AW: /27 the new /24

William Waites wwaites at
Sat Oct 3 10:57:07 UTC 2015

On Sat, 3 Oct 2015 12:42:01 +0200, Baldur Norddahl <baldur.norddahl at> said:

    > 2 million routes will not be enough if we go full /27. This is
    > not a scalable solution. Something else is needed to provide
    > multihoming for small networks (LISP?).

It's not too far off though. One way of looking at it is, for each
extra bit we allow, we potentially double the table size. So with 500k
routes and a /24 limit now, we might expect 4 million with /27. Not
exactly because it depends strongly on the distribution of prefix
lengths, but probably not a bad guess.

Also there are optimisations that I wonder if the vendors are doing to
preserve TCAM such as aggregating adjacent networks with the same next
hop into the supernet. That would mitigate the impact of wanton
deaggregation at least and the algorithm doesn't look too hard. Do the
big iron vendors do this?


William Waites <wwaites at>  |  School of Informatics       | University of Edinburgh             |      HUBS AS60241

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
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