2.2.a Implement and troubleshoot IGMP

2.2.a [i] IGMPv1, IGMPv2, IGMPv3

IGMP Version 1 (IGMPv1) messages are transmitted in IP datagrams and contain the following fields:
Version: 1

● Type: There are two types of IGMP messages, Membership Query and Membership Report.

● Checksum


Membership reports are issued by hosts that want to receive a specific multicast Group Destination Address (GDA). Membership queries are issued by routers at regular intervals to check whether there is still a host interested in the GDA in that segment. Host membership reports are issued either unsolicited (when the host wants to receive GDA traffic first) or in response to a membership query.

They are sent with the following fields:

L2 Information

● Source MAC: Host MAC address

● Destination MAC: Destination MAC for the GDA

L3 Information

● Source IP: IP address of the host

● Destination IP: GDA IGMP Packet

● IGMP data contains, furthermore, the GDA and some other fields

Host membership queries are sent by the router to the all-multicast address: These queries use in the IGMP GDA field. A host for each group must respond to that query, or the router stops forwarding the traffic for that GDA to that segment (after three attempts). The router keeps a multicast routing entry for each source, and links it to a list of outgoing interfaces (interface from where the IGMP report came). After three IGMP query attempts with no answer, this interface is erased from the outgoing interface list for all entries linked to that GDA.

In IGMP Version 2 (IGMPv2), the version field has been removed, and the type field can now accept different values. The types are shown below.

● Membership Query

● IGMPv1 Membership Report

● Version 2 Membership Report

● Leave Group Descriptions of the most important new features added in IGMPv2 are listed below.

Descriptions of the most important new features added in IGMPv2 are listed below.

● IGMP Leave Message: when a host wants to leave a group, it should send a Leave Group IGMP message to destination (instead of leaving silently like in IGMPv1).

● A router can now send a group-specific query by sending a Membership Query to the group GDA instead of sending it to

In IGMP Version 3 (IGMPv3), there is a type field that can have the following values:

● Membership query

● Version 3 Membership Report

An implementation of IGMPv3 must also support the following three message types, for interoperation with previous versions of IGMP:

● Version 1 Membership Report

● Version 2 Membership Report

● Version 2 Leave Group

IGMPv3 adds support for source filtering, that is, the ability for a system to report interest in receiving packets from specific source addresses, or from all but specific source addresses sent to a specific multicast address. This feature is also called Source Specific Multicast (SSM)

In order for a computer to support SSM, it must support IGMPv3.

You can use the following command to see all IGMP default timers.

Switch# show ip igmp interface fastEthernet 0/ 1

FastEthernet0/ 1 is up, line protocol is up

Internet address is 8

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

In a network using PIM stub routing, the only allowable route for IP traffic to the user is through a switch that is configured with PIM stub routing. PIM passive interfaces are connected to Layer 2 access domains, such as VLANs, or to interfaces that are connected to other Layer 2 devices. Only directly connected multicast (IGMP) receivers and sources are allowed in the Layer 2 access domains. The PIM passive interfaces do not send or process any received PIM control packets

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2042-2045).  . Kindle Edition.