Information Technology Reference
In-Depth Information
The topic of IP multicast is extensive. This section provided a high-level overview of how
multicast routing works intradomain. The purpose of this overview was to cover the
fundamentals you need to learn more about interdomain multicast.
Interdomain Multicast
In a multicast domain, the RP has knowledge of all active sources for that domain. When
moving beyond a single multicast domain, knowledge of active multicast sources must be
distributed to other domains. A possible solution is to have a global RP that is shared by the
entire Internet. However, this would not scale for both technical and administrative reasons.
An initial solution located all RPs adjacent to each other at a Multicast Internet Exchange
(MIX) running PIM-DM between them. This solution caused periodic flooding of multicast
source information. However, this solution does not scale and places specific constraints on
RP location. Figure 11-18 shows an example of using a MIX for interdomain multicast.
Figure 11-18
Multicast Internet Exchange
ISP A
PIM Sparse-Mode
ISP B
PIM Sparse-Mode
RP
RP
AS 10888
PIM Dense-Mode
RP
RP
ISP C
PIM Sparse-Mode
ISP D
PIM Sparse-Mode
The next solution devised was to advertise active sources between RPs across multiple
domains. This new paradigm spurred the development of Multicast Source Discovery
Protocol (MSDP). The sole purpose of MSDP is to advertise the existence of remote (S,G)
