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AW: Multicast Assert Messages in PIM-SM - Designated Forwarder? posted 05/21/2008
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Hi Sadiq,

Thanks for this really good explanation. The whole story is quite tricky and
some mechanism really depend on the topology. I've studied the "old good
Doyle" which has some really good explanations.

Also thanks to all the others who replied to my question.


-----Urspr|ngliche Nachricht-----
Von: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] Im Auftrag von
Sadiq Yakasai
Gesendet: Dienstag, 20. Mai 2008 19:29
An: Roger RPF
Cc: ccielab@xxxxxxxxxxxxxx
Betreff: Re: Multicast Assert Messages in PIM-SM - Designated Forwarder?

Hi Roger,

See my comments inline please:

> 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
> 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
> execute the PIM-join?

What is wrong with both routers sending PIM-Joins upstream towards the
RP? This would not be a problem as far as the Join msgs go. It would
only be an issue when the actual mcast traffic comes back in the
opposite direction to the PIM Joins, during which there would be an
Assert process and the winner would become the designated forwarder
(note the small letters and not Designated Forwarder which is a PIM
Bidir terminology).

> When in this process and how exactly starts the forwarder election through
> the assert messages?

The Assert process and election is started when a router forwards
mcast traffic on to a LAN segment for a particular group and it also
notices it is receiving the mcast traffic for the same group.

> 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.
> the traffic into the LAN but that's not the fact or do I mix again
> up?

If the same router is performing both functions, then it sends out the
PIM Join. If 2 different routers are doing each funtion (Querier and
DR) then both of them would send out PIM Joins upstream.

> Who starts the PIM join?

Anyone who receives an IGMP 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"

Check page 484 for a good explanation :)

> 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?

What if you are looking from the perspective of the source of the
mcast traffic?

> Sorry, but I get more confused :o)) Would be really happy if you can
> that in regards of IGMPv2 and PIM-SM.

I hope you will be less confused now :)


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