From: Con Spathas (con@spathas.net)
Date: Wed Oct 31 2007 - 08:28:44 ART
To be honest I haven't really tested it either - however I could see it used
in an environment as follows:
DM used for selected groups and then SM for all other groups (ip pim
sparse-dense mode).
Then should there be a failure with any of the RPs, Auto-RP or BSR
responsible for the SM traffic - this traffic then won't be able to drop
back into DM and ruin your networks day.
I am of the same opinion at the minute as you are that this command appears
to be redundant in a purely SM environment aside from perhaps serving as a
peace-of-mind safety net.
From what I've read this command came about from an old workaround whereby
each router would have the RP statically configured pointing itself just in
case there was an AutoRP failure.
I'd be interesting in reading others comments as well.
Cheers!
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
nicky noname
Sent: Wednesday, 31 October 2007 11:00
To: Cisco certification
Subject: no ip pim dm-fallback
Hello!
I have come across this alot in the documentation ( also on the sample
configs with sparse-mode) and it seems to stress that you must configure
this command to prevent multicast groups from defaulting to dense
mode........regardless of interface configuration and in absense of RP.
Now, this is implying that if I configure "ip pim sparse mode" in the
interface and I DO NOT have "no ip pim dm-fallback" configured.......that if
someone tries to multicast for an particular group that is undefined by an
RP, it will revert to dense mode on the network.
I don't believe this but!!.....I will test today!
some experienced comments...have you tested this. If it is the case, then
you must always put this command in when you want no dense
mode..........even if you have sparse only configured!.....doesn't seem
right thanks Nic
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