Which multicast address for IGMP Join messages ?

[cbeswick]

Hi,

Can anyone tell me the correct multicast address that hosts should send their IGMP join messages to ?

I was under the impression it should be 224.0.0.2 ? the same address that the IGMP leave messages are sent to.

Thanks, Chris.

[Giuseppe Larosa]

Hello Chris,

it depends unsolited IGMP reports are sent to 224.0.0.2 but not IGMP reports sent as an answer to a group specific query that are sent to the group G address

see

http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_customize_igmp_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1055689

IGMP packets are transmitted using IP multicast group addresses as follows:

•IGMP general queries are destined to the address 224.0.0.1 (all systems on a subnet).

•IGMP group-specific queries are destined to the group IP address for which the router is querying.

•IGMP group membership reports are destined to the group IP address for which the router is reporting.

•IGMPv2 leave-group messages are destined to the address 224.0.0.2 (all routers on a subnet).

•IGMPv3  membership reports are destined to the address 224.0.0.22; all  IGMPv3-capable multicast routers must listen to this address. 

Hope to help

Giuseppe

[cbeswick]

Hi Giuseppe,

It is the IGMP join that I am troubleshooting at the moment. I can see these messages but the destination address is the multicast group address. I always thought that join messages had to be sent to the 224.0.0.2 address to inform the routers about the clients that wish to join the multicast group, yet the developer is telling me that it should be destinated to the group address and that only leave messages are sent to 224.0.0.2

If the join message is benig sent to the group address, surely the routers aren't being informed correctly about the client(s) wishing to join the group ?

[Giuseppe Larosa]

Hello Chris,

the unsolicited IGMP report  should be sent to 224.0.0.2 = first IGMP report, otherwise the host has to wait next general query from router to report

after the state is built group address can be used

if it is IGMPv2 use RFC 2236 as a reference for standard behaviour

http://www.faqs.org/rfcs/rfc2236.html

Edit:

see section 9 Message destinations

also unsolicited is sent to group address !

probably a flag bit is set in the packet header in order to have the packet processed by the router ( like router alert)

Hope to help

Giuseppe

[cbeswick]

Hi Guiseppe,

I have read the RFC and although I agree that it states the following:

"When a host joins a multicast group, it should immediately transmit    an unsolicited Version 2 Membership Report for that group"

It doesn't specify to which address the message should be sent.

Then in section 9 it states:

9.  Message destinations

This information is provided elsewhere in the document, but is    summarized here for convenience.

Message Type                Destination Group

---------------------                -------------------------

General Query                                ALL-SYSTEMS (224.0.0.1)

Group-Specific Query               The group being queried

Membership Report                     The group being reported

Leave Message                               ALL-ROUTERS (224.0.0.2

....So according to the above the join message which is actually part of the membership report should have a destination address of the group?