> On Tue, 24 Jul 2012, Sam Wilson wrote:
>
> > [VLAN number in IP address]
>
> I'm averse to using the VLAN number, simply because I have chopped and
> changed VLAN allocations over the years to account for topological changes
> (a process largely invisible to end users), and often those changes
> haven't required IP renumbering (a process very visible to some end users
> or their technical support staff), and I wouldn't want to have introduced
> that relationship without a particularly good reason.
We haven't found it particularly onerous except in the opposite case -
where we've needed to change the IP address range without changing the
VLAN number - but it's only been a problem once that I can think of.
> In (our) IPv4 world, carving out classic /24 chunks of a /16 and matching
> the third octet with the VLANid is common, but starts to break down as
> soon as you start doing networks of sizes other than /24. Once you've
> broken that link for one not-insignificant set of 'exceptions', then it
> seems to me to be a bit pointless introducing that same relationship into
> a potentially pristine new scheme.
We have lots of different sizes of IPv4 subnet, everything from /32 to
/22 (though I don't think we have any /31s). The /22s to /24s and
anything else with zeroes in the 4th octet gets its VLAN numbered by the
3rd octet. There are similar schemes with offsets for various chunks of
the RFC1918 space and everything else gets an arbitrary assignment from
the numbers that are left. It's really not been a problem.
> Putting it another way; while I might chose to take the opportunity to do
> IPv6 numbering in a more logical structured way according to topologies,
> potential for route aggregation, and so on, I'm extremely unlikely to
> start changing the extant VLAN ids to match that neat IPv6 addressing in
> many cases.
Even with 700 IPv4 routes on campus I don't see any major value in doing
route aggregation within the LAN and don't expect to with IPv6.
Sam
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