RE: MTU

Hi Anthony,

Thx a lot for your reply. I always post/check the INE forum first :) no
offense to anybody but it looks friendlier to me

I replied to Alexey's post with a couple of additional questions regarding the
same topic. If you can follow up on it that would be great.

Thx a lot

> CC: ccielab_at_groupstudy.com
> From: asequeira_at_ine.com
> To: hadytannous_at_hotmail.com
> Subject: Re: MTU
> Date: Sun, 9 Aug 2009 15:38:21 -0400
>
> 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 Mon Aug 10 2009 - 13:20:52 ART