Subject: [pim] Differentiating PIM-SM from PIM-DM

From: "P.Jhingran" <[email-masked]>
Date: 4 Feb 2002 06:45:26 -0000

Hi all,
       I have a trivial doubt.What shall happen if PIM-SM and PIM-DM enabled interfaces are connected back to back.
PIM-SM is enabled on router R1's interface 1 while PIM-DM is enabled on interface 2.

        Router R1  <----------> Router R2
          PIM-SM                 PIM-DM
          Interface 1         Interface 2

Is cisco or any other router able to distinguish the above two modes?? The hello format is same for both the modes and now it is up the to administrator to make proper connections!!
regards,
Prashant

---

From: Paras Trivedi <[email-masked]>
Date: Mon, 4 Feb 2002 10:42:12 -0800 (PST)

Sparse or dense used to be a property of the interface in PIMv1.
But, in PIMv2, these days, the sparse or dense is a property of
a group or group range. So, the interface simply needs to run PIM,
regardless of any particular mode. (Although, in some exceptional 
cases it is needed to run an interface in strictly sparse or dense mode.)

Most implementations (at least JunOS and IOS) allow you to
turn on PIM, regardless of the mode, with "sparse-dense mode" command.

Yes, it'd be a misconfiguration to have sparse configuration on
one end of a link and dense, on another. And PIMv2 hello does not
carry this information. So, it cannot be detected automatically.
But, if you absolutely do this, then I guess, the sparse router
will do sparse thing (If DR, send register, send (*,G) join) 
and dense router will do dense thing (flood and expect to prune), for
any combination of source/receiver/transit situation, leading most
likely to chaos.

- Paras.

---

From: John Zwiebel <[email-masked]>
Date: Mon, 04 Feb 2002 12:31:09 -0800

Not exactly chaos.  It depends on the toplogy.

"ONCE UPON A TIME..."

one could configure a dense router and a sparse router
next to one another to transition between a sparse and
a dense domain -- where 'dense' also includes DVMRP areas.

Paras did hint at this when he said "exceptional cases".

The "sparseness" of a group is determined by whether or not
a router knows about an RP for a given multicast group.  If
it does, then the route is sparse.  If it doesn't the route
is dense.

FWIW: although it would appear at first glance that dense-mode
PIM is useful (especially now with "state-refresh"), in practice
it has not proven to be.  Part of the reason is that 
PIM-dense doesn't work well with the "expanding ring search",
partly because of the fact that some routers throw away 
packets with TTL of 1 or 0 if they aren't specifically addressed
to an IP address on that router, resulting in duplicate
packet delivery in some topologies...  And part of the
reason is that application developers don't understand
how PIM-dense works and so the result in certain toplogies
is unexpected.

Also, sparse-mode can "do" everything that dense-mode can and...

in any case, SSM and bi-dir PIM may prove to be much more
appropriate than any of the multicast routing protocols now
used.

2cents

Paras Trivedi wrote:
> _______________________________________________
> Pim-external mailing list
> [email-masked]
> http://mailist.procket.com/mailman/listinfo/pim-external