Internet Exchanges supporting jumbo frames?

William Herrin bill at
Wed Mar 9 19:55:31 UTC 2016

On Wed, Mar 9, 2016 at 9:50 AM, Kurt Kraut via NANOG <nanog at> wrote:
> Thank you for replying so quickly. I don't see why the consensus for an MTU
> must be reached. IPv6 Path MTU Discovery would handle it by itself,
> wouldn't it? If one participant supports 9k and another 4k, the traffic
> between them would be at 4k with no manual intervention. If to participants
> adopts 9k, hooray, it will be 9k thanks do PMTUD.
> Am I missing something?

Hi Kurt,

As far as I know, there is no "discovery" of MTU on an Ethernet LAN.
That's Link MTU, not Path MTU. Unhappy things happen when the
participants don't agree exactly about the layer-2 MTU. It's a layer 2
thing; IPv6 and layer 3 pmtud can't discover that layer 2 problem.
Pmtud discovers when a link *reports* that the MTU is too small, not
when packets vanish as a result of a layer 2 error. As a result,
non-1500 byte MTUs in a network with multiple connected organizations
can be brittle: human beings are bad at each configuring their router
to exactly the same non-standard configuration as everybody else.

Other than that one problem, high MTUs at an IXP are a good thing, not
a bad one. Because IPv4 path MTU discovery is so badly broken, it's
desirable to maintain a minimum MTU of 1500 bytes across the core,
even as packets travel through tunnels, VPNs and other layered
structures that add bytes to the payload.

Greater than 1500 byte MTUs to the customer, on the other hand, are a
bad thing. The aggravate the problems with PMTUd.

Bill Herrin

William Herrin ................ herrin at  bill at
Owner, Dirtside Systems ......... Web: <>

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