HP VPN Firewall Appliances Appendix Protocol Reference

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{ An intra-domain multicast routing protocol discovers multicast sources and builds multicast
distribution trees within an AS to deliver multicast data to receivers. Among a variety of mature
intra-domain multicast routing protocols, Protocol Independent Multicast (PIM) is most widely
used. Based on the forwarding mechanism, PIM has dense mode (often referred to as
"PIM-DM") and sparse mode (often referred to as "PIM-SM").
{ An inter-domain multicast routing protocol is used for delivery of multicast information between
two ASs. So far, mature solutions include Multicast Source Discovery Protocol (MSDP) and
Multicast Border Gateway Protocol (MBGP). MSDP propagates multicast source information
among different ASs. MBGP is an extension of the Multiprotocol Border Gateway Protocol
(MP-BGP) for exchanging multicast routing information among different ASs.
For the SSM model, multicast routes are not divided into intra-domain routes and inter-domain
routes. Because receivers know the position of the multicast source, channels established through
PIM-SM are sufficient for the transport of multicast information.
NOTE:
The firewalls do not support MBGP or IPv6 MBGP.
Multicast packet forwarding mechanism
In a multicast model, a multicast source sends information to the host group identified by the multicast
group address in the destination address field of IP multicast packets. To deliver multicast packets to
receivers located at different positions of the network, multicast routers on the forwarding paths usually
need to forward multicast packets that an incoming interface receives to multiple outgoing interfaces.
Compared with a unicast model, a multicast model is more complex in the following aspects:
• To ensure multicast packet transmission in the network, unicast routing tables or multicast routing
tables (for example, the MBGP routing table) specially provided for multicast must be used as
guidance for multicast forwarding.
• To process the same multicast information from different peers received on different interfaces of the
same device, every multicast packet undergoes a reverse path forwarding (RPF) check on the
incoming interface. The result of the RPF check determines whether the packet will be forwarded or
discarded. The RPF check mechanism is the basis for most multicast routing protocols to implement
multicast forwarding.
For more information about the RPF mechanism, see "RPF check mechanism" or "IPv6 m
ulticast
routing
and forwarding."