Stupid Ipv6 question...

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1

Date: Fri, 19 Nov 2004 10:11:36 -0800
From: Crist Clark <crist.clark@globalstar.com>
Sender: owner-nanog@merit.edu

Lars Erik Gullerud wrote:

>
>
>>/127 prefixes are assumed for point-to-point links, and presumably an
>>organization will divide up a single /64 for all ptp links -- unless they
>>have more than 9,223,372,036,854,775,808 of them.
>
>
> While that would seem logical for most engineers, used to /30 or /31 ptp
> links in IPv4 (myself included)

Aren't most engineers used to the fact that point-to-point links are
not broadcast links and therefore the concept of a network/netmask for
the interface is somewhat useless? In addition, link-local addressing
eliminates many situations where you need to allocate tiny blocks for
p2p links.

Just to introduce a touch of practicality to this discussion, it might
be worth noting that Cisco and Juniper took the RFC stating that the
smallest subnet assignments would be a /64 seriously and the ASICs only
route on 64 bits. I suspect that they influenced the spec in this area as
expending them to 128 bits would have been rather expensive.

In any case, if the prefix length is >64, routing is done in the
CPU. IPv6 traffic for most tends to be light enough that this is not a
big issue today, but the assigning /126 or /127s for P2P links is
really, really not a good idea. the use of 127s also ignore the
possibility of a anycast address.

2

Just to introduce a touch of practicality to this discussion, it might
be worth noting that Cisco and Juniper took the RFC stating that the
smallest subnet assignments would be a /64 seriously and the ASICs only
route on 64 bits. I suspect that they influenced the spec in this area as
expending them to 128 bits would have been rather expensive.

  darn... and we fought so hard last time we had to expunge
  classfull addressing asics/hardware in the late 1990s.
  looks like it crept back into vendor gear. IPv6 was -never-
  supposed to be classful.

--bill

3

While the concept of classes has changed, I'm not so sure that I agree with
the complaint here...

Everything I've seen about the multi TLA/SLA concepts always seem to leave
64 bits at the end for the actual host address, so it would be a logical
step at that point to have the ASICs spun so that 64 bits was the limit for
routing tables.

Perhaps I have had the same assumption/misunderstanding that the programmer
guys have had then?!?!?

Scott

4

Engineers at Juniper seem to be telling me that this is definitively not the case for their M- and T-series routers. Which routers were you referring to?

Joe