Priscilla,
You're correct in that Fears' real fear at this point has not
been answered. ;-> In doing some quick research, I found that as you
mentioned IGMP(costly) and CGMP(a less costly solution) would assist in
providing one the ability to control multicast flooding. This is what I
found...
Even in an design where the host and the server resides on the same
VLAN(segment) IGMP and CGMP still provides the ability to control flooding
of multicast traffic. Specifically, when the host multicasts the IGMP
membership report to the group with the address 224.1.2.3(MAC
0x0100.5E01.0203) and there's no existing mapping in it's CAM table, the
switch will flood the report on all ports in the VLAN. However, any futher
attempts to join that existing group would then be limited to port listed in
the CAM table that are eligible to recieve the multicast traffic for the
group. Chptr 14, pgs 412-442 of Beau Williamson's book Developing IP
Multicast Network provides some really good info on this issue. The author
does note that flat switched LAN designs will present major problems in
gaining/maintaining control of multicast flooding.
I guess this really comes down to the network design as with every other
aspect of building a scalable and efficient network.
Thoughts.. Anyone!
Nigel
> At 09:28 PM 2/1/02, Nigel Taylor wrote:
> >Priscilla,
> > You are correct. Thanks for the added insight.
> >
> >Nigel
>
> You are nice to say this, but you know what I realized?! My answer doesn't
> resolve the quandary either! ;-)
>
> I now think that Fears' real fears had to do with the recipients and the
> server being on the same VLAN. This might cause the switch to forward the
> multicast traffic before it even checks the results of CGMP. The switch
may
> do its default multicast flooding to ports in a VLAN and just make use of
> CGMP to learn about other ports. Am I making any sense? It's late. ;-)
>
> My guess it that the answer is still that CGMP is smart. Once you
configure
> it, the switch knows to not do its normal multicast flooding and instead
> wait to hear from the router regarding which ports should receive the
> multicast flow. Hopefully someone can confirm that.
>
> Priscilla
>
>
> >----- 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
> ________________________
>
> Priscilla Oppenheimer
> http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=34188&t=33964
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