subnet prefix length > 64 breaks IPv6?

Ray Soucy rps at
Wed Dec 28 19:14:14 UTC 2011

I did look into this a bit before.

To be more specific:

IPv6 CEF appears to be functioning normally for prefixes longer than
64-bit on my 720(s).

I'm not seeing evidence of unexpected punting.

The CPU utilization of the software process that would handle IPv6
being punted to software, "IPv6 Input", is at a steady %0.00 average
(with spikes up to 0.02%).

So there would seem to be at least one major platform that is OK.

On Wed, Dec 28, 2011 at 12:51 PM, Ryan Malayter <malayter at> wrote:
> On Dec 28, 9:44 am, Ray Soucy <r... at> wrote:
>> For what its worth I haven't stress tested it or anything, but I
>> haven't seen any evidence on any of our RSP/SUP 720 boxes that would
>> have caused me to think that routing and forwarding isn't being done
>> in hardware, and we make liberal use of prefixes longer than 64
>> (including 126 for every link network).  They might just be under
>> capacity to the point that I haven't noticed, though.  I have no
>> problem getting muti-gigabit IPv6 throughput.
> You can get >10GbE *throughput* from a Linux box doing all forwarding
> in software as well. That's easy when the packets are big and the
> routing tables are small, and the hash tables all fit in high-speed
> processor cache.
> The general lack of deep information about how the switching and
> routing hardware really works for IPv6 is my main problem. It's not
> enough to make informed buying or design decisions. Unfortunately, I
> have over the course of my career learned that a "trust but verify"
> policy is required when managing vendors. Especially vendors that have
> a near-monopoly market position.
> The problem, of course, is that verifying this sort of thing with
> realistic worst-case benchmarks requires some very expensive equipment
> and a lot of time, which is why the lack of solid information from
> vendors and 3rd-party testing labs is worrying.
> Surely some engineers from the major switch/router vendors read the
> NANOG list. Anybody care to chime in with "we forward all IPv6 prefix
> lengths in hardware for these product families"?

Ray Soucy

Epic Communications Specialist

Phone: +1 (207) 561-3526

Networkmaine, a Unit of the University of Maine System

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