Multicast PIM Accept RP

PIM Accept RP is a security feature on Cisco IOS routers that prevents unwanted rendezvous points or multicast groups to become activate in the PIM sparse mode domain. By default a RP will accept all multicast groups in the 224.0.0.0/4 range (the entire class D range) but if we want we can configure our router to allow only PIM join/prune messages towards the groups that we want.

Let me demonstrate this feature using a very simple topology:

R1 R2 Fastethernet Interfaces

Only 2 routers, R1 will be our rendezvous point. Let’s configure this network so that PIM sparse mode is enabled and R1 becomes the RP:

R1(config)#ip multicast-routing          
R1(config)#ip pim rp-address 192.168.12.1
R1(config)#interface fastEthernet 0/0    
R1(config-if)#ip pim sparse-mode 
R2(config)#ip multicast-routing 
R2(config)#ip pim rp-address 192.168.12.1
R2(config)#interface fastEthernet 0/0
R2(config-if)#ip pim sparse-mode 

This is how we enable multicast routing, configure R1 as the RP and enable sparse mode. Let’s take a look what multicast groups R1 will serve:

R1#show ip pim rp mapping 
PIM Group-to-RP Mappings

Group(s): 224.0.0.0/4, Static
    RP: 192.168.12.1 (?)
R2#show ip pim rp mapping 
PIM Group-to-RP Mappings

Group(s): 224.0.0.0/4, Static
    RP: 192.168.12.1 (?)

Both routers agree that R1 is the RP for the entire multicast group range 224.0.0.0/4. Let’s change it so that it only accepts multicast group 239.1.1.1:

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Forum Replies

  1. Hi Rene,

    What is the different PIM accept RP and below configuration? The PIM accept RP applicable for static RP, Auto Rp and BSR?

    Ip PIM send-rp-annouce loopback 0 scope 10 group-list 1

    Davis

  2. Hi Davis,

    PIM accept RP can be used to tell your router not to accept just any RP. I believe it works for AutoRP and BSR, not for static RPs. For example when there are 10 routers in your network advertising themselves as the RP then you can use this command to ignore a couple of them.

    ip pim send-rp-announce is used to configure your router to advertise itself as an RP, the access-list is used to advertise the networks that you want to be a RP for.

    Rene

  3. Got it.

    Thanks Rene.

    Davis

  4. Hi Rene,

    Is 192.168.12.1 the address of the loopback of R1?

    Groeten,

    Hans.

  5. Hi Hans,

    192.168.12.1 is the IP address of R1. I usually use subnets / IP addresses like this:

    192.168.XY.X where X = R1 and Y = R2.

    For loopbacks I use 1.1.1.1/32 for R1, 2.2.2.2/32 for R2 etc.

    Rene

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