From: Andrew Lissitz \(alissitz\) (alissitz@cisco.com)
Date: Thu Nov 17 2005 - 16:02:01 GMT-3
Hey Drew, I hope all is well,
Pretty good questions ... One question relates to "how to maintain the
shared tree" and the other question "how to keep dense mode fallback
from occurring". Dense-mode fall back and the shared trees are not
related. In dense mode, there is not concept of RP or trees... Dense
mode sends the multicast traffic everywhere unless told otherwise.
Q1
Multicast will always try and build the most optimum path; shortest path
tree (SPT). To disable this you need to configure the spt-threshold to
be infinity. This command is a local command and will need to be
configured on every router that would otherwise attempt to build a SPT.
Q2
Lets say you have to configure sparse-dense mode on the interfaces. In
this mode, when a RP can not be found then dense mode would occur. This
is default behavior.
The command you list "no ip pim dm-fall" will work. Setting a RP of
last resort will also work (I need to practice this - never done it).
Putting all interfaces in sparse-mode only would work, except... it
would be very clever to write a lab requirement that says something
like: "Dense-mode can never occur, and the interfaces need to be
configured for sparse-dense"
Does this answer your questions Drew? I have a pretty good ppt I can
share as well if you want it. If anyone on this lists wants it, simply
unicast me and I will send it. It is the same one I previously sent
out.
andrew
-----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/hi
mc_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?
This archive was generated by hypermail 2.1.4 : Thu Dec 01 2005 - 09:12:07 GMT-3