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DES-7200 Configuration Guide Chapter 7 PIM-SMv6
Configuration
7-3
multicast packets in the registration message and send it to the PR in unicast
form.
The essential difference between PIM-SM and PIM-DM is that PIM-SM is based
on explicit join mode and PIM-DM is based on flood/prune mode. For PIM-SM,
the receiver sends a join message to the PR, but the device forwards the packets
of a multicast group only on the interface joining this multicast group. PIM-SM
forwards multicast packets through the shared tree. Each multicast group has a
rendezvous point. The multicast source sends packets to the RP along the
shortest path, and then the RP sends the packets to every receiver along the
shortest path. This process is similar to CBT. However, the core concept is not
used in PIM-SMv6. One of the main advantages of PIM-SMv6 is that it not only
receives multicast packets through the shared tree but also offers the shared
tree-to-SPT transformation mechanism. This transformation consumes a lot
number of resources, even though it reduces network delay and possible block on
the RP. It is suitable for the environment where there are many pairs of multicast
sources yet fewer networks.
PIM-SM distributes multicast packets through the shared tree and SPT. Assume
that other devices do not need to receive these multicast packets, unless
otherwise specified. When a host joins a multicast group, the devices connecting
to the host notify the root (or RP) through the PIM join message. This message is
transferred among these devices in order to set up the structure of a shared tree.
So, the RP records this transmission path and the registration message from the
first hop device (DR) of the sending multicast source, and perfects the shared tree
based on these two messages. Update of leaf messages is enabled on periodic
query message. For the shared tree, the multicast source sends multicast
packets to the RP so that all receivers can receive these multicast packets.
*.G indicates a tree, in which * indicates all sources and G indicates the specific
multicast address. The prune message is also used in the shared tree when leafs
do not need to receive multicast packets.
PIMv2 BSR distributes the group-to-RP message to all devices without the
necessity for configuring RP for every device. The BSR distributes the mapping
message through the hop-by-hop flooding BSR message. First of all, the BSR is
elected among devices. This election procedure is similar to electing the root
bridge in STP by priority. Every BSR device checks the BSR message, and only
forwards the BSR messages with higher or equivalent priority (or higher IP
address). The elected BSR sends the BSR message to the all-PIM-routers
multicast group (ff02::d) with TTL 1. Upon receiving the BSR message, the
adjacent PIMv2 device sends it out in multicast form and then reset TTL to 1. In
this way, the BSR message is sent to all devices hop by hop. Since the BSR
message includes the IP addresses of BSR devices, the candidate BSR can
determine which device is the current BSR device. The candidate RP sends the
candidate RP advertisement and alleges in which address ranges it can become
RP. The BSR stores the advertisement message in its local candidate RP cache,
and notifies all PIM devices of local candidate RPs periodically. Also in this way,
the message is sent to all devices hop by hop.
7.2 PIM-SMv6 Configuration Task
PIM-SMv6 Configuration Preparation：Enable a unicast routing protocol, for
instance OSPFv3, to automatically discover routes.
PIM-SMv6 configuration includes the following tasks, but only the first and the
second tasks are mandatory, others are optional.