Help with DHCPv6 client-identifiers

sthaug at nethelp.no sthaug at nethelp.no
Sat Nov 19 16:28:19 UTC 2011


> > Of course using prefixes wider than /64 doesn't break IPv6. It
> > breaks SLAAC, which is not at all the same thing. [ ... ]
> 
> Well, it depends on what one means by "prefixes wider than /64"
> and "break IPv6".
> 
> A core decision about IPv6 has been that only /64 is *generally
> routable*. That is, inter-site routers (BPv6) are not required
> to route on any longer prefix. Indeed arguably even intra-site
> routers are not required to route on prefixes longer than /64.
> 
> This is not because it would not work for technical reasons;
> conceivably the routable prefix could be anything up to /128.
> 
> It is just a matter of declaring it so as a convention (even if
> no doubt motivated by the efficiencies of having routers only
> keep track of 64 instead of up to 128 bits).

This "convention" has been dead for many years. Routers route on
all 128 bits.

> In practice:
> 
> * It is well possible to make many routers, e.g. Linux based
>   ones, to route on prefixes longer than /64.

Let's turn it around - can you give examples of routers that can
*not* route on the full 128 bits?

(And yes, I am aware of some Cisco products with hardware based
forwarding that cannot do ACLs with full 128 bit IPv6 addresses
and port numbers. But they still *route* on the full 128 bits...)

> * It is rather unwise to do so, because it may trigger corner
>   cases, especially in networks exposed to the Internet.

Then those corner cases need to be found and fixed. However, I
believe your concern is unfounded. Longer than 64 bits prefixes
work just fine in practice. 128 bits, no magic.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no



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