AW: /27 the new /24

Baldur Norddahl baldur.norddahl at
Sat Oct 3 13:18:12 UTC 2015

Except we might very well reach 1+ million routes soon without accepting
longer prefixes than /24. Also route updates is a concern - do I really
need to be informed every time someone on the other end of the world resets
a link?

On 3 October 2015 at 12:57, William Waites <wwaites at> wrote:

> 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?
> -w
> --
> 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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