And you probably also meant to say that the MAC header only has room for one data-link-layer address also. So the IP multicast address is converted to a single MAC multicast address.
Applications on end systems register with the NIC to receive packets addressed to particular multicast addresses. The application may also use the Internet Group Management Protocol (IGMP). IGMP allows a host to join a group and inform routers of the need to receive a multicast data stream. When a user (or software) starts a process that requires the host to join a multicast group, the host transmits an IGMP Membership Report message to inform routers on the segment that traffic for the group should be multicast to the host's segment. Although it is possible that a router is already sending data for the group, the IGMP specification states that a host should send a Membership Report in case it is the first member of the group on the network segment. The router does not need to know how many or which specific hosts on a segment belong to a group. The router just needs to recognize that a group has at least one member on a segment so that the router will forward group traffic to that segment using the IP and MAC multicast addresses for the group. By default, a data-link-layer switch floods multicast frames out every port. The Cisco Group Management Protocol (CGMP) and the IETF IGMP Snooping method allow switches to participate in the process of determining which segments have hosts in a particular multicast group. CGMP is a Cisco-proprietary method that lets a router send a message to switches to tell the switches about Membership Reports and Leaves occurring on their segments. IGMP is an IETF standard that causes no extra traffic, but allows a switch to learn from the IGMP messages sent to routers. In addition to determining which local network segments should receive traffic for particular multicast groups, a router must also learn how to route multicast traffic across an internetwork. Multicast routing protocols provide this function. Multicast routing protocols extend the capabilities of a standard routing protocol, which learns paths to destination networks, to include the capability of learning paths to multicast destination addresses. There are numerous multicast routing protocols, some of which are considered obsolescent at this time. The most commonly-used multicast routing protocol today is the Protocol-Independent Multicast (PIM) protocol. Just wanted to add to your excellent explanations. Priscilla At 05:10 PM 12/13/01, Karen E Young wrote: >Reding this over I realize that I should have explained a little better... > >What I should have said is "An IP header only has room for one destination >address, therefore a MAC must be manufactured for the group rather than a >specific device so that the layer-2 protocol (ethernet, token-ring, etc.) >can deliver to those routers/switches that belong to the group. The >routers/switches can then forward to those group members it has listed if >necessary. > >I should also have mentioned that this means that the NIC needs to be able >to recognize the MAC address associated with any multicast groups the device >belongs to. > >Just shows what happens when you try to do too many things at once.... > > > >Karen >*********** BEGIN FORWARDED MESSAGE *********** > >On 12/13/2001 at 3:27 PM Karen E Young wrote: > >Elmer, > >In fact I have done soem teaching, however, it was the months spent doing >phone-tech-support for an ISP that honed the explanation skills. Most of >our customers didn't know much about computers and felt alot more confident >doing what you tell then to do if you explain WHY in a manner that they can >understand. > >As far as the "you can't fit multiple destination MAC addresses into an IP >header"... I was just explaining why a special multicast MAC address is >required for messages sent to a specific Multicast group address. An IP >header only has room for one dest. MAC, therefore a MAC must be >manufactured for the group rather than a specific device. > >Glad I was able to help, > >Karen > >*********** REPLY SEPARATOR *********** > >On 12/13/2001 at 3:27 PM Elmer Deloso wrote: > > >(Corrected message for an earlier posting.) > >Karen, > >I have a feeling that you've been in some kind of teaching role > >before based on how you explain concepts. This makes the picture > >complete especially when revisiting the previous post by Shawn Kaminski. > >However, when you say "you can't fit multiple destination > >MAC addresses into an IP header" it seems you're referring to > >the device's mapping of the IP-to-MAC address, since there is no > >place in the IP header itself to even contain the value of the > >MAC address being used to frame the IP packet. At least that's what > >Doyle's book shows. If so, then I'm perfectly enlightened now. > >It's good to hear from you again. Thanks to all reponses. > > > >Elmer > > > >-----Original Message----- > >From: Karen E Young [mailto:[EMAIL PROTECTED]] > >Sent: Thursday, December 13, 2001 2:00 PM > >To: [EMAIL PROTECTED] > >Subject: RE: about multicast address! [7:29057] > > > > > >Elmer, > > > >Since an IP address needs to be mapped to a MAC address for delivery, a > >multicast frame needs a destination MAC address in the header. As a > >multicast frame is going to multiple destinations that are probably not > >known to the sender, a special MAC address needs to be used. After all, you > >can't fit multiple destination MAC addresses into an IP header. The base > >MAC > >address used for multicast is 0100.5E00.0000. In order to tailor the MAC > >address to the specific multicast group, the least significant 23 bits of > >the group address are mapped to the least significant 23 bits of the > >multicast MAC address. > > > >The first four bits of the IP address (1110) identify it as a class D > >address (multicast) and will never change. Therefore, they are not > >considered when figuring the IP to MAC conversion. This leaves 28 bits. > >Since you are copying only 23 bits to the MAC address, you are left with 5 > >bits in the address that don't get used in thed MAC. > > > >Multicast IP address: > >1110xxxx.xmmmmmmm.mmmmmmmm.mmmmmmmm > >x = un-used bits > >m = bits copied to the MAC address. > > > >Multicast MAC address: > >0000000100000000.0101111000mmmmmm.mmmmmmmmmmmmmmmm > >m = bits copied from IP address > > > >That help? > > > >Karen > > > >*********** REPLY SEPARATOR *********** > > > >On 12/13/2001 at 12:00 Pm Elmer Deloso wrote: > > > >>Richard, > >>This is an excellent post, but i need a little bit of clarification on... > >>1. I've understood multicast as at Layer 3, so I'm confused when you say > >> that a "25-bit prefix is assigned" for the Layer 2 frame. I can't seem > >>to > >> follow what is happening in multicast addressing between the Layers 2-3 > >> to arrive at this 25-bit prefix. I can't figure out where to place > >this > >> prefix bit setting while looking at the 802.3 frame format on Uyless > >>Black's > >> book on Data Link Protocols. > >>2. You state "there is a short fall of five bits and 2 to the 5 is 32". > >> What is this 2^5 referring to? > >>3. Finally, "are all allocated a MAC of 0100.5e01.0101." Please confirm... > >> is this the Destination MAC on the DA field of the frame? If so, what > >> happens when you have to pass this multicast stream of data from one > >> interface to another, e.g. from mBone -> r1 -> r2 -> multicast enabled > >> Intranet endstations, will the same "multicast MAC address" stay the > >>same? > >> > >>Thanks for your input. > >>Elmer Deloso > >>-----Original message----- > >>From: richard beddow [mailto:[EMAIL PROTECTED]] > >>Sent: Thursday, December 13, 2001 9:18 Am > >>To: [EMAIL PROTECTED] > >>Subject: RE: about multicast address! [7:29057] > >> > >> > >>An IP m'cast address is 32 bits long (as with any IP address), the first > >>for > >>bits are 0x1110 leaving 28 bits. (Still with me :)) > >> > >>Any m'cast ethernet borne frame has a 48 bit MAC (as do all ethernet > >>frames). A 25 bit prefix is assigned leaving 23 bits. > >> > >>As 28 won't go into 23 there must be some duplication, there is a short > >>fall > >>of five bits and 2 to the 5 is 32. Hence and one m'cast MAC represents > >32 > >>IP addresses. > >> > >>For instance > >> > >>224.1.1.1 > >>224.128.1.1 > >>225.1.1.1 > >>225.128.1.1 > >>etc > >>etc > >>238.1.1.1 > >>238.128.1.1 > >>239.1.1.1 > >>239.128.1.1 > >> > >>are all allocated a MAC of 0100.5e01.0101. > >> > >>Hope this is explained OK. > >> > >>RB > >> > >> > >> > >> > >>message Posted at: > >>http://www.groupstudy.com/form/read.php?f=7&i=29095&t=29057 > >>-------------------------------------------------- > >>FAQ, list archives, and subscription info: > >>http://www.groupstudy.com/list/cisco.html > >>Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] >*********** END FORWARDED MESSAGE *********** ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=29155&t=29057 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
