DES-3326 Layer 3 Fast Ethernet Switch User’s Guide

IGMP Versions 1 and 2

Users that want to receive multicast packets need to be able to join and leave multicast groups. This is accomplished using IGMP.

Figure 5-18. IGMP Message Format

The IGMP Type codes are shown below:

Type

Meaning

0x11

Membership Query (if Group Address is 0.0.0.0)

0x1

Specific Group Membership Query (if Group Address

1

is Present)

0x1

Membership Report (version 2)

6

 

0x1

Leave a Group (version 2)

7

 

0x1

Membership Report (version 1)

2

 

 

Table 5-11. IGMP Type Codes

Multicast routers use IGMP to manage multicast group memberships:

An IGMP “report” is sent by a user’s computer to join a group

IGMP version 1 does not have an explicit ‘leave’ message. Group members have an expiration timer, and if this timer expires before a query response is returned, the member is dropped from the group.

IGMP version 2 introduces an explicit “leave” report. When a user wants to leave a group, this report is sent to the multicast router (for IGMP version 2).

Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) periodically to see whether any group members exist on their subnetworks. If there is no response from a particular group, the router assumes that there are no group members on the network, and multicast packets are not forwarded.

The TTL field of query messages is set to 1 so that the queries do not get forwarded to other subnetworks.

IGMP version 2 introduces a few extensions to IGMP version 1 such as, the election of a single multicast querier for each network, explicit ‘leave’ reports, and queries that are specific to a particular multicast group.

The router with the lowest IP address is elected as the querier. The explicit group leave message is added to decrease latency, and routers can ask for membership reports from a particular multicast group ID.

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D-Link DES-3326 manual Igmp Versions 1, Igmp Type Codes