Re: More Multicast Understanding

Karim,

The DR and DF work in different direction. DF is responsible for sending
joins up to the RP and thus forwarding traffic onto the LAN. The DR is
responsible for registering with the RP when a new source is heard. Consider
this topology:

R1-----LAN1-----R2/R3-----LAN2-----R4

R2 and R3 are connected to LAN1 with R1, and LAN2 with R4.
R1 is the RP.
If R4 is a client, then the winner of the ASSERT process between R2 and R3
on LAN2, sends joins up to R1.
If R4 is a server, then the DR on LAN2 sends REGISTERS to R1.

-hth

On Thu, Oct 15, 2009 at 7:50 AM, karim jamali <karim.jamali_at_gmail.com>wrote:

> ---------- Forwarded message ----------
> From: karim jamali <karim.jamali_at_gmail.com>
> Date: Wed, Oct 14, 2009 at 10:04 AM
> Subject: Fwd: More Multicast Understanding
> To: Narbik Kocharians <narbikk_at_gmail.com>, smorris_at_ine.com, Anthony
> Sequeira
> <asequeira_at_internetworkexpert.com>, petr_at_internetworkexpert.com
>
>
>
>
> ---------- Forwarded message ----------
> From: karim jamali <karim.jamali_at_gmail.com>
> Date: Wed, Oct 14, 2009 at 10:04 AM
> Subject: More Multicast Understanding
> To: Cisco certification <ccielab_at_groupstudy.com>
>
>
> Dear Experts,
>
> I have a weird issue which I am not getting in multicast, especially which
> router will forward to the segment in case of two routers for instance. I
> have sent an enquiry on groupstudy and got a great article stating that the
> forwarder on every subnet will be elected based on :
>
> 1)Best Administrative Distance to the source
> 2)Best Metric
> 3)Highest IP address of the two competing routers.
>
> This is A PIM SPARSE MODE SCENARIO
>
> This made perfect sense to me and I understood how forwarders were elected
> on a shared segment, until I got shocked with the fact that seemed very
> strange to me. It seems that the designated router on a segment is what
> determines who will forward on the common subnet. In the attached file, I
> have a streamer,2 backbones devices and a common subnet. On the basis of
> election, it was clear to me that the highest IP address (BB2) will win the
> election and be the forwarder, and by default it was.
>
> show ip igmp int command shows me two things : Who is the Querier on the
> subnet and who is the Designated Router, there is no mention for the
> forwarding router on th segment. Can someone please explain this?
>
> The thing that completely shocked me that wheneve I use the command ip pim
> dr-priority <255> for example on the Querier, it also becomes the
> Designated
> router and it is the one that forwards the multcast traffic on the subnet,
> without any respect to who is the forwarder and how it it elected(3 steps
> above).. Please can someone clarify for me the role of the designated
> router,is it the forwarder on the subnet?
>
> As far as I remember, the DR is the router which gets the stream,and
> notifies the Rendezvous Point (RP) that a stream exists by sending a
> register message. But here I see it affecting how traffic is forwarded on
> the client side.
>
> I would truly be glad if someone can sort this confusion out!
>
> Best Regards,
>
> Karim
>
>
>
>
>
> --
> KJ
>
>
>
> --
> KJ
>
>
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-- 
Bryan Bartik
CCIE #23707 (R&S), CCNP
Sr. Support Engineer - IPexpert, Inc.
URL: http://www.IPexpert.com
Blogs and organic groups at http://www.ccie.net