicast designated router (DR) is 192.168.1.1 (this system)
  IGMP querying router is 192.168.1.1 (this system)
  Multicast groups joined by this system (number of users):
      224.0.1.40(1)

Above we can see that IGMP is enabled and that our router is the querying router. There are no other routers, so that’s an easy way to win the election.

Before we let the hosts join a multicast group, let’s enable debugging on all devices:

R1, H1 & H2
#debug ip igmp 
IGMP debugging is on

On R1, you will see the following message:

R1#
IGMP(0): Send v2 general Query on GigabitEthernet0/1

Like IGMP version 1, the router sends general membership queries every 60 seconds. This is what it looks like in wireshark:

Above, you can see the destination, which is 224.0.0.1 (all hosts multicast group address). Let’s configure our first host to join a multicast group:

H1(config)#interface GigabitEthernet 0/1
H1(config-if)#ip igmp join-group 239.1.1.1

This is what you will see on the console of the host:

H1#
IGMP(0): WAVL Insert group: 239.1.1.1 interface: GigabitEthernet0/1Successful
IGMP(0): Send v2 Report for 239.1.1.1 on GigabitEthernet0/1

Our host is sending a membership report to 239.1.1.1 to tell the router that it wants to receive this multicast traffic. Here you can see the packet in wireshark:

Here’s what R1 thinks of this:

R1#
IGMP(0): Received v2 Report on GigabitEthernet0/1 from 192.168.1.101 for 239.1.1.1
IGMP(0): Received Group record for group 239.1.1.1, mode 2 from 192.168.1.101 for 0 sources
IGMP(0): WAVL Insert group: 239.1.1.1 interface: GigabitEthernet0/1Successful
IGMP(0): Switching to EXCLUDE mode for 239.1.1.1 on GigabitEthernet0/1
IGMP(0): Updating EXCLUDE group timer for 239.1.1.1
IGMP(0): MRT Add/Update GigabitEthernet0/1 for (*,239.1.1.1) by 0

R1 receives the membership report from host 1 and adds an entry for multicast group 239.1.1.1.

Everything you have seen so far is pretty much the same as IGMP version 1. Our router sends general membership queries, and the host sends a report.

When we add a second host, things will change. Let’s configure H2 to join the same group:

H2(config)#interface GigabitEthernet 0/1
H2(config-if)#ip igmp join-group 239.1.1.1

Here’s what we will see on the router:

R1#
IGMP(0): Send v2 general Query on GigabitEthernet0/1
IGMP(0): Received v2 Report on GigabitEthernet0/1 from 192.168.1.102 for 239.1.1.1
IGMP(0): Received Group record for group 239.1.1.1, mode 2 from 192.168.1.102 for 0 sources
IGMP(0): Updating EXCLUDE group timer for 239.1.1.1
IGMP(0): MRT Add/Update GigabitEthernet0/1 for (*,239.1.1.1) by 0

Our router sends another membership query, and it also received the membership report from H2. Here’s what happens now when both hosts receive the query:

H1#
IGMP(0): Received v2 Query on GigabitEthernet0/1 from 192.168.1.1
IGMP(0): Set report delay time to 2.8 seconds for 239.1.1.1 on GigabitEthernet0/1
IGMP(0): Send v2 Report for 239.1.1.1 on GigabitEthernet0/1

H2#
IGMP(0): Received v2 Query on GigabitEthernet0/1 from 192.168.1.1
IGMP(0): Set report delay time to 3.0 seconds for 239.1.1.1 on GigabitEthernet0/1
IGMP(0): Received v2 Report on GigabitEthernet0/1 from 192.168.1.101 for 239.1.1.1
IGMP(0): Received Group record for group 239.1.1.1, mode 2 from 192.168.1.101 for 0 sources
IGMP(0): Cancel report for 239.1.1.1 on GigabitEthernet0/1

Above, you can see that both hosts receive the query from the router. Host 1 sets its report delay time to 2.8 seconds and then sends the membership report.

H2 also receives the query but sets its report delay time to 3.0 seconds. Since it has to wait longer, H1 was able to send the membership report first. When H2 receives the report from H1, then it will cancel sending a membership report itself.

Where did these timers come from? This is one of the new features of IGMP version 2. The router advertises a maximum response time in its queries: