IP Multicast Routing
Anycast RP with MSDP
The final tool covered here for finding a router’s RP is called Anycast RP with Multicast Source
Discovery Protocol (MSDP). Anycast RP is actually an implementation feature more than a new
feature with new configuration commands. As will be explained in the upcoming pages, Anycast
RP can actually use static RP configuration, Auto-RP, and BSR.
The key differences between using Anycast RP and using either Auto-RP or BSR relate to how the
redundant RPs are used. The differences are as follows:
■ Without Anycast RP—RP redundancy allows only one router to be the active RP for each
multicast group. Load sharing of the collective work of the RPs is accomplished by using one
RP for some groups and another RP for other groups.
■ With Anycast RP—RP redundancy and load sharing can be achieved with multiple RPs
concurrently acting as the RP for the same group
The way Anycast RP works is to have each RP use the same IP address. The RPs must advertise
this address, typically as a /32 prefix, with its IGP. Then, the other methods of learning an RP—static
configuration, Auto-RP, and BSR—all view the multiple RPs as a single RP. At the end of the
process, any packets sent to “the” RP are routed per IGP routes to the closest RP. Figure 17-22
shows an example of the process.
Figure 17-22 Learning the RP Address with Anycast RP
Figure 17-22 shows a design using two RPs (RP-East and RP-West) along with Auto-RP. The steps
shown in the figure are as follows:
1. Both RPs are configured with 172.16.1.1/32, and configured to use that IP address for RP
functions. In this case, both are configured to be the RP for all multicast groups.
R1
Mapping
Agent
RP-Announce
172.16.1.1,
224.0.0.0/4
2
4
RP-Announce
172.16.1.1,
224.0.0.0/4
1 RP so far. Mapping:
All groups, 172.16.1.1
RP-Discovery
Messages
2
3
RP-West RP-East
R-W1 R-E1
Configuration:
RP Address: 172.16.1.1
Groups: 224.0.0.0/4
Use Auto RP
Loop1: 172.16.1.1/32
1
RP Learned via Auto RP:
172.16.1.1
5 RP Learned via Auto RP:
172.16.1.1
5
Configuration:
RP Address: 172.16.1.1
Groups: 224.0.0.0/4
Use Auto RP
Loop1: 172.16.1.1/32
1
Sparse-Mode Routing Protocols 629
2. Both RPs act as normal for Auto-RP by sending RP-Announce messages to 224.0.1.39.
3. The Auto-RP mapping agent builds its mapping table with a single entry, because it cannot
tell the difference between the two RPs, because both use IP address 172.16.1.1.
4. The Auto-RP mapping agent acts as normal, sending an RP-Discovery message to
224.0.1.40. It includes (in this case) a single mapping entry: all groups map to 172.16.1.1.
5. All the routers, including routers R-W1 and R-E1, learn via Auto-RP that the single RP for
all groups is 172.16.1.1.
The last step described in the list brings the discussion to the main benefit of Anycast RP. At this
point, the core Auto-RP function of advertising the IP address of the RP is complete. Of course,
the IP address exists on two routers in Figure 17-22, but it could be more than that in other designs.
Because of the IGP routes, when routers in the western part of the network (like R-W1) send
packets to the RP at 172.16.1.1, they are actually sending the packets to RP-West. Likewise, when
routers in the eastern part of the network (like R-E1) send packets to the RP (172.16.1.1), they are
actually sending the packets to RP-East. This behavior is only achieved by using the Anycast RP
implementation option beyond simply using Auto-RP.
The two biggest benefits of this design with Anycast RP are as follows:
■ Multiple RPs share the load for a single multicast group.
■ Recovery after a failed RP happens quickly. If an RP fails, multicast traffic is only interrupted
for the amount of time it takes the IGP to converge to point to the other RP sharing the same
IP address.
The design of Anycast RP creates a problem that must be overcome using MSDP. The problem
relates to the fact that each individual RP builds its own shared tree, but any multicast source sends
packets to one of the RPs. For example, Figure 17-23 shows the same network as Figure 17-22,
but now with a multicast source in the western part of the network. The routers in the west side of
the figure receive the packets as distributed by RP-West via its shared tree. However, the routers
in RP-East’s shared tree do not get the packets because RP-East never gets the packet sent by the
server in the west side.
The solution to this problem is for the RPs to tell each other about all known sources by using
MSDP. MSDP allows RPs to send messages to each other, revealing the IP addresses of each
source for each multicast group. In Figure 17-23, RP-West could tell RP-East about the multicast
source for 226.1.1.1 at unicast IP address 172.16.5.5. Then, RP-East can join the SPT of source
172.16.5.5, group 226.1.1.1, just as it would have done if it had received the multicast traffic
directly from 172.16.5.5.
630 Chapter 17: IP Multicast Routing
Figure 17-23 The Anycast RP Problem (Later Solved with MSDP)
Summary: Finding the RP
This section covers the concepts behind four separate methods for finding the RP. Three are
specific configuration features, namely static configuration, Auto-RP, and BSR. The fourth,
Anycast RP, actually uses any of the first three methods, but with the design that includes having
the RPs use the same unicast IP address to achieve better redundancy features. Table 17-3
summarizes the methods of finding the RP with PIM-SM.
Table 17-3 Comparison of Methods of Finding the RP
Method RP Details Mapping Info
Redundant
RP
Support?
Load
Sharing
of One
Group?
Static Simple reference to
unicast IP address.
— No No
Auto-RP Sends RP-Announce to
224.0.1.39; relies on
sparse-dense mode.
Mapping agent sends via
RP-Discovery to
224.0.1.40
Yes No
R1
1 4
3
3
RP-West RP-East
R-W1 R-E1
H2
2
Forward on my Shared
Tree – that doesn’t
include the East!
I’m part of RP-East’s
Shared Tree – I don’t
get the packets!
Scope of RP-West’s
Shared Tree
Scope of RP-East’s
Shared Tree
Source for 226.1.1.1
IP Address 172.16.5.5
Method RP Details Mapping Info
Redundant
RP
Support?
Load
Sharing
of One
Group?
BSR Sends c-RP
advertisements as
unicasts to BSR IP
address; does not need
sparse-dense mode.
Sends bootstrap messages
flooded over non-RPF path
Yes No
Anycast RP Each RP uses identical
IP addresses.
Can use Auto-RP or BSR
normal processes
Yes Yes