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 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=47601&t=47591 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
