Re: [pim] Some thoughts on draft-mirsky-pim-bfd-p2mp-use-case

Hi

I've been thinking about this some more. Usually the DR election is
based on the neighbor's you have received hello from, which only
requires one way reachability. p2mp BFD would by just using multicast,
verify one way reachability in the same direction. If the BFD packets
don't make it through, then hello's from that neighbor would
presumably not make it either. So this should be fine.

I was asking what action to take when a failure is detected. I agree a
new DR should be elected, but it should be more specific. I think it
should expire the neighbor (or at least exclude that neighbor from
consideration as a DR).

Regarding using BFD not just for the DR. There could potentially be
other cases where pim could benefit from quickly knowing a neighbor
has gone down. E.g. if assert winner is down. Might it make sense for
all pim neighbors to send the hello option for bootstrapping, and make
it optional for the other neighbors whether they join the session? It
could be left to the admin whether to just use if for DR, or also for
other routers.

Stig

On Fri, Jun 22, 2018 at 9:00 AM, Mankamana Mishra (mankamis)
<mankamis@cisco.com> wrote:
> Hi Stig
> In general should we not only be concerned about DR and BDR ? Even if link failure happens for non DR/BDR PIM routers, with respect to multicast we do not need to take any action. But yes, we would have increase the scope of this document to cover cases where DR load balancing is applied and we want to use BFD protection for all of the DR in shared LAN.
>
> Mankamana
>
>> On Jun 21, 2018, at 1:44 PM, Stig Venaas <stig@venaas.com> wrote:
>>
>> Hi
>>
>> Posting as a WG participant.
>>
>> This looks useful to me and I believe it would scale better than the
>> current use of p2p BFD for pim. Below are some questions I have.
>>
>> Unidirectional link failures
>> I believe in pim we assume that if a link fails in one direction, it
>> also fails in the other direction. Several things could go wrong
>> otherwise. Is it sufficient to use BFD for discovering unidirectional
>> failures? p2mp BFD seems to be only in one direction normally.
>> However, if we need bidirectional, is it useful to have the head query
>> the tails and see if it gets responses from all neighbors? What should
>> be the behavior if a failure is detected? E.g. if a link is
>> partitioned (in both directions), it is beneficial to have a DR in
>> each partition.
>>
>> BFD only for DR?
>> The draft currently assumes that it is only needed for the DR. Do we
>> in pim need to detect other neighbors going down? I believe there are
>> such cases. Even if one uses p2mp BFD for all pim routers, I believe
>> it will scale better than the current full p2p mesh.
>>
>> Stig
>>
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