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Re: [v6ops] DHCPv6/SLAAC Make Hosts Confusing-//RE: new draft: draft-liu-bonica-v6ops-dhcpv6-slaac-problem

Mark ZZZ Smith <markzzzsmith@yahoo.com.au> Tue, 29 October 2013 19:17 UTC

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Date: Tue, 29 Oct 2013 12:16:48 -0700
From: Mark ZZZ Smith <markzzzsmith@yahoo.com.au>
To: Andrew Yourtchenko <ayourtch@cisco.com>
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Cc: "v6ops@ietf.org" <v6ops@ietf.org>, Ted Lemon <mellon@fugue.com>, "Ole Troan (otroan)" <otroan@cisco.com>, Dave Thaler <dthaler@microsoft.com>, "draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org" <draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org>
Subject: Re: [v6ops] DHCPv6/SLAAC Make Hosts Confusing-//RE: new draft: draft-liu-bonica-v6ops-dhcpv6-slaac-problem
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----- Original Message -----
> From: Andrew Yourtchenko <ayourtch@cisco.com>
> To: Mark ZZZ Smith <markzzzsmith@yahoo.com.au>
> Cc: Lorenzo Colitti <lorenzo@google.com>; "draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org" <draft-liu-bonica-v6ops-dhcpv6-slaac-problem@tools.ietf.org>; "v6ops@ietf.org" <v6ops@ietf.org>; Ted Lemon <mellon@fugue.com>; Ole Troan (otroan) <otroan@cisco.com>; Dave Thaler <dthaler@microsoft.com>
> Sent: Wednesday, 30 October 2013 1:05 AM
> Subject: Re: [v6ops] DHCPv6/SLAAC Make Hosts Confusing-//RE: new draft: draft-liu-bonica-v6ops-dhcpv6-slaac-problem
> 
> Hi,
> 
> On Tue, 29 Oct 2013, Mark ZZZ Smith wrote:
> 
>>  Hi,
>> 


<snip>

> 
>> 
>> 
>>  - "While architecturally "pure", the reliability and timing 
> of the 
>> routing resiliency provided by the RAs is far below those achieved by 
>> FHRP protocols which are used in today's networks predominantly." 
> This 
>> seems to suggest that you can't run VRRP/HSRP if you're using RAs. I 
> 
>> don't think they're mutually exclusive, as long as the RAs come from 
> the 
>> virtual router link-local address, rather than the routers' physical 
>> interface link-local addresses.
> 
> No, this is a long-winded way to say that RA as a routing 
> mechanism is useless a lot of today's networks' requirements, not that 
> it 
> is incompatible. :-)
> 

So I'm afraid I'm still confused by this.

The reason why you need VRRP/HSRP etc. is because ND NUD isn't acceptably fast enough to detect that a default gateway has gone away. It might be possible to lower the NUD timers such that they detect default gateway death much quicker, however you then have all of the hosts actively probing the default gateway very quickly, creating load on its control plane.

VRRP/HSRP avoids this control plane load by making the the switch between routers transparent or nearly transparent to the hosts, without the need to lower NUD timers. It could be viewed as a NUD special case and optimisation.

This all has nothing to do with RAs or DHCPv6 - the need for VRRP is to overcome a performance limit in ND NUD, and that limitation would still exist in a DHCPv6 only model without VRRP/HSRP. So I don't understand why DHCPv6 is or woud be "better" and RAs are "worse" in this scenario.


Regards,
Mark.

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