Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB
OL-21227-02
Chapter 1 Cisco MWR 2941 Router Overview
Features
• Auto-RP allows load splitting among different RPs and arrangement of RPs according to the
location of group participants.
• Auto-RP avoids inconsistent, manual RP configurations that can cause connectivity problems.
Multiple RPs can be used to serve different group ranges or serve as backups to each other. For Auto-RP
to work, a router must be designated as an RP-mapping agent, which receives the RP-announcement
messages from the RPs and arbitrates conflicts. The RP-mapping agent then sends the consistent
group-to-RP mappings to all other routers. Thus, all routers automatically discover which RP to use for
the groups they support.
Note If you configure PIM in sparse mode or sparse-dense mode and do not configure Auto-RP, you must
statically configure an RP.
Note If router interfaces are configured in sparse mode, Auto-RP can still be used if all routers are configured
with a static RP address for the Auto-RP groups.
To make Auto-RP work, a router must be designated as an RP mapping agent, which receives the RP
announcement messages from the RPs and arbitrates conflicts. The RP mapping agent then sends the
consistent group-to-RP mappings to all other routers by dense mode flooding. Thus, all routers
automatically discover which RP to use for the groups they support. The Internet Assigned Numbers
Authority (IANA) has assigned two group addresses, 224.0.1.39 and 224.0.1.40, for Auto-RP. One
advantage of Auto-RP is that any change to the RP designation must be configured only on the routers
that are RPs and not on the leaf routers. Another advantage of Auto-RP is that it offers the ability to
scope the RP address within a domain. Scoping can be achieved by defining the time-to-live (TTL) value
allowed for the Auto-RP advertisements.
Each method for configuring an RP has its own strengths, weaknesses, and level of complexity. In
conventional IP multicast network scenarios, we recommend using Auto-RP to configure RPs because
it is easy to configure, well-tested, and stable. The alternative ways to configure an RP are static RP,
Auto-RP, and bootstrap router.
Bootstrap Router
Another RP selection model called bootstrap router (BSR) was introduced after Auto-RP in PIM-SM
version 2. BSR performs similarly to Auto-RP in that it uses candidate routers for the RP function and
for relaying the RP information for a group. RP information is distributed through BSR messages, which
are carried within PIM messages. PIM messages are link-local multicast messages that travel from PIM
router to PIM router. Because of this single hop method of disseminating RP information, TTL scoping
cannot be used with BSR. A BSR performs similarly as an RP, except that it does not run the risk of
reverting to dense mode operation, and it does not offer the ability to scope within a domain.
Multicast Source Discovery Protocol
In the PIM sparse mode model, multicast sources and receivers must register with their local rendezvous
point (RP). Actually, the router closest to a source or a receiver registers with the RP, but the key point
to note is that the RP “knows” about all the sources and receivers for any particular group. RPs in other
domains have no way of knowing about sources that are located in other domains. Multicast Source
Discovery Protocol (MSDP) is an elegant way to solve this problem.
MSDP is a mechanism that allows RPs to share information about active sources. RPs know about the
receivers in their local domain. When RPs in remote domains hear about the active sources, they can
pass on that information to their local receivers. Multicast data can then be forwarded between the