Superfluous advertisement (was: Friday's Random Comment)

["Jakob Heitz (jheitz)" <jheitz () cisco com>]

A use case for a longer prefix with the same nexthop:

   F
  / \
 D   E
 |   |
 B   C
  \ /
   A

Suppose A is a customer of B and C.
B has a large address space: 10.1.0.0/16.
B allocates a subset to A: 10.1.1.0/24.
B advertises the longer prefix to its backup provider C.
C propagates it to E and then to F.
B MUST advertise both 10.1.0.0/16 and 10.1.1.0/24 to D.
D MUST propagate both of them to F.
Otherwise, if F only receives 10.1.0.0/16 from D, then
F will have the longer match 10.1.1.0/24 to E,
but E is only the backup route.

Thanks,
Jakob.

> -----Original Message-----
> Date: Fri, 29 Apr 2016 08:17:41 -0400
> From: Alain Hebert <ahebert () pubnix net>
> To: "'NANOG list'" <nanog () nanog org>
> Subject: Friday's Random Comment - About: Arista and FIB/RIB's
> Message-ID: <00ea292f-e779-25ad-ce89-eae897e9516d () pubnix net>
> Content-Type: text/plain; charset=utf-8
>
>     While following that Arista chat...  That reminded me of that little
> afternoon project years ago.
>
> So I decided to find new hamsters, fire up that VM, refresh the DB's and
> from the view point of a tiny 7206VXR/G1 with 2 T3 peers...
>
>     The amount of superfluous subnet advertisement drop to ~120k from
> ~166k from the previous snapshot.
>
>     And this is the distribution by country.
>
>       country       | superfluous
> --------------------+-------------
>  United States      |     28254
>  Brazil             |     10012
>  China              |      7537
>  India              |      6449
>  Russian Federation |      4524
>  Korea, Republic of |      4062
>  Saudi Arabia       |      3297
>  Australia          |      2989
>  Indonesia          |      2878
>  Hong Kong          |      2251
>  Thailand           |      2093
>  Canada             |      2019
>  Taiwan             |      1955
>  Ukraine            |      1877
>  Singapore          |      1856
>  Bulgaria           |      1488
>  Argentina          |      1436
>  Japan              |      1403
>  Mexico             |      1351
>  Chile              |      1271
>
>     (Damn Canada, can't break top 10 again).
>
>     PS: "Superfluous" is a nice way to say that the best path of a
> subnet is the same as his supernet.  And yes I'm aware of the Weekly
> Routing Report, I was just curious to see it by country =D.
>
> -----
> Alain Hebert                                ahebert () pubnix net
> PubNIX Inc.
> 50 boul. St-Charles
> P.O. Box 26770     Beaconsfield, Quebec     H9W 6G7
> Tel: 514-990-5911  http://www.pubnix.net    Fax: 514-990-9443