3com 3031 Instruccion De Instalación
PIM Overview
615
PIM-SM depends on the current unicast routing table to perform the RPF check
rather than an unicast routing protocol.
rather than an unicast routing protocol.
PIM-SM Working Principle
The operation of PIM-SM includes neighbor discovery, establishing the rendezvous
point tree (RPT), multicast source registration and SPT switching. Neighbor
discovery mechanism is the same as that of PIM-DM and will not be addressed
here.
point tree (RPT), multicast source registration and SPT switching. Neighbor
discovery mechanism is the same as that of PIM-DM and will not be addressed
here.
1 Establishing the Rendezvous Point Tree (RPT)
A leaf router locates the corresponding RP when it notices through IGMP packets
that a host directly connected to it joins in the multicast group G. It then sends the
join message to the upstream node towards the RP. Each router along the path
between the leaf routers and the RP generates a (*, G) entry in the forwarding
table, covering all packets sent to the multicast group G regardless of the sources.
When receiving a packet sent to multicast group G, the RP forwards it through the
established path to the leaf router and then to the host. In this way, an RP-rooted
tree (RPT) is built as shown in the following figure.
that a host directly connected to it joins in the multicast group G. It then sends the
join message to the upstream node towards the RP. Each router along the path
between the leaf routers and the RP generates a (*, G) entry in the forwarding
table, covering all packets sent to the multicast group G regardless of the sources.
When receiving a packet sent to multicast group G, the RP forwards it through the
established path to the leaf router and then to the host. In this way, an RP-rooted
tree (RPT) is built as shown in the following figure.
Figure 148 RPT diagram
2 Multicast source registration
The multicast source S sends a multicast packet to the multicast group G. When
the PIM-SM multicast router directly connected to S receives the packet, it will
encapsulate the packet into a register packet and send it to the corresponding RP
in unicast form. If there are multiple PIM-SM multicast routers on a network
segment, the Designated Router (DR) will be responsible to send the multicast
packet.
the PIM-SM multicast router directly connected to S receives the packet, it will
encapsulate the packet into a register packet and send it to the corresponding RP
in unicast form. If there are multiple PIM-SM multicast routers on a network
segment, the Designated Router (DR) will be responsible to send the multicast
packet.
3 SPT switching
When a multicast router detects that the multicast packet with the destination
address of G from the RP is sent at a rate greater than the threshold, the multicast
router will send a join message to the node of a higher level toward the source S,
which results in switching from the RPT to the SPT.
address of G from the RP is sent at a rate greater than the threshold, the multicast
router will send a join message to the node of a higher level toward the source S,
which results in switching from the RPT to the SPT.
Multicast Source S
RPT
join
Multicast source registration
RP
Receiver