[c-nsp] IPv6 p2p transit link addressing

[Billy Guthrie]

cisconsp at secureobscure.com wrote:
> I have been working through a timeless topic: p2p transit link addressing.
>
> After reading http://tools.ietf.org/tools/rfcmarkup/rfcmarkup.cgi?rfc=3627
> and http://tools.ietf.org/html/draft-ietf-ipv6-addr-arch-v4-04 as well a
> networkers presentation, various blogs, etc. I thought it was time to
> consult people who might actually be dealing with this in production today
> in hopes that I can learn from any potential mistakes.
>
> Can I simply avoid the problem of p2p links altogether with unnumbered from
> an ipv6 loopback address?
> I fear that link local addressing will ruin traceroute information.
>
> Any definitive answers would be greatly appreciated. I would prefer not to
> burn up all of my routable address space with p2p /64's however.
>
> Thanks for your time,
>
> John
>
>
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/
>
>   
There has been some heated debates on the subject with no real 
definitive answer; I have seen /64s, /112, /120, /126 and /127s.

Steve Leibson takes a shot at putting it in real world terms:

*/ So we could assign an IPV6 address to EVERY ATOM ON THE SURFACE OF 
THE EARTH, and still have enough addresses left to do another 100+ 
earths. It isn't remotely likely that we'll run out of IPV6 addresses at 
any time in the future! */

The point being is that this analogy is simply to show that there are an 
awful lot of addresses and that you should not be too concerned with 
conserving address space; however, this is to be taken with a grain of 
salt. I would highly recommend that your address policies should avoid 
the assignments in a unnecessarily wasteful fashion because it does 
start to deteriorate from 2128 to 264 (from 340,282,366,920,938,463,46
3,374,607,431,768,211,456 to 18,446,744,073,709,551,616. In all 
honestly, if you are a large network, then you were more than likely 
allocated a /32 which only gives you a 8-16 bit improvement (16 bit 
improvement if all you assign are /48s). Once you start planning a 
practical address plan, the IPv6 allocation that you were assigned is 
not that big; nothing more than a class B from the v4 land.

Here is some additional reading and references:

*In RFC5375 Section 2.4.1 and 2.4.2*
"Address conservation requirements are less stringent in IPv6, but they 
should still be observed."

*In RFC5375 B.2. Considerations for Subnet Prefixes Longer than /64*
"The following subsections describe subnet prefix values that should be 
avoided - B.2.1. /126 Addresses"

*In RFC3177 Section 3 (Address Delegation Recommendations) *
"/64 when it is known that one and only one subnet is needed by design."

*http://www.getipv6.info/index.php/IPv6_Addressing_Plans 
<http://www.getipv6.info/index.php/IPv6_Addressing_Plans> - Point 10*.
"IETF expects that you will assign a /64 for point-to-point links"

In my professional opinion, allocate a /64 and assign a /126 to a point 
to point link; do not assign anything else out of that /64.
I am sure there will be tons of response to your question and you will 
not get on definitive answer; you as an engineer will need
to take all the information and based on your design and topology will 
need to make that decision.

Good Luck

Billy