Subject: RE: Cisco's DVMRP Discovery ?

From: Arlie Davis <[email-masked]>
Date: Wed, 5 Mar 1997 12:32:04 -0500

Cisco routers use RPF to decide if the packet was received on the
correct interface, regardless of the flavor of the peer on that
interface.  (There could be multiple peers, too.)  If you do have peers
(on any interface) that are either native DVMRP (mrouted) or Ciscos
exchanging DVMRP routes, then the router will first use the DVMRP table
for RPF calculation, then if the destination is not found there, it will
check its unicast routing table.

For native DVMRP peers connected via DVMRP-mode tunnels, Ciscos use
mrouted-style prune/graft messages.  For PIM-speaking peers (other
Ciscos), Ciscos use PIM mechanisms.  For dense mode, this resembles but
is not identical to mrouted-style prune/graft.  Sparse mode is entirely
different.

> but that the forwarding of that packet is done using 
> PIM's mechanish 

Basically, yes.

> instead of trying to find out if the 
> router is on the shortest path between a neighbor 
> who is to receive the multicast packet and the 
> source of such packet?

This is getting confusing.  If you still have a question, could you
break it down into smaller chunks, or state your question more clearly?

-- arlie

>-----Original Message-----
>From:	Jesus Maria Arango - Ing. Comunicaciones FSM LTDA.
>[SMTP:[email-masked]]
>Sent:	Wednesday, March 05, 1997 2:19 AM
>To:	Arlie Davis
>Cc:	[email-masked]'
>Subject:	RE: Cisco's DVMRP Discovery ?
>
>On Wed, 5 Mar 1997, Arlie Davis wrote:
>
>> No, you are wrong.
>> 
>> Two Ciscos connected by either a GRE tunnel or a physical interface use
>> PIM, either sparse mode or dense mode, to do multicast routing.
>> Additionally, the two machines can be configured to exchange DVMRP
>> routes, and can be configured to prefer DVMRP routes over their own
>> unicast routing tables.
>> 
>> However, they still use the mechanisms of PIM to determine what traffic
>> is sent across the link.  They do not use the DVMRP prune/graft
>> messages.  This is the fundamental difference.
>
>Does this mean that a cisco router uses the DVMRP table only to decide if
>the incoming packet came on a tunnel that is on the shortest path to the
>source but that the forwarding of that packet is done using PIM's
>mechanish instead of trying to find out if the router is on the shortest
>path between a neighbor who is to receive the multicast packet and the
>source of such packet?
>
>- Jesus
>