BGP PIC (Prefix Independent Convergence) Core & Edge

[ReneMolenaar]

Hi Dominique,

@syncope988

P1 and P2 are (more/very) essential for IGP OSPF to find the routes between PE routers
When P1 fails, does the BGP table on P2 change just a little or do not change at all ? (for me do not change at all, but i am not sure)

The BGP entries in P2 won’t change since from P2’s perspective, nothing changes:

P2#show ip bgp

     Network          Next Hop            Metric LocPrf Weight Path
 * ia 8.8.8.8/32       7.7.7.7                  0    100      0 8 i
 *>i                   6.6.6.6                  0    100      0 8 i
 * ia 88.88.88.88/32   7.7.7.7                  0    100      0 8 i
 *>i                   6.6.6.6                  0    100      0 8 i

The next hop doesn’t change:

P2#show ip cef 6.6.6.6
6.6.6.6/32
  nexthop 192.168.56.6 GigabitEthernet0/4

In this topology, it’s not needed to enable hierachical FIB. It doesn’t hurt just to enable it on all routers though. I think on IOS XE, it’s even enabled by default.

The key thing to remember is that PIC core is useful when the path to get to the next hop changes. In this topology, that’s not the case for P2. Let’s imagine that in between P2 and PE3/PE4 we have a P3 and P4 router, P2 uses P3 as the path to get to PE3/PE4. When P3 fails, P2 has to reconverge to P4. That’s a scenario where a hierarchical FIB on P2 would be useful.

Hope this helps!

Rene