Subject: Multiple Prefixes in RA

From: Vijayrajan ranganathan <[email-masked]>
Date: Thu, 1 Oct 2009 17:30:37 +0530

Hi,
Perhaps a naive question, but can somebody mention some practical use
cases for advertising multiple prefixes in a Router Advertisement?

Thanks & Regards
Vijay

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From: TJ <[email-masked]>
Date: Thu, 1 Oct 2009 08:35:05 -0400

Off the top of my head: A link that has multiple prefixes assigned to it;
perhaps a Global and a ULA or simply multiple Globals ...

/TJ

On Thu, Oct 1, 2009 at 8:00 AM, Vijayrajan ranganathan <[email-masked]>wrote:

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/TJ

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From: Christopher Morrow <[email-masked]>
Date: Thu, 1 Oct 2009 09:37:16 -0400

> perhaps a Global and a ULA or simply multiple Globals ...

right, like the original 'how to multihome in ipv6', one router
interface, 1 prefix from each upstream provider, auto-conf enabled ==
many prefixes in RA (or perhaps many RA's each with their own
prefix/default-route info, which may get confusing 'am I on X or Y or
Z?')

-chris

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From: Tim Chown <[email-masked]>
Date: Thu, 1 Oct 2009 14:42:59 +0100

Or temporary use of multiple prefixes for renumbering (rfc4192).

Some rogue RA deprecation tools might also use this (eg. rafixd/ramond).

I am not aware of ULAs being used commonly (read: not aware of use
in any campus environment as yet).

tim

-- 
Tim

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From: Brian E Carpenter <[email-masked]>
Date: Fri, 02 Oct 2009 09:40:43 +1300

Anyway, the bottom line is: running with multiple simultaneous
prefixes is absolutely standard use of IPv6 (and highly
unusual with IPv4). This is one of the features that people
will figure out how to use, as time goes on.

    Brian

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From: Mark Andrews <[email-masked]>
Date: Fri, 02 Oct 2009 10:23:48 +1000

In message <[email-masked]>, Vijay
> cases for advertising multiple prefixes in a Router Advertisement?

ULA + Gobal

Renumbering

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: [email-masked]

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From: [email-masked] (Arnaud Ebalard)
Date: Thu, 01 Oct 2009 16:55:43 +0200

Hi,

> cases for advertising multiple prefixes in a Router Advertisement?

I have an ULA prefix, a 6to4 prefix and a native IPv6 prefix advertised
internally on my networks. RFC 3484 does the rest.

Cheers,

a+

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From: Alexandru Petrescu <[email-masked]>
Date: Fri, 02 Oct 2009 16:49:27 +0200

> cases for advertising multiple prefixes in a Router Advertisement?

Let me give some more reasons which are non-standard.  One could qualify 
them as research.

I put several prefixes in a single RA.  RAs are exchanged by routers as 
they'd exchange routes with a dynamic routing protocol.  This is done in 
order to propagate routes.  It's not the main objective of RAs, but 
works ok.  Well, partially.

Alex

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From: Mark Smith <[email-masked]>
Date: Sun, 4 Oct 2009 12:21:24 +1030

On Fri, 02 Oct 2009 16:49:27 +0200

> works ok.  Well, partially.

What OS/environment are you using?

The reason I ask is because I've got a slightly odd scenario where
having a router listen to RAs (in my specific case, it's own RAs,
containing a 6to4 address derived from a dynamic IPv4 address
configured via PPP) and configure it's interface addresses from it would
be useful, but as far as I know if an interface is designated as a
forwarding interface, RAs are ignored.

I've forgotten the exact operational details, however Appletalk
supported routers announcing network numbers to each other
("cable-ranges"), and having the receiving router automatically
configure it's addresses. I seem to remember that the receiving router
might even automatically announce that learned prefix into the the IGP.

Regards,
Mark.

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From: Alexandru Petrescu <[email-masked]>
Date: Mon, 05 Oct 2009 11:19:15 +0200

Mark Smith a écrit :
> On Fri, 02 Oct 2009 16:49:27 +0200 Alexandru Petrescu
> <[email-masked]> wrote:
> 
>>> Hi, Perhaps a naive question, but can somebody mention some
>>> practical use cases for advertising multiple prefixes in a Router
>>> Advertisement?
>> Let me give some more reasons which are non-standard.  One could
>> qualify them as research.
>> 
>> I put several prefixes in a single RA.  RAs are exchanged by
>> routers as they'd exchange routes with a dynamic routing protocol.
>> This is done in order to propagate routes.  It's not the main
>> objective of RAs, but works ok.  Well, partially.
>> 
> 

Linux and radvd modified.

> configured via PPP) and configure it's interface addresses from it
> would be useful, but as far as I know if an interface is designated
> as a forwarding interface, RAs are ignored.

I think (not sure) radvd may have an option in the conf file allowing
the router to auto-configure an address based on the on-link prefix
present in the RA, even if it's a forwarding interface.

> configure it's addresses. I seem to remember that the receiving
> router might even automatically announce that learned prefix into the
> the IGP.

A-ha, good to know in a certain env the receiving router receiving an RA
may propagate that route in the IGP.

The routers I'm using don't auto-configure an address based on the
received RA; they install routes in the rt tables, towards the prefixes
present in the RA.  These prefixes are not the prefixes "on-link" on
which the RAs are sent, but are prefixes of the 'other' interfaces of
the routers.

This is for a star-like topology, where by definition the link-layer
doesnt'form loops... otherwise the technique may not survive loop forming.

Alex

> Regards, Mark.