Subject: [magma] IGMP on multi-subnet networks

From: "Fredrik Nyman" <[email-masked]>
Date: Mon, 09 May 2005 23:42:56 +0200

People,

I've run into a discussion about how to handle IGMP queries in a 
multi-subnet network.  

Three hosts share the same ethernet broadcast media. Each of 
them belong to a different IPv4 subnet.

 Host A --------!
 192.168.1.1    !
                !
 Host B --------!
 172.16.1.1     !
                !
 Host C --------!
 10.0.0.1       !
                !------ Router
                        192.168.1.254/24
                        172.16.1.254/24
                        10.0.0.254/24

The Router is the IGMP Querier. Each of the hosts are members of 
the same group, 230.1.1.1. 

What would be the correct behavior:

a) The router sends out three IGMP General/Group-Specific 
queries, one for each subnet with its IP address in each subnet 
as source address in the IP header.

b) The router sends out one IGMP General/Group-Specific query 
using any one of its configured IP addresses as source address 
in the IP header.  

There seems to be host implementations out there that drops 
Queries if they are not sourced from the same IP subnet as the 
hosts joined interface belongs to (forcing routers to behave 
according to a) for the host to work properly). 

I can't find a statement in the RFCs for IGMP that deals with 
this particular situation, but my interpretation of the wording 
(here from IGMPv3 RFC section 4.1.12) is that b) is correct.

"*However*, a system MUST accept and process any Query whose IP 
Destination Address field contains *any* of the addresses (unicast or multicast) 
assigned to the interface on which the Query arrives. "

What's your opinion on this?

-- 
Fredrik Nyman
PacketFront Sweden AB
http://www.packetfront.com/
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