Subject: Re: Multicast pruning problem?

From: Bill Fenner <[email-masked]>
Date: Thu, 1 May 1997 15:30:20 PDT

Bill Fenner <[email-masked]> wrote:
>I'd suggest configuring the following on every multicast-enabled
>interface of every Cisco you own:
>
>        ip dvmrp accept-filter 1 neighbor-list 2

As usual, my message was probably too concise.

This command will prevent the Cisco from establishing a peering
relationship with any DVMRP routers on the given interface.
Configuring your router in this way WILL PARTITION THE NETWORK ALONG
THIS LINK if you have DVMRP peers; see below for options on how to deal
with this.  I recommend configuring it on all multi-access links
because when a Cisco peers with a DVMRP router on a multi-access
network, the Cisco ignores the prunes that the DVMRP router sends,
causing all traffic to be forward in the Cisco->DVMRP direction.

This can cause severe problems if the Cisco->DVMRP direction is towards
a leaf of the MBone, since it means that all MBone traffic will be
forwarded on this link.  It can cause less severe problems if the
Cisco->DVMRP direction is towards the MBone, since all it means is that
all of the traffic sourced beyond the Cisco will be forwarded to this
link.

If the link is a required part of your topology, you have two choices:

1) Use a mesh of tunnels instead of the physical medium to peer.  This
is wasteful in bandwidth (for multiple copies of the packets), router
CPU (to do replication that the network could do), and configuration
(to keep the mesh configured if another router shows up).  Alternately,
you can use the physical medium to let your DVMRP routers peer and
create a mesh of tunnels from the Ciscos to one of the DVMRP routers,
or use the physical medium to let the Ciscos run PIM and create a mesh
of tunnels from the DVMRP routers to one of the Ciscos.  This is still
wasteful but slightly less so since you still get a small amount of the
benefit of the replication of the physical medium.

2) Live with the lack of pruning.  This is not recommended, as the
amount of multicast traffic is steadily increasing and if you choose
this option you will receive *all* of it.

  Bill