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Latest post 11-19-2008 12:01 PM by awilkins. 2 replies.
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  • 11-16-2008 4:03 PM

    Multicast and 'ip pim dr-priority XXX' on serial interface.......

    I'm working on Lab 4 right now - Section 6.4 : Inter-AS Multicast

     

    Can someone explain to me why we had to put on the 'ip pim dr-priority 255' on the Serial interface of R2 going to R4 in order to give R4 the ability to ping the group 224.1.1.1 on R1? 

     

    I did advertise the Vlan 456 network to AS13 and realized that the group 224.1.1.1 was reachable when sourced from the E0/0 interface of R4.  But, not until I did the 'ip pim dr-priority 255' on R2's interface to R4 could R4 get a response on it's serial interface.

    Does it have something to do with the way the frame-relay link was set up with the frame-relay map ip x.x.x.x yyy broadcast?

     

    Thanks in advance.

    • Post Points: 20
  • 11-18-2008 5:39 PM In reply to

    Re: Multicast and 'ip pim dr-priority XXX' on serial interface.......

    Hi awilkins,

    R2 and R4 are pim neighbours. When there is no explicit dr-priority value configured on PIM neighbours, the priority value is equal on both (I think the default is 1). The DR is then elected based on IP address (highest wins).

    Its the job of the DR to encapsulate multicast messages to the RP in a PIM-SM setup. Most important (and relevant in this case) is the SOURCE REGISTER message to the RP.

    Here R4 has the highest IP address and will be DR by default. The DR cannot send a register message for itself. Since the task asks us to configure R4 to send traffic to the group address, we have to configure R4 to send a REGISTER message to DR (or RP - whichever is metrically closer). So we force it to concede the DR status to R2 by applying the ip pim dr-priority 255 command on R2's interface.

    You may not come accross this in real-world scenarios - its only because you are testing your Multicast shared tree using Routers/router interfaces that this was necessary.

    Hope this explains it to you.

    Cheers

     

    **edited 21/11/08 - The DR will send a REGISTER msg (rather than a JOIN), since R4 was pinging the group on R1. ie R4 source, R1 is Receiver. But the logic remain same. Cheers.

     

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    • Post Points: 20
  • 11-19-2008 12:01 PM In reply to

    Re: Multicast and 'ip pim dr-priority XXX' on serial interface.......

    Makes perfect sense to me now!  Thanks!

    On Tue, Nov 18, 2008 at 7:42 PM, sp_quest <bounce-sp_quest@ieoc.com> wrote:

    Hi awilkins,

    R2 and R4 are pim neighbours. When there is no explicit dr-priority value configured on PIM neighbours, the priority value is equal on both (I think the default is 1). The DR is then elected based on IP address (highest wins).

    Its the job of the DR to encapsulate multicast messages to the RP in a PIM-SM setup. Most important (and relevant in this case) is the (*,G) JOIN message to the RP.

    Here R4 has the highest IP address and will be DR by default. The DR cannot send a join message for itself. Since the task asks us to send to configure R4 to send traffic to the group address, we have to configure R4 to send a JOIN to DR (or RP - whichever is metrically closer). So we force it to concede the DR status to R2 by applying the ip pim dr-priority 255 command on R2's interface.

    You may not come accross this in real-world scenarios - its only because you are testing your Multicast shared tree using Routers/router interfaces that this was necessary.

    Hope this explains it to you.

    Cheers

     

     




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    • Post Points: 5
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