Fwd: More Multicast Understanding

---------- 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
Blogs and organic groups at http://www.ccie.net