re: [RRG] Opportunistic Topological Aggregation in the RIB->FIB Calculation?

[Xu Xiaohu <xuxh@huawei.com>]

> 1. Determine which exit has the most routes pointing at it.
> 2. Add 0.0.0.0/0 pointing at that exit.
> 3. Mark the routes that point in that direction and aren't cutouts of
> a larger route as non-FIB. These won't be imported into the FIB.
> 4. Mark the rest of the routes for import into the FIB.

Let me take an example, route 1.1.1.0/24 is pointed to interface A, while
route 1.1.0.0/16 is pointed to interface B. if most routes are pointed to
interface A, in your theory, those routes with interface A as exit
interface, including route 1.1.1.0/24, will be replaced by 0.0.0.0/0 in the
FIB. This will cause a wrong route for packets to 1.1.1.0/24. Is my
understanding correct?

Xiaohu XU

> 5. Look at the routes in the two /1's inside that /0, considering both
> the marked and unmarked ones. Do any of the /1's have the majority of
> the routes pointing towards a different exit than the /0? If so, add
> the /1 and change the markings on the subroutes accordingly.
> 6. If both /1's point a different direction than the /0, remove the /0.
> 
> 7. Inside each /1, repeat steps 5 and 6 using /2's.
> 8. Repeat for /3, /4, ... /8
> 
> 
> In theory, this should result in identical routing for all routes
> which are actually in the RIB while cutting the FIB size
> significantly. In the worst case you should see FIB reduction equal to
> 1/(number of BGP interfaces) of the RIB table. Near the edge you
> should normally get much more.
> 
> The obvious weakness of this approach is that packets which would
> ordinarily generate a host-unreachable will instead tend to fall into
> an odd routing loop until the TTL expires. Other than that, would it
> work?
> 
> 
> Comments? Criticism? Further thoughts? It is Monday morning and I
> haven't thoroughly vetted the idea so it may be DOA.
> 
> Regards,
> Bill Herrin
> 
> 
> --
> William D. Herrin ................ herrin@dirtside.com bill@herrin.us
> 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
> Falls Church, VA 22042-3004
> 
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