From: Scott Morris (swm@emanon.com)
Date: Fri Jan 02 2004 - 23:06:36 GMT-3
The RP is the "know it all" of the multicast network. It prevents every
router from needing to maintain state for every single multicast group that
exists. In our lab networks, we generally could care less about this, but
in real life, this could get ugly!
Yes, an edge multicast router may decide to not go with the RP. But only
AFTER it joins the shared tree and starts to get multicast traffic. Only
then does it learn where the source actually is, and can examine its routing
table to determine whether it really is on the shortest path tree (SPT) or
not. If not, then it will create a source distribution tree directly to the
source.
So while yes it's true, it's not always the case. Just depends on where
source and receivers are located compared to the RP.
So specifically to your questions:
1. Only for the *,G tree and MAYBE for the S,G tree.
2. Only after seeing multicast traffic and IF it determines that the source
is closer to itself than the RP is.
HTH,
Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, CISSP,
JNCIS, et al.
IPExpert CCIE Program Manager
IPExpert Sr. Technical Instructor
swm@emanon.com/smorris@ipexpert.net
http://www.ipexpert.net
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Yasser Aly
Sent: Monday, December 29, 2003 6:39 PM
To: ccielab@groupstudy.com
Subject: Role of Rendezvouz point in multicast
Dear All,
I am confused about the role of RP in multicast sparse-mode. I thought
that a shared distribution tree will be created where sources register
themselves with the RP and receivers send request to RP.
However after reading the following lines from a PDF speaking about
configuring RP, I got confused.
Kindly have a look at the lines below and let me hear your feedback.
My questions mainly are:
1) Will multicast traffic pass by the PR all the time on its way from source
to receiver or not ?
2) Is it true that after the first hop router of the receiver learns about
the source, it will send a join message directly to the source, creating a
source-based distribution tree ??
Your thoughts are highly appreciated
---------
An RP acts as the meeting place for sources and receivers of multicast data.
In a PIM-SM network,
sources must send their traffic to the RP. This traffic is then forwarded to
receivers down a shared distribution tree. By default, when the first hop
router of the receiver learns about the source, it will send a join message
directly to the source, creating a source-based distribution tree from the
source to the receiver. This source tree does not include the RP unless the
RP is located within the shortest path between the source and receiver.
In most cases, the placement of the RP in the network is not a complex
decision. By default, the RP is needed only to start new sessions with
sources and receivers. Consequently, the RP experiences little overhead from
traffic flow or processing. In PIM-SM version 2, the RP requires less
processing than in PIM-SM version 1 because sources must only periodically
register with the RP to create state.
-----------------------
Regards,
Yasser
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