Re: MTU

Anthony,
Referring to Point 1 - System MTU as mentioned in the post is it used to
modify the mtu size of the only ROUTED PORTS - FE and GE does this mean the
mtu sizes are not modified for frames leaving trunk ports that carry traffic
from SVI's eg: VLAN interfaces. In other words if I'm peering with a
switch's SVI does that mean modifying system mtu on a switch will not affect
frames leaving the switch originated from the SVI. I might be redundant but
just making my question clear to you.

Pearson

On Mon, Aug 10, 2009 at 1:08 AM, Anthony Sequeira <asequeira_at_ine.com> wrote:

> 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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>>
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>
>
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Received on Mon Aug 10 2009 - 01:21:16 ART