  Interactions between the intra- and inter-domain routing protocols received
little attention despite playing an important role in forwarding transit
traffic. More precisely, by default, IGP distances are taken into account by
BGP to select the closest exit gateway for the transit traffic (hot-potato
routing). Upon an IGP update, the new best gateway may change and should be
updated through the (full) re-convergence of BGP, causing superfluous BGP
processing and updates in many cases. We propose OPTIC (Optimal Protection
Technique for Inter-intra domain Convergence), an efficient way to assemble
both protocols without losing the hot-potato property. OPTIC pre-computes sets
of gateways (BGP next-hops) shared by groups of prefixes. Such sets are
guaranteed to contain the post-convergence gateway after any single IGP event
for the grouped prefixes. The new optimal exits can be found through a single
walk-through of each set, allowing the transit traffic to benefit from optimal
BGP routes almost as soon as the IGP converges. Compared to vanilla BGP,
OPTIC's structures allow it to consider a reduced number of entries: this
number can be reduced by 99\% for stub networks. The update of OPTIC's
structures, which is not required as long as border routers remain at least
bi-connected, scales linearly in time with its number of groups.
