Re: BSR and dm-fallback

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 - 09:23:36 ART