Operation Manual – Multicast
H3C S3600 Series Ethernet Switches-Release 1510 Chapter 1 Multicast Overview
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II. Multicast routing protocols
A multicast routing protocol operates between multicast routers to establish and
maintain multicast routes and forward multicast packets accurately and effectively. A
multicast route establishes a loop-free data transport path from a data source to
multiple receivers. The task of multicast routing protocol is to establish a distribution
tree structure. Multicast routers can establish the data transmission path (namely,
distribution tree) in many ways.
Like unicast routes, multicast routes come in intra-domain routes and inter-domain
routes. Intra-domain multicast routes are quite mature now. Protocol independent
multicast (PIM) is the most commonly used protocol currently. It can cooperate with any
unicast routing protocol.
1.3 Forwarding Mechanism of Multicast Packets
In a multicast model, a multicast source host transports information to the host group,
which is identified by the multicast group address in the destination address field of an
IP data packet. Unlike a unicast model, a multicast model must forward data packets to
multiple external interfaces so that all receiver sites can receive the packets. Therefore
the forwarding process of multicast is more complicated than that of unicast.
In order to guarantee the transmission of multicast packets in the network, multicast
packets must be forwarded based on unicast routing tables or those specially provided
to multicast (such as an MBGP multicast routing table). In addition, to prevent the
interfaces from receiving the same information from different peers, routers must check
the receiving interfaces. This check mechanism is reverse path forwarding (RPF) check,
which is the basis of performing multicast forwarding for most multicast routing
protocols.
Based on source addresses, multicast routers judge whether multicast packets come
from specified interfaces; that is, RPF check determines whether inbound interfaces
are correct by comparing the interfaces that the packets reach with the interfaces that
the packets should reach. If the router resides on a shortest path tree (SPT), the
interface that multicast packets should reach points to the multicast source. If the router
resides on a rendezvous point tree (RPT), the interface that multicast packets should
reach points to the rendezvous point (RP). When multicast data packets reach the
router, if RPF check passes, the router forwards the data packets based on multicast
forwarding entries; otherwise, the data packets are dropped.
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