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=34099&t=33964 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
