Subject: How to run with a minmal DVMRP table using IOS

From: Arlie Davis <[email-masked]>
Date: Tue, 18 Feb 1997 17:04:33 -0500

I've gotten so many messages about running with a minimal DVMRP table
that I wrote up a quick page on how to do it, with most of my
configuration.

The URL is http://tis-www.thepoint.net/minimal-dvmrp.htm .  If you have
any comments on this / suggested changes, let me know.

-- arlie

---

From: Toerless Eckert <[email-masked]>
Date: Wed, 19 Feb 1997 00:26:11 +0100 (MET)

> any comments on this / suggested changes, let me know.

There are some obstacles as to your URL's description.

1. I don't know why i would want to have 

   "tunnel sequence-datagrams"

   because even though it's a tunnel it's still datagram service and as
   such misordering is a normal fact of life but still better than
   packet loss.

2. I also don't know why i should set

   "no ip mroute-cache"

   True, there have been a lot of problems with mroute-cache over the years,
   but i'm not aware of any problem still surviving on this battlefield.
   Even if there was one, i'd rather quickly report it. At least i don't
   know how to run my routers without "mroute-cache", they'd simply overload.
   (I am not quite sure about the speedups on tunnels though, meaning that
    i've never willingly disabled it there to test).

3. Providing only a dvmrp-default route to some stub-area in the mbone
   is basically a nice thing that the cisco can do. Nevertheless you
   should be aware, that not all multicast-software is able to run correct
   multicasting with only a defaul-route supplied. I know definitely
   that it is not supported in release 11 of Bay Networks dvmrp software
   and i am quite persuaded that it won't work with current 3com  software
   either (meaning: with 3com i still don't know a working support channel
   to get real answers to technical questions). (no, please don't start
   a war over this ;-). Nevertheless: it seems to work for all of my
   tunnels going down to other ciscos (what a surprise) and for mrouted3.8
   downstream. I guess that's the majority of mbone routers.

4. To make life much more easier, i always recommend not only to use
   an

   "ip unnumbered <if>" 

   command, but also to use the same address as the tunnel source address
   of all tunnels. This way, you've got the same simple config as with
   mrouted, at least when it comes to the ip-addresses locally being used
   for multicasting, i.e.:

   tunnel source <if>

5. I don't think that it is such a great idea to go for dense-mode PIM
   with some DVMRP unicast-routing for your local area network. Just for this
   crap to go away, PIM was made. In my experience the most painless
   thing with IOS multicasting was always sparse-mode PIM in my local
   network. I could just configure it everywhere and never had to worry
   about any useless multicast traffic or dvmrp routing loops. The
   worst thing that can happen there, as to my experience is that
   multicast traffic does not arrive if something is misconfigured.

6. One should clearly make a distinction between a full-mbone router
   and a stub router. In your configuration it seemed to me as
   if the one router carrying the full mbone routing table gotan

   "ip mroute 0.0.0.0 0.0.0.0"

   into his routing tables too. I would not recommend this. Either you carry
   the full mbone-dvmrp routing tables, then you should not have a 0.0.0.0
   multicast default route, or you do not, but then you should get the
   dvmrp-default route through dvmrp, and not through a statically
   configured "mroute".

7. If you start out using a multicast-default route, you should always
   very carefully check that RPF is working as you like it. If you do
   not have a full mbone-dvmrp routing table and you have something like:

   ip route  0.0.0.0 0.0.0.0 188.1.13.9 10
   ip mroute 0.0.0.0 0.0.0.0 tunnel     1

   cisco% show ip rpf 123.123.0.0 ! something not in your routing table

   will still show toward the unicast route, so basically:

   a "mroute" command will work only if the same network is in the
   unicast-routing table, with the exception of a 0.0.0.0 route where
   it will only work as a dvmrp route (to my experience).

Best regards
	Toerless

-- 
Toerless Eckert
Universitaet Erlangen Nuernberg
RRZE / IMMD4
Martensstrasse 1
D-91058 Erlangen, Germany
Tel.: +49 9131 85 7278
FAX: +49 9131 85 8732
[email-masked]
/C=De/A=D400/P=Uni-Erlangen/OU=Informatik/S=Eckert/G=Toerless/