Re: MTU

The system mtu command for the switch impacts the ports (both routed
and switched) of this device globally. This includes your VLAN
interfaces. Thanks for the clarification.

Warmest Regards,

Anthony J. Sequeira, CCIE #15626
http://www.INE.com

Test your Core Knowledge today!
Q: What switch feature detects misconfigured etherchannel ports and
puts those ports in error-disabled state?
A: EtherChannel Guard
More Info: http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/stp_enha.html

On Aug 9, 2009, at 3:51 PM, Pearson wrote:

> 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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Received on Sun Aug 09 2009 - 16:07:43 ART