Subject: Questions on IGMP

From: Ajit Bapat <[email-masked]>
Date: Wed, 11 Aug 1999 10:15:38 -0700 (PDT)

Hello ,

    I have a couple of questions on IGMP, as I am reading RFC 2236.

1. In RFC 2236, the term 'interface' seems to point to a physical interface
   of a router.  The igmpCacheTable in <draft-ietf-idmr-igmp-mib-10.txt>
   also seems to index entries with {igmpCacheAddress, igmpCacheIfIndex}. 

     I am thinking of a case when there are multiple IP subnets configured on 
   a physical interface.  
   - Should IGMP run separately on each subnet ?  
   - Do all IP subnets on a physical interface share/use the same multicast 
     router ? 
   - Is it possible / useful for each subnet to use a different multicast 
     router ?
   - Does IGMP ignore sender's IP address in all its received packets ?

2. How does IGMPv2 use Router Alert option ? - RFC 2113 says it uses.

	Thanks for your time.

Ajit Bapat
[email-masked]

---

From: Bill Fenner <[email-masked]>
Date: Wed, 11 Aug 1999 16:24:37 -0700

>   - Should IGMP run separately on each subnet ?  

No.

>   - Do all IP subnets on a physical interface share/use the same multicast 

Yes.

>   - Is it possible / useful for each subnet to use a different multicast 

Without complex coordination, that would result in multiple copies
of the same packet when there are group members of the same group on
different subnets on the same wire.

>   - Does IGMP ignore sender's IP address in all its received packets ?

No, it's suggested to ensure that the sender's IP address belongs
to one of the subnets associated with the reception interface.

>2. How does IGMPv2 use Router Alert option ? - RFC 2113 says it uses.

As RFC2236 says, all messages described in the document contain a
Router Alert option in the IP header.  The option is useful mostly for
Report messages, which are addressed to the group being reported but
should be received by routers.

  Bill