Ericsson ECN330, User Manual

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Configuring the ECN330-switch
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1553-KDU 137 365 Uen D 2006-06-16
bootstrap messages, they process the message and forward it out all interfaces,
except for the interface on which this message was received. If a router receives
a bootstrap message with a BSR priority larger than its own, it stops 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