The stupidity of trying to "fix" DHCPv6

Steve Clark sclark at
Fri Jun 10 19:21:27 UTC 2011

On 06/10/2011 09:37 AM, Ray Soucy wrote:
> 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
> you?

You are the moron - this stuff happens and wishing it didn't doesn't stop it. Get a clue!

> Honestly.  This whole argument is getting ridiculous.
> On Fri, Jun 10, 2011 at 9:32 AM, Leo Bicknell<bicknell at>  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 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
>>> technology.
>> 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.
>> Oops.
>> 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
>>   features.
>> 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 - CCIE 3440
>>         PGP keys at

Stephen Clark
Sr. Software Engineer III
Phone: 813-579-3200
Fax: 813-882-0209
Email: steve.clark at

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