From: Roger RPF (rpf@bluemail.ch)
Date: Tue May 20 2008 - 08:40:19 ART
Hi Sadiq,
Thanks for your message...but I think not everything is clear to me :o(
If we have two routers on a LAN (and use IGMPv2 and PIM-SM), one of them
will be the active IGMP querier (router with the lowest IP address). Which
one of the two routers will send the PIM join towards the RP? The IGMP
querier or the other router? If it is not the IGMP querier, the other router
listens to the IGMP join from the host and start's the PIM join, isn't it?
If that's the case, how does the IGMP querier know that he does not have to
execute the PIM-join?
When in this process and how exactly starts the forwarder election through
the assert messages?
So far I was always thinking that the IGMP querier will also initiate the
PIM-join towards the RP and also be the designated forwarder, i.e. forwards
the traffic into the LAN but that's not the fact or do I mix again something
up?
Who starts the PIM join?
In Jeff Doyle Vol2 Page 410 he says:" The querier is responsible for
forwarding the multicast stream. The other router or routers only listen,
and they begin forwarding the stream only if the querier fails"
I do also not understand what you mean with
"While the Designated Router is responsible for delivering mcast traffic
**FROM** a LAN segment"
I mean, we do not send multicast traffic off the LAN, it's only a "one way
direction" into the LAN, isn't it?
Sorry, but I get more confused :o)) Would be really happy if you can clarify
that in regards of IGMPv2 and PIM-SM.
Thank you
Roger
-----Urspr|ngliche Nachricht-----
Von: Sadiq Yakasai [mailto:sadiqtanko@gmail.com]
Gesendet: Dienstag, 20. Mai 2008 12:05
An: Roger RPF
Cc: ccielab@groupstudy.com
Betreff: Re: Multicast Assert Messages in PIM-SM - Designated Forwarder?
HI Roger,
I think you are mixing up two different roles and direction as far as
MCAST traffic forwarding goes:
The Designated Forwarder is responsible for delivering MCAST traffic
from the source (or RP) **TO** the LAN segment (having more than one
entry/exit point) where the receivers of the traffic are located. The
Assert message is used to elect the router to be used as a DF (higher
IP address wins).
While the Designated Router is responsible for delivering mcast
traffic **FROM** a LAN segment (whether there is only one or multiple
exit points). You are right in that in IGMPv1, the DR is the same as
the Querier automatically.
While in IGMPv2, a DR could be different from the Querier. In this
case, the DR is elected based on the highest IP adress while the
Querier election is based on the Lower IP address.
Hope that clears up the air a little bit more.
Sadiq
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