At 6:36 PM -0400 6/27/02, Priscilla Oppenheimer wrote: >At 04:42 PM 6/27/02, Lopez, Robert wrote: >>At what OSI layer do IP multicasts lie? Reading through CCO has made me >>more doubtful in my choices. > >IP multicasts are sent to a layer 3 IP multicast address. That address is >converted to a data-link-layer multicast address. The Internet Assigned >Numbers Authority (IANA) owns a block of MAC-layer addresses that are used >for group multicast addresses. The range of addresses for Ethernet is >0x01:00:5E:00:00:00 through 0x01:00:5E:7F:FF:FF. When a host sends a frame >to an IP group that is identified by a Class D address, the host inserts >the low-order 23 bits of the Class D address into the low-order 23 bits of >the MAC-layer destination address. The top 9 bits of the Class D address >are not used. The top 25 bits of the MAC address are 0x01:00:5E followed by >a zero bit (00000001 00000000 01011110 0 in binary). > >IP multicast gets used for many purposes and those purposes may be at >different layers: > >Sending routing updates (EIGRP, OSPF, RIPv2) - Layer 3 >Establishing routing protocol neighbor relationships (EIGRP, OSPF) - Layer 3 >Sending multimedia streaming audio or video - Layer 7 with some help from >Layer 6 (MPEG or whatever), Layer 5 (RTSP), and Layer 4 (UDP) >Finding services (Service Location Protocol) - Layer 7 >Joining groups (IGMP) - Layer 3 >Determining a dynamic L3 address assignment (IPv6) - Layer 3
You're not saying, are you, that IP multicast exists at layers above 3, are you? I think it is correct to say that a higher-layer protocol may assume that a lower-layer protocol will require use of a layer 3 multicast service, but doesn't itself implement multicast. The upper layer entity (in strict OSI terms) need not have direct access to the multicast network layer service, but potentially could indirectly request that functionality through higher-layer service interfaces. Without looking at the Transport Service Specification, I can't remember if it has the semantics, with the Connectionless Transport Service, of multicasts. My general recollection is that you use a network service address and let the Network Service figure out the semantics. > >There's probably lots of others too! > >Layer 2 multicasts are used for IP multicast, but for many other purposes >too, such as BPDU, CDP, VTP, DISL, AppleTalk Name Binding Protocol (NBP) >lookups, etc. > >Priscilla > > >>TIA >> >>Robert >________________________ > >Priscilla Oppenheimer >http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=47624&t=47591 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
