From: simon hart (simon@harttel.com)
Date: Sun Oct 16 2005 - 05:02:03 GMT-3
Hi all,
I think everyone has done an eloquent job in explaining the difference
between IGMP and PIM Joins, however I do not believe that this goes to the
heart of Ashok's question.
When a router's interface is enabled for either pim sparse or pim
sparse-dense mode (not sure about dense will have to lab it up). Then that
interface will immediately send out igmp joins to any listening multicast
router on that subnet, as well as sending out pim hello's.
Why does it send out IGMP joins. Well the router is in sparse mode and
therefore needs to know how to get to the Rendevouz point. The only way it
knows how to do this is to join the discovery and announcement groups of
224.0.1.40 and 224.0.1.39, thus it will send out an IGMP join.
The router that recieves this IGMP join from its PIM neighbor will send a
PIM Join to the Rendevouz point so that the initiating router can join the
group.
Remember that if the router is not next to the discovery agent, then
everything breaks in sparse mode as the initiating router does not know how
to get to the RP - this is the catch 22 that pim sparse-dense mode is
trying to overcome as is autorp listener
Hope that helps
Simon
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Andrew Lissitz (alissitz)
Sent: 16 October 2005 05:11
To: Jian Gu; Ashok M A
Cc: ccielab@groupstudy.com; C&S GroupStudy
Subject: RE: IGMP join & PIM join
Hey Group,
Jian says it well, igmp join messages are spoken by hosts. PIM messages
are sent by the PIM protocol (I know that sounds simple ...) which is
typically run only by routers. PIM is a control plane protocol used by
routers to set up and control multicast traffic.
When you configure an interface for multicast, you must configure the
PIM protocol. This command enables the protocol on the interface as
well as tells PIM how to behave on that interface; sparse, sparse-dense,
and dense mode. PIM is not smart enough to know how to behave, you are
telling it to try and operate as configured. Notice I said try ... If
you configured sparse-dense mode, but your router can not find the RP
... What will happen? It certainly will not be sparse mode, but will
fall back to dense mode ... Zzzis not good.
When a router receives a igmp join message on it's interface, it then
knows that it has a receiver requesting traffic for a group. What does
it do now with this knowledge? Well in the case of sparse mode, it
needs to tell the RP that it is interested in receiving traffic for this
group. It will do this by using the PIM protocol. In the case of dense
mode, it will be sure that it will NOT send a prune messages for the
requested group.
Ashok, I am not sure I answered your question. Are you also thinking of
config options, such as igmp join-group?
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jian Gu
Sent: Saturday, October 15, 2005 3:18 PM
To: Ashok M A
Cc: ccielab@groupstudy.com
Subject: Re: IGMP join & PIM join
They are very different, IGMP joins are from end host to last hop
router, when last hop router receives a IGMP join, it will initiate a
PIM join upstream to RP and create (*,G) entry locally, PIM joins are
between routers.
On 10/14/05, Ashok M A <ashok_ccie@yahoo.co.in> wrote:
>
> Hi GS,
>
> This might be silly question; but i can't explanin this well.
>
> What is the differnece between IGMP join and PIM join messages?
>
> Confusion arises as IGMP will be automatically enabled by configuring
> PIM sparse or dense or sparse-dense mode over an interface.
>
> TIA,
> Ashok
>
>
>
>
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