Lack of IPv6 traffic stats makes judging progress difficult

Gert Doering gert at
Fri Jun 3 20:56:11 CEST 2011


On Fri, Jun 03, 2011 at 02:24:18PM -0400, Todd Snyder wrote:
> please forgive my ignorance, I'm a systems guy in a networking world, tasked
> with doing some ipv6 monitoring and trying to learn as I go.  What is the
> difference and why would it be deceiving?  

Well, Cisco gear has multiple ways a packet that comes in on interface A
and goes out on interface B can be handled, like "the forwarding hardware
will grab it and move it over" or "something in the packet is too 
complicated for the ASIC, so it's passed to the CPU, stared at, and then
moved forward".

In an ideal world, the difference should not matter (except for the CPU
load, and possible additional latency).

In the real world, some vendors take shortcuts, like "we have full hardware
support for IPv6!!" and then they forget to implement hardware counters for
packets moved via the hardware forwarding engine.  Worse, on the Cisco 6500
and 7600, the IPv6 packets moved by the HW engine are counted as IP*v4*
packets.  It's lying to you -> "deceiving"

... and if the CPU touches the packet, the counting is done by "software",
and that one has working IPv6 counters, so you'll see some stats go up, but
they are not representative for the real amount of IPv6 traffic going
*through* the box.

> How would one know where the stats are coming from?

By staring at numbers that make no sense, and then opening TAC cases
to figure out why they are the way they are...

If you want to verify the numbers for yourself - take an otherwise idle 
link (so you have a clean baseline), push through some Gbytes of IPv6 
traffic, look at the counters, then push some Gbytes of IPv4 traffic,
and look again.  Do not try "ping the router" - that's CPU-handled, and
will be counted differently...

(If I ever have time, I'll write a book "the story of my life with Cisco
counter bugs" - but I'm already forgetting the specifics, like "what 
was the trigger that made IOS 11.2 show 3.5 Mbit/s average on a E1 
line" :-) )

Gert Doering
        -- NetMaster
did you enable IPv6 on something today...?

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