Re: [pim] BSR update

[Dino Farinacci <dino at cisco.com>]

>> Slide 6, Unicast BSMs
>> 
>> New revision says that they must be sent from the primary address,
>> and also only accepts them from the primary. I believe Dino said this
>> was problematic. But I must say now that I'm not sure why. Could you
>> clarify Dino? Or anyone else?

    The problem with unicast BSMs, as sent by the DR on a LAN when a new
    neighbor comes up, is if the ARP cache is not primed with the new 
    neighbor, then the BSM is *typically* dropped by the local system (when
    the ARP implementation does not queue packets). 

    Since the PIM neighbor adjacency and the unicast routing protocol adjacency
    are coming up at the same time, it is typically a race on which one sends 
    the first unicast packet to the newly booted router. And since there is 
    less chit-chat in PIM for bringing up an adjacency (versus OSPF or, IS-IS 
    which never sends a unicast packet), it's the BSM that triggers the ARP
    request.

    So I am questioning the probablistic usefulness of unicast BSMs. I think
    a simple solution is to link-local multicast them. But the new neighbor
    needs to not RPF-check these. So either:

    o The new neighbor accepts all BSMs within a short-time of discovering
      the DR, or

    o Set a bit in the BSM to unconditionally accept (and not forward) the
      BSM. This would be useful from another repsect in that the new neighbor
      wouldn't have to determine if the DR is sending it. Especially, in the
      case where the new neighbor is just becoming the DR.

Dino



    

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