v6 subnet size for DSL & leased line customers

sthaug at nethelp.no sthaug at nethelp.no
Thu Dec 27 10:57:09 UTC 2007

> Personally, I'm not a big fan of DHCPv6. First of all, from a  
> philosophical standpoint: I believe that stateless autoconfiguration  
> is a better model in most cases (although it obviously doesn't support  
> 100% of the DHCP functionality). But apart from that, some of the  
> choices made along the way make DHCPv6 a lot harder to use than DHCP  
> for IPv4. Not only do you lack a default gateway (which is actually a  
> good thing for fate sharing reasons) but also a subnet prefix length  
> and any extra on-link prefixes. So even if you do address  
> configuration with DHCPv6 you need RAs for that other information.  

Which is probably going to make IPv6 deployment slower in service
provider environments.

> There's also tons of ways to complicate your life by mixing stateless  
> autoconf and DHCPv6, especially since most systems don't support  
> DHCPv6 unless you install additional software. Last but not least,  
> DHCPv6 has a stateful mode that's appropriate for address assignment  
> or prefix delegation, and a stateless mode that's more efficient for  
> the configuration of information that's not client-specific.  
> Unfortunately, it's up to the client to initiate the desired mode.  
> Then there are the M and O bits in RAs that tell you whether to do  
> DHCPv6 but a number of DHCPv6 aficionados look like they want to  
> ignore those bits.

Again, this is something that's going to slow down deployment in
service provider environments. Providers are comfortable with the IPv4
DHCP model (which is definitely not stateless) and many of us would
like to keep this.

> What this all boils down to is that if you want to deploy DHCPv6 you  
> need to install software on a lot of systems and modify a lot of  
> configurations. If you're going to do all that, it's easier to simply  
> configure this stuff manually. The only downside to that is that it's  
> not compatible with easy renumbering, but then, you need to do more  
> than just automate address assignment to support easy renumbering.

"Configure this stuff manually" may work for a small number of
customers. It is highly undesirable (and probably won't be considered
at all) in an environment with, say, 1 million customers.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no

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