From: keith tokash (ktokash@hotmail.com)
Date: Thu Feb 14 2008 - 00:21:45 ARST
"The no ip pim dm-fallback command only prevents interfaces configured
as pim sparse-dense mode from becoming dense interfaces if RP
configuration is lost."
This is the only time you care. If a RP exists for a group then it is treated
as sparse. By telling the router not to fall back to dense mode you are
enforcing a policy that says you would rather have that multicast stream die
in a horrible flaming bit bucket than spray it out in dense mode.
I have had the same confusing experience reading multicast articles, so I'll
throw my understanding out here for harpooning. If you configure the
interface for sparse-mode only, then you will never send dense mode, with a
single exception. The exception is if you configure auto-rp listener, which
will forward the two Auto-RP groups in dense mode, but no others ever. I
believe the point of the "no dm-fallback" command is to allow a sparse-only
environment without making everyone change their interface configurations.
Sounds a bit odd, but Cisco wouldn't have done it if customers (or one really
big one) didn't yell for it.
So to avoid dense mode 100%
1. Use sparse mode on interfaces
2. Don't use Auto-RP
3. If you have a sparse-dense interface, use no dm-fallback
Again, if any of this is wrong, someone can pounce right about ... now.
With a few exceptions, secrecy is deeply incompatible with democracy and with
science.
--Carl Sagan
> Date: Mon, 11 Feb 2008 15:11:58 -0800
> From: kvapzarov@yahoo.com
> Subject: How to Prevent Dense Mode Fallback
> To: ccielab@groupstudy.com
>
> Is configuring all multicast interfaces as pim sparse-mode enough to prevent
the router from falling back to dense mode? I read ten different articles on
just this topic and the more I read the more confused I get. I have the
feeling that some of the authors are confused themselves too. The Cisco
on-line documentation is also unclear. Once it says that by configuring BSR
we absolutely prevent falling back to dense mode another time they state just
the opposite. The no ip pim dm-fallback command only prevents interfaces
configured as pim sparse-dense mode from becoming dense interfaces if RP
configuration is lost.
> I guess my question is if the task is to ensure that no DM fallback occurs
what configuration will fulfill the task?
>
>
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