Chapter 20
| Multicast Routing
Overview
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advertising itself as a BSR candidate. Eventually, only the router with the highest
BSR priority will continue sending bootstrap messages.
Rendezvous Point (RP) – A router may periodically sends PIMv2 messages to the
BSR advertising itself as a candidate RP for specified group addresses. The BSR
places information about all of the candidate RPs in subsequent bootstrap
messages. The BSR and all the routers receiving these messages use the same hash
algorithm to elect an RP for each multicast group. If each router is properly
configured, the results of the election process will be the same for each router. Each
elected RP then starts to serve as the root of a shared distribution tree for one or
more multicast groups.
Designated Router (DR) – A DR advertising the highest priority in its hello
messages is elected for each subnet. The DR is responsible for collecting
information from the subnet about multicast clients that want to join or leave a
group. Join messages from the DR (receiver) for each group are sent towards the RP,
and data from multicast sources is sent to the RP. Receivers can now start receiving
traffic destined for the client group from the RP, or they can identify the senders
and optionally set up a direct connection to the source through a shortest path tree
(SPT) if the loading warrants this change over.
Shared Tree – When many receivers join a group, their Join messages converge on
the RP, and form a distribution tree for the group that is rooted at the RP. This is
known as the Reverse Path Tree (RPT), or the shared tree since it is shared by all
sources sending to that group. When a multicast source sends data destined for a
group, the source’s local DR takes those data packets, unicast-encapsulates them,
and sends them to the RP. When the RP receives these encapsulated data packets, it
decapsulates them, and forwards them onto the shared tree. These packets follow
the group mapping maintained by routers along the RP Tree, are replicated
wherever the RP Tree branches, and eventually reach all the receivers for that
multicast group. Because all routers along the shared tree are using PIM-SM, the
multicast flow is confined to the shared tree. Also, note that more than one flow can
be carried over the same shared tree, but only one RP is responsible for each flow.
Shortest Path Tree (SPT) – When using the Shared Tree, multicast traffic is
contained within the shared tree. However, there are several drawbacks to using
the shared tree. Decapsulation of traffic at the RP into multicast packets is a
resource intensive process. The protocol does not take into account the location of
group members when selecting the RP, and the path from the RP to the receiver is
not always optimal. Moreover, a high degree of latency may occur for hosts
wanting to join a group because the RP must wait for a register message from the
DR before setting up the shared tree and establishing a path back to the source.
There is also a problem with bursty sources. When a source frequently times out,
the shared tree has to be rebuilt each time, causing further latency in sending
traffic to the receiver. To enhance overall network performance, the switch uses the
RP only to forward the first packet from a source to the receivers. After the first
packet, it calculates the shortest path between the receiver and source and uses
the SPT to send all subsequent packets from the source directly to the receiver.
When the first packet arrives natively through the shortest path, the RP sends a
register-stop message back to the DR near the source. When this DR receives the
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