[c-nsp] Sanity check OSPF/BGP

[adamv0025 at netconsultings.com]

> Drew Weaver
> Sent: Thursday, October 8, 2020 2:01 PM
> What I expect to happen is:
> 
>               The route to the peering edge router's loopback interface is
> withdrawn when OSPF/OSPFv3 closes.
>               The core router will close the BGP session when the route to
the dead
> peering edge router is withdrawn and will begin using one of the 5 other
> copies of the same route that it has.
>

Number of things come to mind since you provided no details regarding the
setup

Case A)
If all 5 peering points are not advertising best-external prefixes -then
there's only a single path for each of the 700K prefixes in the entire AS
via one of the 5 peering points.
-in case one peering point fails all prefixes it offered a best path for
will be withdrawn from all BGP speakers in the AS at OSPF convergence
speeds, but then the remaining 4 peering points needs to realize they now
have the overall best path for a given prefix and start advertising it to
all BGP speakers in the AS -tedious process that converges at "BGP-speed".

Case B)
If all 5 peering points are advertising best-external prefixes and all BGP
speakers in the AS already have all 5 paths available in RIB, but none of
the BGP speakers has hierarchical FIB so there's a direct correlation
between a prefix and it's NH, 
-in case one peering point fails all prefixes it offered a best path for
will be withdrawn from all BGP speakers in the AS at OSPF convergence
speeds, but now each BGP speaker will need to painstakingly update its FIB
on a prefix by prefix bases for each of the each of the 700K prefixes. 

Case C)
If all 5 peering points are advertising best-external prefixes and all BGP
speakers in the AS already have all 5 paths available in RIB, and all BGP
speakers not even have hierarchical FIBs but also PIC-CORE enabled where a
backup path for each prefix is programmed in FIB,
-in case one peering point fails all prefixes it offered a best path for
will be withdrawn from all BGP speakers in the AS at OSPF convergence speeds
and each BGP speaker will then just need to change 5 HN pointers to point to
remaining 4 peering points in FIB.


Note,
The above assumes full mesh between all BGP speakers or otherwise assumes
the RR infrastructure emulates full-mesh with regards to prefix distribution
to all BGP speakers in the AS via one of the several available mechanisms.  


Adam