Re: [NDP] Router autoconfiguration with RS/RA

Alexandru Petrescu <alexandru.petrescu@gmail.com> Fri, 06 June 2008 21:17 UTC

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Date: Fri, 06 Jun 2008 23:16:57 +0200
From: Alexandru Petrescu <alexandru.petrescu@gmail.com>
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To: "Hemant Singh (shemant)" <shemant@cisco.com>
Subject: Re: [NDP] Router autoconfiguration with RS/RA
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Hemant Singh (shemant) wrote:
> Silviu,
> 
> A router can receive an RA on the router's upstream

Yes it can.  It uses it to report whether some things went wrong, log 
stuff, but don't act.

> and use this RA to autoconfigure the ipv6 address on interface(s) of
>  the router.

Usually no, it can not.  A particular case of a Mobile Router away from
home can auto-configure an address on its egress interface with
stateless autoconf.  But a non-mobile router (not implementing rfc3963)
can't and it shouldn't.

A router is something that forwards packets.  A linux router can't
auto-configure an address once one sets the forwarding=1.  A Cisco
router I have doubts, but it doesn't mean it follows rfc.

> Such a router interface configuration is no different from how a host
>  interface statelessly autoconfigures as per ND RFC 4861 and 4862. 
> However, ND RFC's do not mandate what does a router implementation do
>  for sending RA, configuring network prefixes in the router
> downstream direction - these are conceptual variables that a router
> vendor is left to do what they want to do.

Not sure what you mean left to vendors? there are some precisely defined
corner cases for configuring routers downstreams like DHCPv6-PD followed
eventually by Router Renumbering.

And, in most cases, assigning addresses to routers is part of a network
planning procedure performed by humans on paper, designed and
redesigned before being deployed; some call it architecting the
network.  That's a good reason for manually (or via SNMP, or other
proprietary tool) to configure addresses on routers, and not with
stateless autoconf.
> 
> As to answering your question which was:
> 
> "Why wouldn't a router be authorized to send Router Sollicitation 
> messages?"
> 
> here is my reply.
> 
> As far as the interface on the router has no RA configured, and the 
> interface is configuring an IPv6 address using stateless 
> autoconfiguration or even manual configuration, this interface is OK
>  to send an RS in the router downstream. However, soon as any RA 
> configuration for router downstream is configured on the network 
> interface, then ND prohibits a router to send any RS.
> 
> Furthermore, I totally agree with Remi on his reply to this question
>  of yours:
> 
> "The same question for autoconfiguring the prefix it advertises on 
> its subnets."
> 
> You cannot mix router upstream and downstream operations in random 
> fashion. IPv6 stateless autoconfiguration does not support prefix and
>  router configuration of an upstream router. One should be careful 
> discussing router downstream vs. router upstream directions for 
> address configuration, routing configuration, and IPv6 ND RA 
> configuration.

Well I wouldn't even talk upstream/downstream, just routers and maybe 
default-free routers are very special.

Alex


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