Priscilla,
You are correct. Thanks for the added insight.
Nigel
----- Original Message -----
From: "Priscilla Oppenheimer"
To:
Sent: Friday, February 01, 2002 2:03 PM
Subject: Re: multicast / CGMP towards the multicast server [7:33964]
> No offence, but that answer doesn't remove the quandary. The entire switch
> is a segment from the router's point of view. The router receives the IGMP
> Join and now knows that packets for that multicast group must be sent out
> that interface to that Ethernet segment. All devices on the switch are out
> that interface, however.
>
> What Fears fears is that the router won't be smart enough to tell the
> switch that not all devices connected to the switch should receive the
> multicast stream.
>
> But fear not, Fears. CGMP is smarter than you might think. Here's how I
> understand it. Correct me if I'm wrong, please (anyone).
>
> As you know, when a host wants to join an IP multicast group, it sends an
> IGMP Join message. The Join specifies the host's MAC address and the IP
> multicast group that it wants to join.
>
> When a router receives the IGMP Join, it creates a CGMP message that
> contains the MAC address of the host and the multicast group address. The
> router sends the CGMP message to a well-known address that all switches
> listen to. When a Catalyst switch receives the CGMP message from the
> router, the supervisor engine responds by modifying the forwarding table
> automatically. In other words, it now knows the specific port that must
> receive the multicast stream. Other hosts on different ports may Join
also,
> and the switch will add them to the table.
>
> This is different from IGMP Snooping, by the way. From what I understand,
> IGMP Snooping allows the switch to proactively snoop into IGMP packets and
> figure out which ones are Joins. IGMP Snooping requires more powerful (and
> more expensive) switching hardware (firmware).
>
> Priscilla
>
> At 10:18 PM 1/31/02, Nigel Taylor wrote:
> >Michael,
> > Of course this would depend on if the multicast server and
the
> >host connected on the same switch was assigned to the same vlan(broadcast
> >domain). Just some quick points to mention..
> >
> >Routers by default will not forward multicast traffic. However, if you
> >enabled a multicast routing protocol(PIM, DVMRP) then this is possible.
The
> >important thing here is that IGMP is used by hosts to inform routers of
> >their intent to become part of a multicast stream. This depends on your
> >implementation of the multicast protocol. IGMPv2 has been improved to
> >support leaves from a multicast group which is not supported in IGMPv1.
> >This way the host is able to notify the source of it's intent to leave
the
> >multicast group. This is will allow the routers to prune the multicast
> >traffic from the segment removing the unnecessary traffic, providing no
> >other host on the segment remains a member of the multicast stream
> >
> >A good title as recommended by a number of folks on the list is
Developing
> >IP Multicast Networks
> >Author: Beau Williamson. ISBN: 157870779
> >
> >HTH
> >
> >Nigel
> >
> >
> >
> >---- Original Message -----
> >From: "Fears Michael S SSgt 50 CS/SCBBN"
> >To:
> >Sent: Thursday, January 31, 2002 4:59 PM
> >Subject: multicast / CGMP towards the multicast server [7:33964]
> >
> >
> > > If a multicast server is connected to a Cisco Switch running CGMP, and
> > > several hosts are connected to the same switch, will a router turn off
> the
> > > switch ports for the users that are not requesting the multicast?
> > >
> > > So, will CGMP work back towards the multicast server?
> > >
> > > Fears
> ________________________
>
> Priscilla Oppenheimer
> http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=34159&t=33964
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