CIDR Aggregation Tool

Avi Freedman freedman at
Mon Nov 27 04:00:49 UTC 1995

Well, after observing the debates about What To Do About The Routing Table
Size, I decided to work on some informal tools to examine the current
routing table space for possible aggregations.

Right now, the raw data is the routing table from the Net Access MAE-East

It only searches for aggregates /15 < x < /25 right now - ignoring some
fairly obvious aggregation-suggestions in the /8 < x < /16 range.

The results are up at and the some of the caveats 
are listed on the web page, but are:

1) When the Net Access MAE-East router has multiple identical routes that 
   point to the same next-hop (192.41.177.x), the system only uses the one 
   first listed in the cisco 'sho ip bgp summ' output - even though that's
   not always the 'best' route.

2) When an AS advertises both an aggregate and a specific, the specific 
   is 'dropped' by the aggregator. If the input is:
   {,}, the output will be:
   {} ( will be dropped).

3) The source of data is the Net Access MAE-East router routing table, 
   and we don't peer with all parties at MAE-East directly - thus, it's 
   possible that the system catches aggregations that are impossible
   because they're announced to a 3rd party via different paths - but
   an informal look around doesn't appear to indicate that.

4) The system goes by next-hop rather than by AS-Path.  There seems to
   be a good correlation, but ...

Also, the final disclaimer: I've examined the results and they *look* 
reasonable.  But I haven't written tester-tools to attempt to verify
by another algorithm that the results are correct.

Here's the table at the end of the web page:

                           Before   After 
                           Run      Agg. Run ibm             81     68 digex           98     98         949    775 nsn.nasa       126    112 ans           2146   1425 agis           539    304 uscyber          2      2 interpath        2      2 net99          309    246 mci              2      2 mci           8963   6828 pipex          367    308 netcom         370    348 psi              1      1 icp           4499   3712 sprintlink    6429   4694 psi           1491   1098 alternet      3971   3148          96     88         122    101   suranet        470    445  internex         1      1  ios             10      8  cais           184    158  hlc            354    243  energis          2      2  delphi           3      3

A final note:  We're open to {Additional static sources of route tables at
MAE-East; Additional static sources of route tables at {MAE-West, Pennsauken,
or the PacBell NAP}; and dynamic (i.e. the vty password so we can run
a sho ip bgp summ every so often) sources at any of the MAEs or NAPs.

If we get good feedback, we may automate this to run overnight and keep
a history.

Avi Freedman
freedman at

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