Digisol DG-GS4826S Management Manual page 597

C 22 | Multicast Routing
HAPTER
Overview
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 register-stop message, it stops sending
register messages to the RP. If there are no other sources using the shared
tree, it is also torn down. Setting up the SPT requires more memory than
when using the shared tree, but can significantly reduce group join and
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