Subject: MIB implementation

From: Todd Hayton <[email-masked]>
Date: Tue, 19 Jun 2001 11:15:52 -0400 (EDT)

Hello,

I've recently been tasked with implementing the multicast routing MIB (rfc
2932) and have a question about the indexes used in one of the tables.
Specifically, I'm confused about the ipMRouteNextHopAddress index of the
ipMRouteNextHopTable.  

The name ipMRouteNextHopAddress seems to imply that the index is actually
the IP address of the next receiver on the outgoing interface. However,
the rfc makes the following comment on this index:

"The address of the next-hop specific to this entry.  For most interfaces,
this is identical to ipMRouteNextHopGroup. NBMA interfaces, however, may
have multiple next-hop addresses out a single outgoing interface."

So, I guess I'm unclear about how this is a useful index, as
ipMRouteNextHopGroup already specifies the group being sent to. The
distinction seems to be for NBMA interfaces, but that part of the
description I'm honestly a bit lost on.

If anybody has any clarifications, I'd really appreciate it.

Regards,
Todd H

---

From: Bill Fenner <[email-masked]>
Date: Wed, 20 Jun 2001 09:46:52 -0700

>The distinction seems to be for NBMA interfaces

It is.  For interfaces that have a layer 2 multicast or broadcast,
the ipMRouteNextHopAddress is simply the same as ipMRouteNextHopGroup.
However, if you have an interface that does not have a layer 2 multicast
or broadcast, and yet you have multiple neighbors, you have to replicate
the packet on that network once to each interested neighbor.  (Maybe all
of the multicast-capable systems on the link, maybe some subset).  In this
situation, ipMRouteNextHopAddress is used to list the relevant neighbors.

  Bill