Subject: RE: Cisco's DVMRP Discovery ?

From: "Jesus Maria Arango - Ing. Comunicaciones FSM LTDA." <[email-masked]>
Date: Wed, 5 Mar 1997 11:36:14 +0500 (GMT)

On Tue, 4 Mar 1997, Dave Rubin wrote:

> You cannot connect two ciscos via a DVMRP tunnel, only a GRE tunnel 
>  running PIM.
> 
> If both sides are configured to do "dvmrp unicast-routing" then DVMRP 
> routes will flow over the tunnel and PIM-DVMRP interoperability will
>  occur.
> 

Tell me if I'm wrong... 

In this case of a GRE tunnel configured to do "dvmrp unicast-routing", the
2 cisco routers would be exchanging DVMRP routes and the forwarding of
multicast packets comming on either interface of the tuunel would be
computed by the DVMRP protocol and also the prunes and grafts would be
comming from DVMRP. This means that the 2 routers are not speaking PIM at
all. They would consider the other router a DVMRP machine. So there is no
sense in saying "GRE tunnel running PIM". Am I right ?

- Jesus

---

From: "Daniel A. Alvarez" <[email-masked]>
Date: Wed, 05 Mar 1997 09:26:29 -0800

> sense in saying "GRE tunnel running PIM". Am I right ?

	NO NO NO

	All "dvmrp unicast routing" does is propagate the dvmrp unicast
	table across the PIM cloud so PIM can use this information for
	its RPF check.  All these means is that the dvmrp unicast route
	takes precedence over the local unicast route table.

	So if you have:

	mrouted		cisco1		cisco2
	+----+		+----+		+----+
	|    +==========+    +==========+    +- e1
	+----+DVMRP tun +----+GRE tunnel+-+--+
					  | e0

	Lets say source 100.1.1.1 from the mbone is transmitting to 
	a group joined in e1.

	Since 100.1.1.1 is an external source cisco2 doesn't have
	a route for it and the default route points to e0.

	Now when cisco2 receives the traffic from 100.1.1.1 on
	the GRE tunnel it drops the packet because the RPF check
	fails since it thinks source 100.1.1.1 is reachable through
	e0.

	Now if we configure "dvmrp unicast" on both ends of the 
	GRE tunnel the dvmrp unicast route table is propagated to
	cisco2.  Now when packets from 100.1.1.1 arrive at cisco2
	on tun0 the RPF check matches the entry on the dvmrp unicast
	route and it forwards the packet.

	So the cisco's don't run DVMRP, they use the dvmrp unicast
	routes for RPF checks.

	Hope this clears things up a little.

dan

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From: "Jesus Maria Arango - Ing. Comunicaciones FSM LTDA." <[email-masked]>
Date: Wed, 5 Mar 1997 16:02:05 +0500 (GMT)

> 	mrouted		cisco1		cisco2

Can you configure "dvmrp unicast" on a fisical interface or is it only for
tunnels? 

- Jesus

---

From: Chris White <[email-masked]>
Date: Wed, 05 Mar 97 13:34:52 PST

> Can you configure "dvmrp unicast" on a fisical interface or is it only for

Yes "ip dvmrp-unicast" can be configured on physical interfaces. Please take
a look at the documents in ftp://ftpeng.cisco.com/ipmulticast.

-Chris

---

From: "John M. Zwiebel" <[email-masked]>
Date: Wed, 05 Mar 1997 15:28:56 -0800

Jesus:

The answers to your questions are well documented in the web page that we've
provided to you on a number of occassions.  If you don't want to take the
time to read how to deploy this feature, please be kind enough to the 
rest of the members of the MBONE to not deploy it.

Thank you

z

---

From: "Daniel A. Alvarez" <[email-masked]>
Date: Wed, 05 Mar 1997 15:30:59 -0800

> Can you configure "dvmrp unicast" on a fisical interface or is it only for

	It can be configured on physical interfaces as well.

dan