subnet prefix length > 64 breaks IPv6?

sthaug at sthaug at
Wed Dec 28 14:50:45 UTC 2011

> > Can you please name names for the "somewhat less efficient" part? I've
> > seen this and similar claims several times, but the lack of specific
> > information is rather astounding.
> Well, I do know if you look at the specs for most newer L3 switches,
> they will often say something like "max IPv4 routes 8192, max IPv6
> routes 4096". This leads one to believe that the TCAMs/hash tables are
> only using 64 bits for IPv6 forwarding, and therefores longer prefixes
> must be handled in software.

It might lead you to believe so - however, I believe this would be
commercial suicide for hardware forwarding boxes because they would no
longer be able to handle IPv6 at line rate for prefixes needing more
than 64 bit lookups. It would also be an easy way to DoS such boxes...

> This may very well not be true "under the hood" at all, but the fact
> that vendors publish so little IPv6 specification and benchmarking
> information doesn't help matters.

Cisco actually has published quite a bit of info, e.g.

"Delivering scalable forwarding Performance: up to 400 Mpps IPv4 and
200 Mpps IPv6 with dCEF"

They have also published EANTC tests which include IPv6 forwarding rates.

Steinar Haug, Nethelp consulting, sthaug at

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