From: Gregory Gombas (ggombas@gmail.com)
Date: Thu Nov 01 2007 - 12:28:59 ART
"if all of the interfaces in a VRF are configured with PIM sparse
mode, no dense mode fallback is achieved by default. If all of the
interfaces are already configured as sparse, even though the group
mode falls to dense mode, the traffic does not get flooded (due to the
sparse characteristic of the interface). But the established flows
might be torn down and the state of the network could become
indeterministic. The main advantage of no dense mode fallback in this
case would be deterministic behavior."
I'm not sure what they mean about deterministic in this case...any ideas?
Years ago if you were running auto-rp you needed to run sparse-dense
mode. Therefore "no dense mode fallback" command made sense, but today
with the ip pim autorp listener command you can run in pure sparse
mode so I'm not sure why its even needed.
Also whats really interesting is the no dense mode fallback command
came out in 12.3(4)T whereas the ip pim autorp listener came out in
12.2(7). You would think it would be the other way around.
Here is the link if interested:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801d1e18.html
On 11/1/07, Toh Soon, Lim <tohsoon28@gmail.com> wrote:
> Hi Con,
>
> I do agree with you. The "ip pim dm-fallback" command is kinda redundant in
> pure sparse mode environment. IMHO, without this command, if a group were to
> fallback to DM, it can't operate since all interfaces are in SM.
>
> I have tried configuring "no ip pim dm-fallback" in an SM environment. When
> I tried to enable it back, I received the error message "Can not have
> fallback mode as dense, all interfaces are in pim sparse mode".
>
> Are we going to see any differences in the outputs of "deb ip pim" with or
> without DM fallback?
>
> Further comments/opinions are most welcome.
>
>
> Thank you.
>
> B.Rgds,
> Lim TS
>
>
>
>
> On 10/31/07, Con Spathas <con@spathas.net> wrote:
> >
> > 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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