PIM Sparse Mode, Cisco Auto-RP Protocol & Configuration

PIM Sparse mode requires a Rendezvous Point (RP) to work. We have to manually configure one or many RPs. Then we can manually configure all the routers in the network to tell what is the RP or we can use 2 protocols to advertise the RP information.

1. Cisco Auto-RP
2. BSR (Bootstrap Router)

In this post I am going to explore Cisco Auto-RP protocol. To get basic idea about PIM Sparse mode please go though the following post.

PIM Sparse Mode Operation & Behavior of Multicast Routing Table

Let's take the following topology..

In Auto-RP, there are 2 special router roles.

1. RP
2. RP Mapping Agent

RP of course is the Rendezvous Point, Mapping Agent is the router which collects information about the RPs and make the primary RP to a specific group and advertise it to other routers. So it is the most important router in the Auto-RP enabled network.

Assume all other PIM Sparse mode configuration (interface level) is already completed, and note that unlike the manual configuration, the loopback interface which is configured as the RP and Mapping Agent must be enabled with PIM Sparse mode too..

Let's configure R1 as the RP and R2 as the RP Mapping Agent.

R1(config)#ip pim send-rp-announce loopback 0 scope 10

Scope 10 means the TTL of these packets are 10.

The above command makes the R1's loopback 0 address (1.1.1.1/32) the IP address of the RP..
As soon as it is configured, following will be the routing table of R1,

This 224.0.1.39 is the reserved IP for Cisco Auto-RP Rendezvous Point announcements.

R2(config)#ip pim send-rp-discovery loopback 0 scope 10

The above command makes the R2's loopback 0 address (2.2.2.2/32) the IP address of the Mapping Agent..

As soon as the above command is entered, the routing table of R2 will be like the following..

This means it has started listening to RP announcements. After a short while, the tunnel will be flapped and the routing table will be like the following..

Now it has these new entries. Because it is listening to 224.0.1.39, it has identified the 1.1.1.1 as the RP. Now the R1's routing table will be like the following..

It has also identified 2.2.2.2 as the RP Mapping Agent. All the routers are listening to this by default.
So R3 should learn this too ryt? Let's see..

It has learned about the RP Mapping Agent. Also as a result of being neighbor to a Mapping Agent, it has an entry listening to 224.0.1.39 which is RP announcements.

Let's see RP Mappings learned by R3;

Looks like it knows that 1.1.1.1 is the RP.

Let's see the routing table of R4;

Well, looks like R4 does not know anything about any of the above. This is because R4 is not directly connected to Mapping Agent and the RP Mapping packets are not forwarded by R3 because of the Sparse mode of operation. Sparse mode only forwards the traffic if it is requested by a downstream listener..

In order to solve this we either have to configure R3 in Sparse-Dense mode or we have to configure ip pim autorp listener command globally in R3. What this command does at back end is that it will flood 224.0.1.39 and 224.0.1.40 traffic in Dense mode.

R3(config)#ip pim autorp listener

 

Now the RP Mappings are learned, let's send some feed from IPTV_Svr and take a look at the routing table of R4..

 Now the feed from IPTV_Svr will get to the PC..