Posts tagged ‘auto-rp’

May 27, 2012

Auto-RP and usage of “ip pim autorp listener”

by costiser

After reading several forums with people arguing about on what routers should we use the “ip pim autorp listener” command, I have decided to write this post explaining things in my own way (of course, with a lot of pictures, as usual). Before starting, just a few words about Auto-RP:  it’s a legacy Cisco proprietary method for selecting the RP that uses two functional roles: the Candidate RPs (C-RP) and the Mapping Agent (MA).

Candidate RPs (C-RP):

    • routers willing to be RP
    • announce themselves to the MA via 224.0.1.39

Mapping Agent (MA):

    • decide who will be the RP from the Candidates RP
    • will inform the rest of the network about the elected RP via 224.0.1.40

Auto-RP uses in fact a DENSE mode fashion for distributing the announces and discoveries. For this:

  • MA must listen for 224.0.1.39
  • all multicast speaking routers must listen for 224.0.1.40

 

Let’s see how all works. Consider a topology with 4 routers connected in a daisy-chain fashion: R1 <–> R2 <–> R3 <–> R4.


1) configuring “ip pim sparse-mode” on all 4 routers will automatically install a (*,224.0.1.40) on all of them ( at least in IOS 12.4 – not tested in other IOSes !!! )


2) configure R1 to be a Candidate-RP (“ip pim send-rp-announce Lo 0“)

=> R1 announce itself as candidate RP via 224.0.1.39 in a DENSE-mode fashion (to all its neighbors):

!! R2 will not forward these announcements !!

3)  configure R2 to be a Mapping Agent (“ip pim send-rp-discovery Lo 0“)

=> R2 selects the RP from all the candidates (in our case only R1)
=> R2 informs everybody about the selected RP via 224.0.1.40 in a DENSE-mode fashion
=> R2 also forwards the announcements that it received from R1 (via 224.0.1.39) !! – this is because there may be redundant MAs in the network
 
 !! R3 will not forward these announcements !! 

4)  configure R3 to be a redundant Mapping Agent (“ip pim send-rp-discovery Lo 0“)

=> R3 receives the announcements sent by R1 (because it happens to be neighbor with the MA-1, R2, that forwards them)
=> R3 selects R1 as RP
=> R3 informs everybody about the selected RP via 224.0.1.40 in a DENSE-mode fashion
=> same as R2, also R3 forwards the announcements that it received from R1 (via 224.0.1.39) !!
At this moment, all 4 routers know the RP via Auto-RP with the following note:
  • R1 sees the source of info as being 2.2.2.2 (R2 = MA)
  • R2 sees the source of info as being 1.1.1.1 (R1 = C-RP)
  • R3 sees the source of info as being 1.1.1.1 (R1 = C-RP)
  • R4 sees the source of info as being 3.3.3.3 (R3 = redundant MA)

Another important note:  I did not use the “ip pim autorp listener” on any of them !!!  I only took advantage of the fact that this command is needed only on routers that are in transit of those groups (routers that need to forward the DENSE information for 224.0.1.39 & 224.0.1.40).

If a 5th router had been connected to R4, then in order for R5 to receive the Auto-RP messages we would have needed to configure “ip pim autorp listener” on R4.
Conclusion:  ip pim autorp listener” is not needed on routers that are adjacent to the MA !
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