Added note for anyone who may be interested:: The well-known CGMP multicast MAC address is: 0x0100,0cdd,dddd,
dj ""Nigel Taylor"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > 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=34220&t=33964 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
