[j-nsp] weird MTU size on "show interface"

[Wojciech Owczarek]

This is just the way Juniper do things, just that it's not consistent
across all of their platforms. I've once been told by a Juniper
engineer that  1514 vs. 1518 MTU value is  displayed (on the EX
platform at least) because the 4 bytes of CRC are not taken into
account as the CRC is not part of the media MTU (which kind of makes
sense). Excerpt from "Configure the Media MTU" in Juniper docs:

"The actual frames transmitted also contain cyclic redundancy check
(CRC) bits, which are not part of the media MTU. For example, the
media MTU for a gigabit Ethernet interface is specified as 1500 bytes,
but the largest possible frame size is actually 1504 bytes; you need
to consider the extra bits in calculations of MTUs for
interoperability."

So while not really a problem, this does cause some confusion.

HTH,

Cheers,
Wojciech


-----Original Message-----
From: juniper-nsp-bounces at puck.nether.net
[mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Felix
Schueren
Sent: 01 October 2010 06:59
To: juniper-nsp at puck.nether.net
Subject: Re: [j-nsp] weird MTU size on "show interface"

On 01.10.10 07:26, Michel de Nostredame wrote:
> Hi,
>
> I was checking my EX4200 trying to resolve a strange connection
> problem with my vendor through a Metro Ethernet.
> During that time I found another weird situation (it is not related to
> the metroEthernet connection).
>
> I setup two topology to test
>
> EX4200.ae0 ===(LACP,trunk)=== ae0.M10
>
> On above setup, the interface link-level MTU on EX4200 side (interface
> ae0) shows MTU 1514. But on M10 the interface link-level MTU shows
> 1518.
> The 1518 is what I expect to have as a VLAN trunk interface should be.
>
the EXes do their (routed) vlans like most switches - in a weird way :)

I think this is a display bug - I see the same values (using 9.6) of
EX1514, MX1518 in a similiar setup.

I just tested this, and if I put an EX port in "family inet" mode, then
the display is correct, if I use "family ethernet-switching", then it's
incorrect but works anyway. The "interface vlan" shows an MTU of 1518,
and all my physical ports (ae or no) show 1514 yet do pass 1500 byte
frames correctly, regardless of whether I use vlan-tagging or pure
access ports. The displayed MTU is 1514 without vlan-tagging, and it
stays 1514 even with vlan-tagging.

Kind regards,

Felix

--
Felix Schüren
Head of Network

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Wojciech Owczarek