From: Salau, Yemi (yemi.salau@siemens.com)
Date: Thu May 15 2008 - 07:55:14 ART
Just to add to Miguel's fine comments:
Cisco IOS Command Reference says:
"ip dm-fallback" is enabled by default on "sparse-mode" pim interface.
http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_04.htm
l#wp1013545
"Command 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."
The problem is, due to congestion or for some wierd reasons which is not
uncommon eg. due to service provider's issues etc., the PIM routers can
loose RP information about certain or all groups which can then result
in a lot of flooding of megabits or sometimes gigabits of traffic down
the multicast path tree. Experienced Multicast Specalists will tell how
serious this can be, hence why we do "no ip dm-fallback" on
"sparse-mode" with Cisco boxes. I'm not too sure this is reflected in
RFCs though, because I know we don't do same for Foundry and Nortel
boxes. I think it's definately a Cisco thing/way of doing things.
This is somehow linked to the reason why we don't do sparse-dense-mode
on the field anyways.
Many Thanks.
Yemi Salau
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Miguel Trejo
Sent: Thursday, May 15, 2008 2:19 AM
To: mac ccie
Cc: Cisco certification
Subject: Re: no ip pim dm-fallback
First thing is to understand that the interface PIM mode is independent
from
the group mode.
Given that and following your example if all interfaces are in
sparse-mode
and a a new group is found which has no RP the group will fall into
dense
mode without that command. I won't be forwarded out to any interface
since,
as you stated, all are in sparse-mode however the group will be kept in
the
mroute table with no output interfaces. Still there is difference in the
forwarding behavior but may be a difference on some cases.
On Sun, May 11, 2008 at 1:30 PM, mac ccie <mac2008.ccie@gmail.com>
wrote:
> forgot to mention the reason why did I ask this question , I see " no
ip
> pim
> dm-fallback" command included in Anycast example in Doc Cd which have
only
> sparse mode configured. Possibly an over configuration??
>
>
>
http://www.cisco.com/en/US/docs/ios/12_4/ip_mcast/configuration/guide/mc
bbasic.html#wp1137898
>
> Thanks,
> Mac.
>
> ---------- Forwarded message ----------
> From: mac ccie <mac2008.ccie@gmail.com>
> Date: May 11, 2008 11:48 PM
> Subject: no ip pim dm-fallback
> To: Cisco certification <ccielab@groupstudy.com>
>
>
> Hi GS,
>
> Is "no ip pim dm-fallback" related to sparse mode? I have only sparse
mode
> configurd in my network and I think this command is only relevant if
> sparse-dense mode is used. In which case all those groups not having
RP
> will
> use dense mode. With no ip pim dm-fallback these groups will not be
allowed
> to use DM.
>
> Any suggestions please?
>
>
> Thanks,
> Mac.
>
>
>
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