Re: igmp querier , assert winner and DR confusion on election

Hi there

u have 3 phases of elections:

1-Querier elections, who will handle the communication between a group of
hosts and a group of routers on the same segment
2-Designated Router Elections, who will handle the quey message (Multicast
signalling)
3-Desingated Forwarder, who will handle Multicast traffic (video stream for
example)

First the Querier Election Process.
At start up each router sends a query message to the all systems group
224.0.0.1 from its own interface address. The router with the lowest ip
address is elected IGMP querier.

Second the PIM DR Election Process
The router with the highest ip address is elected as PIM DR. This selection
process can also be influenced by configuring a pim DR priority. By default
all routers have priority 1, hence highest ip address wins by default.
However if DR priority is used then highest DR priority wins.

The show igmp interface command can be used to show elected DR and querier

*Third the Forwarder Election Process ( u will need to use Assert messaging
as an election mechanism)*
These Assert Messages are used to elect the PIM forwarder using the
following three rules:

   1. The router generating an Assert with the lowest Administrative
   distance is elected the forwarder. The AD would only differ if the routes
   to R5 where from different routing protocols. If the Administrative
   Distances are the same then we move to step 2.
   2. The best unicast routing metric is used to break a tie if the
   Administrative Distances are the same. The combination of AD and the
   unicast routing metric is referred to as a tuple. If metrics are the
same
   them we move on to step 3.
   3. The device with the highest IP Address will be elected as the PIM
   Forwarder.

*Note:*u can make the three routers the same router with some manipulation
in priority or IGP, but they are NOT the same functions, its administrative
issue to be the same router or not

BR/Mark

On Thu, Dec 9, 2010 at 12:32 AM, ehtesham ali
<conect2ehtesham_at_gmail.com>wrote:

> thanks Babola ,
> sure experts will help ! eagerly waiting for their comments ! just to
> point
> out that DR is ONLY used for Registration and sending IGMP join.
>
> Being DR doesn't mean that it will always be forwarder .
>
> Regards
> On Wed, Dec 8, 2010 at 11:29 PM, Bobola Oke <okebobola_at_gmail.com> wrote:
>
> > I highly recommend you study TCP/IP routing vol2. Deff Doyle does a
> > fantastic job of explaining these clearly.
> >
> > For assert elections, the router with the lowest cost to the source
> always
> > wins.
> >
> > The designated router is the forwarder. (I'm not sure on this... I think
> I
> > need to refer to the book myself :D). The IGMP querier checks if there
> are
> > hosts that want to receive multicast traffic. The forwarder does the
> > forwarding :D
> >
> > If all the cost to reach the source were to be the same. R3 would be the
> > designated router (forwarder) and R1 would be the querier. I think the
> rule
> > changes if IGMP version 1 and PIM v1 is being used.
> >
> > I'm sure the experts can help further on this! Just trying to do my
> bit...
> >
> >
> >
> >
> > On Wed, Dec 8, 2010 at 4:54 PM, imran ali <immrccie_at_gmail.com> wrote:
> >
> > > hi group ,
> > >
> > > R1 , R2 , R3 are connected on a multiaccess segment sharing ip address
> > > from
> > > 10.1.123.X /24 subnet where X is the router no .
> > >
> > > now when it comes to cost R2 is having lowest cost to reach the SOURCE
> .
> > >
> > > All the router has equal cost to Rp. igmp ver is 2
> > >
> > > question :
> > >
> > > 1) who will be elected as assert forwarder ? DR ? Igmp Querier ?
> > >
> > > 1) I believe all three roles are seprate or is there any overlap of
> > roles?
> > >
> > > i have read some where that igmp querier is the one who forwards the
> > > traffic on LAN but in my case R2 should be
> > > the Mcast data forwarder .
> > >
> > > Regards
> > >
> > >
> > > Blogs and organic groups at http://www.ccie.net
> > >
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> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
> > --
> > Bobola Oke
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
> > _______________________________________________________________________
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>
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Received on Fri Dec 10 2010 - 22:32:25 ART