[j-nsp] improving global unicast convergence (with or without BGP-PIC)

[Michael Hare]

Hello,

Sorry if this is an easy question already covered.  Does anyone on list have an understanding of what happens in the FIB in the following circumstance?

Simplified topology;
* Router 1 RIB default points to reject
* Router 1 RIB has default free feed from attached eBGP neighbor A
* Router 1 RIB has default free feed from attached iBGP neighbor B (add-path)

I guess what I'm trying to understand, from the perspective of improving upstream convergence for outbound packets from our AS, if my default route pointed to a valid next hop of last resort am I likely to see an improvement (reduction) in blackholing on router 1 during topology changes?  The thought being that if Router 1 FIB invalidates next-hop A quickly (en masse) packets could match default route with valid next-hop while FIB is being re-programmed with more specifics via B?

I am aware of indirect-next-hop being default on MPC but my understanding is this will not work for directly connected eBGP peers?  So if session with A drops (BFD, link, whatever) are routes with next hop to neighbor A deprogrammed nearly atomically due to some level of indirection or are routes considered one by one until all routes (~600K) have been processed?  I suspect the latter but perhaps looking for verification.

I am aware of BGP PIC but not yet running 15.X [when internet is not in VRF].  I am willing to accept that if BGP PIC is the best approach to improving this scenario an upgrade is the best path forward.  I'd be curious to hear from anyone who is on 15.1 [or newer] and using MPC4 in terms of perceived code quality and MPC4 heap utilization before/after.  

Historically the AS I primarily manage has been default free (default pointing to reject), but I'm considering changing that to improve convergence (aware of the security considerations).  As for our "real" topology, adding up all the transit and peering we have our RIB is nearing 6M routes.  We are not doing internet in a VRF.  Our network has add-path 3 enabled.  In some cases our peers/upstreams are on unprotected transport that is longer than I'd like.  Providing a ring and placing the router closer would be nice but not necessarily in budget.

I haven't yet approached our account team to ask about this.

Thanks in advance for any suggestions or pointers for further reading.

-Michael