Multicast Source Discovery Protocol (MSDP)

Multicast Source Discovery Protocol (MSDP)
In PIM sparse mode, the routers closest to the sources and receivers register with the RP, so the
RP knows about all the sources and receivers for any group. But it is possible that several RPs
may need to be created, resulting in several PIM SM domains. Naturally, the RPs don’t know
about multicast sources in other domains. Multicast Source Discovery Protocol (MSDP) was
developed to address this issue.
ISPs offering multicast routes to their customers faced a dilemma. Naturally, they didn’t
want to have to rely on an RP maintained by another ISP, but they needed to access multicast
traffic coming from the Internet. MSDP allows them to each run their own RP. RPs peer
together using a TCP-based connection that allows them to share information about active
sources inside their own domains.
ISPs have the option of which sources they will forward to other MSDP peers, or which
sources they will accept, using filtering configurations. PIM SM is used to forward traffic
between the RP domains.
Host A
Host F
Host B Host C
Host D Host E
Member
224.2.125.125
Member
224.2.125.125
Source for
224.2.125.125
Member
224.2.125.125
Router 1
Router 2
RP
Router 3
Host G
Router 5
Router 4
Prune
request
IGMP remove request
E0 removed from
forwarding cache
3
1
2
E0
S0
Planning and Preparing for Using IP Multicast 619
ISPs have no problem with this peering relationship. ISP border routers already
establish peering relationships with neighboring ISPs, running Border Gateway
Protocol (BGP) version 4 to exchange routing information as part of the
Internet architecture. ISPs with such peering relationships have regular meetings,
and their inter-ISP links are part of their commercial raison d’etre. MSDP
peering is simply an addition to the agenda.