From: Scott Morris (swm@emanon.com)
Date: Thu Nov 17 2005 - 17:30:46 GMT-3
Two different pieces actually....
DM Fallback refers to the process when you are running sparse-dense mode and
an RP (or shared tree) cannot be found for the group you are looking for,
then dense mode will work. So in that case, there never is a shared tree to
keep traffic on.
In sparse mode once you hit the RP and shared tree, the spt-threshold will
keep things on that tree instead of finding the better source tree someplace
along the way.
HTH,
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Drew
Whitaker
Sent: Thursday, November 17, 2005 12:48 PM
To: Cisco certification
Subject: Preventing DM Fallback
This question for the group revolves around the idea of how to keep traffic
on a shared tree.
To disable dense mode fallback, I can type 'no ip dm-fallback' (thus keeping
traffic on a shared tree). Yet, in my reading, I have also come across the
command 'ip spim spt-threshold infinity' which, according to the Cisco
documentation, "causes all sources for the specified group to use the shared
tree." By using the shared tree, doesn't this also prevent DM fallback like
the 'no ip dm-fallback' command?
Also, in the "IP Multicasting Technology Overview" document (
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/himc_c
/
ch05/mcbcncpt.htm#wp1075142)
it says you can create a sink RP (RP of last resort) to ensure that no
groups (other than PIM v1 224.0.1.39 <http://224.0.1.39> and
224.0.1.40<http://224.0.1.40>groups) resort to a source tree. So wouldn't
this do it as well?
I am trying to understand the differences between these three methods. Are
these three different methods to prevent multicast traffic from using a
source tree?
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