Multicast Routing
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OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
Alcatel-Lucent
A multicast receiver expresses its interest in receiving traffic destined for a
multicast group. Typically, it does this using IGMP.
One of the receiver's local routers is elected as the Designated Router (DR) for
that subnet. On receiving the receiver's expression of interest, the DR sends a
PIM Join message towards the RP for that multicast group. All the DR Routers of
receivers send join message towards RP.
This is known as the RPTree (RPT), and is also known as the shared tree
because it is shared by all sources sending to that group. Join messages are
resent periodically so long as the receiver remains in the group. When all
receivers on a leaf-network leave the group, the DR will send a PIM Prune
message towards the RP for that multicast group.
A multicast data sender just starts sending data destined for a multicast group.
The sender's local router (DR) takes those data packets, unicast-encapsulates
them, and sends them directly to the RP. The RP receives these encapsulated
data packets, decapsulates them, and forwards them onto the shared tree. The
packets then follow the RPTree, being replicated wherever the RP Tree branches,
and eventually reaching all the receivers for that multicast group. The process of
encapsulating data packets to the RP is called registering, and the encapsulated
packets are known as PIM Register packets.
After receiving packets from source RP sends join message towards source and
this forms source specific Shortest Path Tree (SPT). This is to reduce the
encapsulation and decapsulation overhead. For many receivers, the route to
source via the RP may not be shortest. To obtain lower latencies, router on
receiver LAN joins SPT once it receives some packets over RPTree. This is called
as switching to SPT from RPT.
INTERNET GROUP MANAGEMENT PROTOCOL (IGMP)
The IGMP version 2 is described in RFC 2236.
IGMP is used by IP hosts to report their multicast group memberships to any
immediately-neighboring multicast routers. Multicast routers use IGMP to learn
which groups have members on each of their attached physical networks.
Routers periodically (Query Interval) send a General Query on each attached
network for which this router is the Querier to solicit the membership information.
A General Query is sent to the all-systems multicast group (224.0.0.1)
When a host receives a General Query, it sets delay timers for each group
(excluding the all-systems group) of which it is a member on the interface from
which it received the query. This delay timer is set to value selected in the range
(0, max response time of query).
When a host receives a Group-Specific Query, it sets a delay timer to a random
value selected from the range (0, Max Response Time) for the group being
queried if it is a member on the interface from which it received the query.
When a group's timer expires, the host multicasts a version 2 Membership
Report to the group, with IP TTL of 1. If the host receives another host's Report
(version 1 or 2) while it has a timer running, it stops its timer for the specified
group and does not send a Report, in order to suppress duplicate Reports.
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