Re: MTU

Hi Hady!

I want to make sure you saw this response from Alexey Tolstenok here
at INE:
Hi Hadi,

First of all let's stay in the scope of CCIE R&S lab equipment while
discussing it ;)

1. System MTU. It is changeable only on switches. It is a global
setting which is used to set the maximum _frame_ size on switched FE,
GE and routed ports. The switch does not support setting the MTU on a
per-interface basis. Usually, system MTU shouldn't be changed until
you configure q-in-q feature.

2. Interface MTU. It is the maximum unit (with all L2 and L3 headers)
that can be processed by physical port hardware.

It is changeable only on bult-in FE and GE ports of ISRs starting from
12.4(20)T IOS. Command is "mtu" under interface configuration mode.
You don't need to care about interface MTU until you configure some
additional services on these ports (EoMPLS, for example).

In other cases interface MTU adjusts automatically (for example,
enabling dot1q on router ethernet interface extends interface MTU
automatically from 1518 to 1522 bytes)

I suppose we will not face the task where we should change interface
MTU (in the current CCIE R&S lab).

3. IP MTU - maximum payload of IP packet (without IP and, of course,
L2 headers)

By default it is 1500 bytes. Could be changed through "ip mtu" command
in interface config mode. When it is the case? For ex., if you enable
additional services and cannot adjust interface MTU to let the whole
frame (IP MTU + all headers (IP, Ethernet, MPLS headers)) be processed
by egress physical port of your device. So you can decrease IP MTU
size on this interface to avoid dropping frames issues.

On Aug 9, 2009, at 7:24 AM, <hadytannous_at_hotmail.com> <hadytannous_at_hotmail.com
> wrote:

> Hi,
>
> What's the difference between the following types of MTUs ?
>
> 1- System MTU
>
> 2- Interface MTU
>
> 3- IP MTU
>
> they are a bit confusing to me. I read on a website online that the
> ethernet
> MTU is the same as the IP MTU !!! is that the case or they are
> mistaken ?
>
> Thank you in Advance,
>
>
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Received on Sun Aug 09 2009 - 15:38:21 ART