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information. This solution allows RPs to be located anywhere in the network and lets
providers have independent RPs.
The other challenge is that not all autonomous systems in the Internet support multicast
traffic. This means that an alternative source of RPF information besides the unicast routing
table needs to be available for building MDTs. If the RPF for a specific (S,G) pointed
toward an AS that does not support multicast, the MDT could not be built back to the
source, and IP multicast would not function for that (S,G). The solution was the introduc-
tion of IPv4 multicast NLRI in BGP as a new SAFI. This multicast NLRI would provide an
alternative database for performing RPF checking.
Multicast Source Discovery Protocol
The MSDP protocol provides a mechanism for interdomain multicast connectivity. The
functionality provided by MSDP is the advertisement of active (S,G) information to remote
domains. In a PIM-SM domain, the RP has information on all active (S,G) entries for that
domain. The interdomain (S,G) advertisement challenge boils down to advertising active
(S,G) entries between RPs in all participating PIM-SM domains.
MSDP is specified to operate over both TCP and UDP on port 639; however, all
implementations use TCP. The control information advertised through MSDP is called a
Source Active (SA) message. An SA contains three primary pieces of information:
Address of the multicast source
Multicast group address the source is sending to
IP address of the originating RP
The RP for a particular (S,G) is the only router that can originate the SA message for that
(S,G). A non-RP router is unable to originate SA messages and can provide only SA transit.
The mechanism in MSDP to prevent the looping of SA messages is called peer-RPF . The
peer-RPF function differs from RPF with respect to MDT creation. The peer-RPF function
ensures that only SA messages received from an MSDP that is logically closer to the orig-
inator of the SA are accepted. When an MSDP router receives a new SA that passes the
peer-RPF check, it floods that SA to all the MSDP peers other than the one on which it was
received. The MSDP router caches all received SA messages. If an SA is received that is
already in the cache, the MSDP router does not flood that SA. This prevents periodic
reflooding of the same SA information.
The ability to perform peer-RPF requires MSDP to understand the logical topology back to
the originating RP. This understanding of the logical topology allows MSDP to determine
which peer is closest to the originating RP. MSDP does not contain any information inter-
nally to provide this topology; instead, MSDP relies on BGP. For MSDP to rely on BGP for
topology determination, the MSDP peering topology should mirror the BGP peering topol-
ogy. The peer-RPF rules are presented in the section “mBGP/MSDP Interaction.”
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