HP-UX Routing Services Administrator's Guide HP-UX 11i v2, HP-UX 11i v3 (B2355-91153, November 2011)

You can use IP multicast addresses only as destination addresses, and they must never appear in
the source address field of a datagram. Internet Control Message Protocol (ICMP) error messages
are not generated for multicast datagrams.
Because IP Internet addressing is a software manifestation of the underlying physical network, you
must map IP addresses to physical addresses that the hardware comprising the network understands.
Normally, IP multicast addresses are mapped to 802.3 or Ethernet multicast addresses. The IP
multicasting addressing scheme, similar to Ethernet’s scheme, uses the datagram’s destination
address to indicate multicast delivery.
When an IP multicast address is mapped to an Ethernet multicast address, the low-order 23 bits
of the IP multicast address are placed into the low-order 23 bits of the special Ethernet multicast
address. The hexadecimal value of the special Ethernet multicast address is 01-00-5E-00-00-00.
The resultant Ethernet address, however, is not unique, because only 23 out of the 28 bits
representing the multicast address are used.
Multicast Groups
A multicast group comprises hosts with an intention to join the multicast group by listening to the
same IP multicast address. Group membership is dynamic, that is, a host may join or leave a group
at any time. A host may be a member of one or more groups simultaneously. Additionally, a host
is allowed to send multicast datagrams to a group without being a member of the group.
You can assign multicast addresses to transient groups because the multicast address are often
temporary. A typical transient group scenario is when users run an application that dynamically
registers to specific multicast addresses, which are discarded later when all members of the group
have left. Some multicast addresses may be assigned to permanent groups that always exist, even
when their membership is empty.
Both hosts and mrouted routers that participate in IP multicasting use the Internet Group
Management Protocol (IGMP) to communicate multicast group information among themselves.
Hosts use IGMP to inform mrouted routers that they are joining a group. mrouted routers use
IGMP to pass multicast routing information to other mrouted routers, and to check whether a host
is still an active group member.
The underlying TCP/IP stack must support ICMP to participate in IP multicasting. While IGMP
defines a standard for communicating information, it does not define a standard for how the
multicast information is propagated among multicast routers. Consequently, DVMRP enables
multicast routers to efficiently communicate group membership information among themselves.
DVMRP uses IGMP messages to carry routing and group membership information. DVMRP also
defines IGMP message types that enable hosts to join and leave multicast groups, and that allow
multicast routers to query one another for routing information.
The gated Routing Daemon
gated (pronounced “gate D”) is a routing daemon that updates routing tables in internetwork
routers. Developed at Cornell University, gated handles the Routing Information Protocol (RIP),
External Gateway Protocol (EGP), Border Gateway Protocol (BGP), Open Shortest Path First (OSPF)
routing protocol, and the Router Discovery Protocol (RDP), or any combination of these protocols.
Routing protocols are designed to find a path between network nodes. If multiple paths exist for
a given protocol, the shorter paths are usually chosen. Each protocol has a cost or a metric that it
applies to each path. In most cases, the lower the cost or metric for a given path, the more likely
a protocol will choose it.
NOTE: You cannot use the HP System Management Homepage (HP SMH) to configure gated.
Upon startup, gated reads the kernel routing table on the local machine. gated maintains a
complete routing table in the user space, and keeps the kernel routing table (in the kernel space)
synchronized with this table.
10 Overview