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1.
The multicast source registers with the nearest RP. In this example, Source registers with RP 1,
with its multicast data encapsulated in the register message. When the register message
arrives at RP 1, RP 1 de-encapsulates the message.
2.
Receivers send join messages to the nearest RP to join in the RPT rooted as this RP. In this
example, Receiver joins the RPT rooted at RP 2.
3.
RPs share the registered multicast information by means of SA messages. In this example, RP
1 creates an SA message and sends it to RP 2, with the multicast data from Source
encapsulated in the SA message. When the SA message reaches RP 2, RP 2 de-encapsulates
the message.
4.
Receivers receive the multicast data along the RPT and directly join the SPT rooted at the
multicast source. In this example, RP 2 forwards the multicast data down the RPT. When
Receiver receives the multicast data from Source, it directly joins the SPT rooted at Source.
The significance of Anycast RP is as follows:
•
Optimal RP path
—A multicast source registers with the nearest RP to build an optimal SPT. A
receiver joins the nearest RP to build an optimal RPT.
•
Load balancing between RPs
—Each RP maintains part of the source/group information
within the PIM-SM domain and forward part of the multicast data. This achieves load balancing
between different RPs.
•
Redundancy backup between RPs
—When an RP fails, the multicast source that previously
registered with the RP or the receivers that previously joined the RP will register with or join
another nearest RP, achieving redundancy backup between RPs.
IMPORTANT:
•
Be sure to configure a 32-bit subnet mask 255.255.255.255 for the Anycast RP address. Namely,
be sure to configure the Anycast RP address into a host address.
•
An MSDP peer address must be different from the Anycast RP address.
MSDP support for VPNs
The interfaces on the multicast routers in a VPN can set up MSDP peering relationship between
each other. By exchanging SA messages between MSDP peers, multicast transmissions in a VPN
between different PIM-SM domains can be implemented.
To support MSDP for VPNs, a multicast router that runs MSDP maintains an independent set of
MSDP mechanism for each VPN that it supports:
•
SA cache
•
Peering connection
•
Timers
•
Sending cache
•
Cache for exchanging PIM messages.
The information in one VPN is isolated from another, and MSDP and PIM-SM messages can be
exchanged only within the same VPN.
Protocols and standards
•
RFC 3618,
Multicast Source Discovery Protocol (MSDP)
•
RFC 3446,
Anycast Rendezvous Point (RP) mechanism using Protocol Independent Multicast
(PIM) and Multicast Source Discovery Protocol (MSDP)
