Configuring the ECN330-switch
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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 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 ECN330-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 data transmission delays. The ECN330-
switch can also be configured to use SPT only for specific multicast groups, or
to disable the change over to SPT for specific groups.
6.21.5.1 Enabling PIM-SM Globally
PIM-SM is used to route multicast traffic to nodes which have requested a
specific multicast service through IGMP. The Designated Router (DR) near the
receiver sends a join message the Rendezvous Point (RP) responsible for the
specified multicast group. If the RP has already established a reverse path tree
back to the source, it immediately starts forwarding packets to the DR; otherwise
it sends a join message back toward the source and waits until traffic arrives
from the source before forwarding traffic down the shared tree. However, note
that if the RP does not receive a Register message back from the source
requested by the host group, it will take no action.
To use PIM-SM, it must be enabled globally for the router as described below,
and for each interface that will support multicast routing as described in the next
section. Also note that IGMP must be enabled to allow the router to determine
the location of group members.
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