Re: BSR and dm-fallback

Thanks a lot for providing such a detailled reply.

2009/4/29 Mohammed M Moustafa <mmma_at_gawab.com>

> Hi Mike,
>
> Totally agree with you, Cisco documentation regarding this point is a
> little confusing.
>
> I was in this dilemma before, your understanding is correct (IMHO), as long
> as you are not using sparse-dense interfaces thus the "no ip pim
> dm-fallback" command is of no need (but please read the email to its end),
> try labing it out, moreover to practically prove this, when I tried
> disabling and enabling this command on a router with all interfaces in
> sparse mode I get the following (the router is a Cisco 3640 running the
> 12.3(14)T7 code - using dynamips):
>
> Rack1R3(config)#ip pim dm-fallback
> Can not have fallback mode as dense, all interfaces are in pim sparse mode
>
> Moreover in the "PIM Dense Mode Fallback Prevention in a Network Following
> RP Information Loss" document:
>
> http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gtautorp.html#wp1045491
>
> I quote "By default, if all interfaces in a multicast VPN routing or
> forwarding instance are configured with the ip pim sparse-mode command,
> there is no dense mode fallback because dense mode groups cannot be formed
> over interfaces configured for sparse mode."
>
> Although within the Cisco documentation command reference for this command:
>
> http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_04.html#wp1013545
>
> I quote "PIM dense mode fallback is enabled. That is, a multicast group in
> the absence of rendezvous point (RP) information will fall to dense mode,
> regardless of the interface mode configuration."
>
> My opinion is that the answer lies in this paragraph from the first
> document:
>
> http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gtautorp.html#wp1050843
>
> "By default, PIM dense mode fallback is enabled. That is, a multicast group
> in the absence of rendezvous point (RP) information will fall to dense
> mode, regardless of the interface mode configuration.
>
> However, 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. "
>
> To recap, this means that with all interfaces in sparse mode, although the
> group mode will fall to dense mode, the traffic can't be forwarded in dense
> mode over the sparse mode interfaces, and thus there will be no traffic
> flow, and this might introduce indeterministic behavior and here comes the
> benefit of this command in this case.
>
> Practical labing does the proof of concept for you, so stick with it, and
> try reading more than one document and verifying their contents.
>
> Have a nice day.
>
> BR,
> Mohammed Mahmoud.
>
>
>
> Mike Leske <mike.leske_at_googlemail.com> wrote on 28 Apr 2009, 03:17 PM:
> Subject: BSR and dm-fallback
> >Hi group,
> >
> >I have a question regarding the command "no ip pim dm-fallback".
> >My understanding is, that this one can be used on interfaces configured in
> >"ip pim sparse-dense-mode" to prevent the interface to fall back to
> >dense-mode operation if a RP cannot be found.
> >
> >Further, the IP Multicast configuration Guide says: "A BSR performs
> >similarly as an RP, except that it does not run the risk of reverting to
> >dense mode operation, and it does not offer the ability to scope within a
> >domain." (page 55)
> >
> >However, section "Configuring Sparse Mode with a Bootstrap Router" on page
> >80/81 also has the command "no ip pim dm-fallback" included. Do we really
> >need that command when running BSR over sparse-mode?
> >
> >Thanks
> >Mike
> >
> >
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Received on Wed Apr 29 2009 - 22:00:38 ART