The stupidity of trying to "fix" DHCPv6
rps at maine.edu
Fri Jun 10 08:37:11 CDT 2011
You really didn't just write an entire post saying that RA is bad
because if a moron of a network engineer plugs an incorrectly
configured device into a production network it may cause problems, did
Honestly. This whole argument is getting ridiculous.
On Fri, Jun 10, 2011 at 9:32 AM, Leo Bicknell <bicknell at ufp.org> wrote:
> In a message written on Fri, Jun 10, 2011 at 01:03:01PM +0000, Bjoern A. Zeeb wrote:
>> On Jun 10, 2011, at 10:10 AM, sthaug at nethelp.no wrote:
>> > Several large operators have said, repeatedly, that they want to use
>> > DHCPv6 without RA. I disagree that this is stupid.
>> I wonder if it's just a "violation" of rule #1: stop thinking legacy!
>> People are used to what they have done for a decade or two. It's hard to
>> see the change and results in "why is this all so different and complicated?".
>> It's hard to open ones mind for the new, but it is essential to do with new
> The problem in this case is that the failure modes are significantly
> different. Some folks have learned this the hard way.
> It's a very easy scenario to reconstruct. Consider the "branch
> office router" in a typical corporate enviornment. We're talking
> a device with one WAN port, and one LAN port. Configure it for
> dual stack, speaking IPv4, and in IPv4 configure it the typical
> corporate way with a "DHCP Helper" forwarding requests over the WAN
> to a central DHCP server. In IPv6, configure it with RA's, the
> supposed "better" way.
> Now, take the 100% working branch router and have it sent back to
> corporate. Maybe they got a bigger router, maybe the office closed.
> A network engineer gets the router and is tasked with making it
> ready to redeploy.
> The network engineer plugs it into the switch on his desktop, plugs in a
> serial cable, turns it on and steps out to get a coffee while it boots.
> He's planning to erase the configuration and then load new software over
> the network.
> As soon as the router boots the IPv6 network fails for all the users on
> his subnet. IPv4 keeps working fine.
> What happened? Well, the router sent IPv6 RA's as soon as it came
> up, and every workstation instantly started using them. In IPv4,
> the router received DHCPv4 requests and forwarded them per the
> helper address, except that its WAN port is down, and thus it in
> fact didn't send them anywhere.
> The important points:
> - IPv4 "failed safe" with the DHCP config. This "rogue device" will
> never disrupt the IPv4 configuration. DHCP snooping isn't even needed
> in your switches, since it never returns a response.
> - IPv6 "failed immediately" with the RA configuration. What's worse is
> if you simply turn the device off after you realized you took down the
> entire network devices will continue to be broken for 2-4 hours until
> the RA's time out. The only method to mitigate is to deploy RA guard
> on all of your switches, which probably means replacing 100% of your
> hardware with new stuff that can do that, and then deploying new
> The fact of the matter is that the failure modes of these two
> protocols are vastly different operationally. The DHCP failure
> semantics are not only better understood, but cause less disruption
> to the network. Even a properly rouge DHCP server will only damage
> _new_ clients coming up on a network, existing folks will work just
> fine. Contrast with RA's which instantly break 100% of the users.
> Even more annoying is that if I use RA's for the default gateway,
> I still have to run DHCPv6 anyway. If I don't my boxes don't have
> DNS servers, NTP servers, know where to tftpboot, etc. It's not a
> choice of one or the other, it's I always run DHCPv6, do I need
> RA's or not.
> Given the failure modes I would much prefer to run with RA's turned off
> completely, and have DHCPv6 able to provide a default gateway just as it
> works in IPv4.
> My opinion comes not from "thinking legacy", indeed my employer has been
> fully dual stacked since 2003. My opinion comes from the fact that in
> the 8 years of operational experience we have RA's are significantly
> more fragile, and IMHO not ready for widespread IPv6 deployment.
> Leo Bicknell - bicknell at ufp.org - CCIE 3440
> PGP keys at http://www.ufp.org/~bicknell/
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