IPv6 Deployment for the LAN

Owen DeLong owen at delong.com
Fri Oct 23 03:50:04 UTC 2009

On Oct 22, 2009, at 4:12 PM, Karl Auer wrote:

> On Thu, 2009-10-22 at 11:03 -0400, Kevin Loch wrote:
>>> If, on the other hand, the REAL desire is to have a DHCP server  
>>> break
>>> the tie in the selection between several routers that advertise  
>>> their
>>> presence, that wouldn't be unreasonable.
>> In some configurations not all hosts are supposed to use the same
>> router.  We need the _option_ to specify a default gateway and
>> have the override any RA's a host may see.
> It would be a tool, and if someone wants to use a tool, they can. It
> won't be my thumb they hit :-)
> But I can't see how a DHCP server can know enough about the routers to
> be able to send out useful discrimination information. So it will have
> to be manually entered, or come from an IPAM, or...
Current practice in the environments I know that are doing this is  
that groups
of hosts are maintained in a database (including MAC addresses) and
this database is used to build the DHCP configuration.  The host group
is assigned a default router address which is actually a VRRP group  

The routers then elect an appropriate VRRP active/standby  
configuration and
the hosts route via the Active router for their VRRP group.  If the host
administrators find that a host needs to be part of a different VRRP  
for whatever reason, there are tools at their disposal to address that  
DHCP lease times can be short since the addresses are actually static
anyway (yes, lots of people use DHCP to assign static addresses in
production environments because it allows table-driven central  
of host assignment).

> Nor can I see how the DHCP server can identify the routers to the host
> except by their addresses, and these can change or be removed without
> the DHCP server finding out.
In most environments I know, there are addresses reserved for the VRRP
groups that the routers participate in and the router administrators are
well aware of the damage they will bring if they change them without
extensive planning and notice.

> The only way I can see it working is if the host were smart enough to
> compare the DHCP router discrimination info with the information it  
> has
> received via RA and delete mismatches, or possibly just revert to  
> using
> RA information if any mismatches at all are detected. That would be an
> item the DHCP server could specify as well - what to do in case of a
> mismatch. It could even be specified on a per-router basis, though the
> whole thing seems to be getting a bit unwieldy now.
That would be a terrible choice because you have eliminated one of the
key reasons that some installations need DHCP to assign router  
instead of RA.  While what you propose is probably technically cleaner
from a pure protocol design perspective, the reality is that pure  
design is not how the real world thinks or operates.  In the real world,
one must make the protocol adapt to the business rules and other
odd parameters that don't always make logical sense from a protocol
design perspective. This is one such example when you have different
administrative groups responsible for hosts and routers.

<SARCASM>I know it is rare to find an enterprise where the network
infrastructure is not run by the same group that does the systems

But in many of these organizations, this means that having the
router specify the default gateway to the host is not going to work
well for the systems administrators. In today's world, they don't
have to worry about this and, the network group, surprisingly,
is pretty good at keeping the VRRP groups numbered as they
are supposed to be (usually .1, .2, etc. or .254, .253, .252, etc.,
or whatever the first/last addresses of a segment happen
to be).

> The DHCP servers will not be on the same subnets as all the routers
> involved, so they can't sniff the RAs themselves - unless we set up an
> RA relay... hmm.
They don't need to.

> I don't see DHCP-delivered router preferences as being something that
> will "break the Internet". In the vast majority of cases they will be
> unnecessary. For those that do need it though, and if it can be done,
> why not?
Why do router preferences instead of just routers?  Sure, the DHCP  
doesn't know which router should be doing the routing, but, VRRP can
take care of that as it does today.  The DHCP server just needs to  
the VRRP group.


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