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  1. Hello Hussien

    Which router will be chosen depends on what is called the PIM Assert mechanism. You can find detailed information about this at the following lesson:

    When you set the priority of a router to 0, this means that it will never become a DR.

    I hope this has been helpful!


  2. Hi Hussien,

    It’s the job of the DR (Designated Router) to forward the PIM join upstream.


  3. Hi Hussein,

    In my example, R4 was only a receiver so it didn’t participate in PIM, which makes R2 or R3 the designated router. When you enable PIM on R4 and it is a DR, it will have two equal unicast routes to so it has to make a choice.

    RPF does not use both routes but uses the PIM neighbor with the highest IP address to go upstream.

    Let’s look at an example. I’ll turn R4 into a PIM router by enabling it on the FastEthernet0/0 interface. I’ll also create a loopback that will have IGMP join on it so that R4 is the DR for the loopback segment:

    R4(config)#interface FastEthernet 0/1
    R4(config-if)#no ip igmp join-group
    R4(config-if)#ip pim sparse-mode 
    R4(config)#router ospf 1
    R4(config-router)#network area 0
    R4(config)#interface loopback 0
    R4(config-if)#ip address
    R4(config-if)#ip pim sparse-mode 
    R4(config-if)#ip igmp join-group

    Here’s how R4 reaches the RP:

    R4#show ip route | include via
      Known via "ospf 1", distance 110, metric 3, type intra area, from, 00:04:57 ago, via FastEthernet0/0
      *, from, 00:04:57 ago, via FastEthernet0/0

    There are two routes but RPF only chooses one:

     R4#show ip rpf                
    RPF information for ? (
      RPF interface: FastEthernet0/0
      RPF neighbor: ? (
      RPF route/mask:
      RPF type: unicast (ospf 1)
      Doing distance-preferred lookups across tables
      RPF topology: ipv4 multicast base, originated from ipv4 unicast base

    It selects R3 since that’s the highest PIM neighbor IP address. If you want to influence this, you have a couple of options:

    * Don’t enable PIM on R2’s connection to R3/R4
    * Change the IP address on R2 so that it’s higher than R3’s IP address.
    * Create a static mroute

    The static mroute is a decent option but you’ll lose redundancy since you have to manually change it.

    Hope this helps!


  4. Its seems relatively straightforward when explained. However, may become less so when starting to apply in real world :slight_smile: . One real world example - client is facing redundant pair of routers with HSRP. Will both routers still participate in DR election? How its going to work then if router with lower IP is the active router? Looks like the joins may not be sent as DR (being passive) will not receive client request and active router will receive it but will not send join to RP, since its not a DR. If thats the case does it mean in HSRP case (which is very often) we always need to manually increase priority on the active router to make it DR and ensure registration with RP?

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