[Int-area] Where to aggregate, where to drop

[Dino Farinacci <[e-mail]>]

Tony, sorry for the delay on this. Thanks for your patience and for poking me about it.

For the list, I had a question about Tony bringing up *where* to do proxy aggregation and what the tradeoffs are. I asked a question about having a discussion about it. Lets use his slide below for reference to list the tradeoffs.




When a provider proxy aggregates, it means they will summarized more specific routes they have stored in their routing table. Like ISP-A above has routes P.1, P.2, and P.3. When ISP-A advertises a P prefix, it is indicating it can reach all more specifics, even though it may not have a full-set of those more specifics that are covered by P.

So here are the options:

(1) ISP-A advertises P to ISP-B (and may also advertise more specifics to other peers for policy reasons).
(2) ISP-A advertises P.1, P.2, and P.3 to ISP-B and ISP-B advertises P to its peers.

The questions is *where is the best place to aggregate*. Its a tradeoff on routing table savings and how far a packet can travel to either (1) get delivered to the destination or (2) get dropped by a router that doesn't have a more-specific for the destination (thereby wasting resources from the source to the drop point).

(1) If ISP-B aggregates P to the links you see above to stub peers, then the stubs can load-split on P and ISP-B can drop packets for say P.4 and deliver packets for P.3. The drop can happen at the PE router relatively soon.

(2) If ISP-B gets P from ISP-A and advertises to the links you see above to stub peers, then P.3 and P.4 packets move through ISP-B where the edge router in ISP-A delivers to P.3 and drops for P.4.

All things not considered and looking at this specific topology, I would vote for (1).

Another issue, which I think Tony brought up is, if P gets sent to peers, hijackers have a better opportunity to hijack routes by injecting more-specifics to direct traffic to a bad acting honey pot. This has been known for a long time though so we are not bringing up a new issue.

Another question is, if all more-specfics are not stored (due to major link failure), is the aggregate withdrawn from the routing system. That is, if you want less route flapping, you may just want to keep P advertised. That optimizes FIB add/delete entropy everywhere that wants to store P. I would rather have hardware routers drop packets fast, then to have route oscillation.

Comments?

Thanks,
Dino