From: Scott Morris (swm@emanon.com)
Date: Sun Oct 16 2005 - 09:18:40 GMT-3
Actually that's not the case...
If you look at some of the detail in the messages that are going out, you'll
find that whenever you implement PIM on an interface, it sends out PIM
messages to discover any other PIM neighbors on that interface. You'll do
things like figure out who the PIM DR is for that interface.
It will send out IGMP messages to do the same thing, figure out who is going
to be in charge and be the querier. But I haven't ever seen it use IGMP to
try to join these groups unless you issue an "igmp join-group" command on an
interface. A router with "ip multicast-routing" enabled is inherently able
to do IGMP on multiaccess interfaces, this doesn't mean it sends out an igmp
join message though!
In dense mode (which is how AutoRP works by default) there is no joining or
leaving (granted, IGMP is a separate process), but internally on a non-MA
and non-RP, you won't even see the groups present in anything until traffic
is seen being flooded around via dense mechanisms. Then you'll see these
groups in the mroute cache. Up until that time, the router is pleasantly
ignorant and just sits there. No igmp joins for those groups, no nothing.
The autorp listener is done to prevent future networking disasters.
Everyone agrees that dense mode kinda sucks. The problem with pim
sparse-dense is that it says we TRY to use sparse, but if we don't know of
an RP, can't reach it, or don't get a good answer then dense is fine. So if
we have any multicast traffic going on either before we know of the RP or if
the RP doesn't respond in a timely fashion, the PIM router thinks dense mode
is cool for ANY group. So we would have the potential for many other dense
groups working on the network than we may intend, and there's no other way
to filter.
So autorp listener was created so that we could put the routers in sparse
mode only, where if an RP isn't available then NOTHING goes (viewed as a
cleaner solution than flooding). Yet, the listener command will override
this for ONLY the AutoRP groups. That still puts a non-MA and non-RP router
into a listening mode though, where if any discovery packets or
announcements aren't forthcoming, there's no igmp joining going on. At
least that's the packet traces I've conducted in 12.2 devices.
I can't think of a good reason why that would change. *shrug*
HTH,
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
simon hart
Sent: Sunday, October 16, 2005 4:02 AM
To: Andrew Lissitz (alissitz); Jian Gu; Ashok M A
Cc: ccielab@groupstudy.com; C&S GroupStudy
Subject: RE: IGMP join & PIM join
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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