MTU Question

Unanswered Question

If I have two switches connected to each other and one of the switches interfaces are configured for an MTU of 1546, while the other is configured with an MTU of 9216...what problems do I run into. I am also in an MPLS enviroment.


The problem that I'm thinking about is I have a switch that has a system MTU of 1546...my other switch only gives me the option of having the default MTU of 1500 or configuring 'mtu 9216'.


Will this cause my any issues?

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Giuseppe Larosa Fri, 10/10/2008 - 11:50

Hello Jeff,

mtu 9216 is jumbo frames.

What is important here is the L3 MTU on all L3 interfaces if the devices with MTU 9216 act only as L2 switches you are fine: no one is going to send packets that will use more then 1546 - MPLS stack bytes.

This is the check that you need to do.

If some device has L3 SVI interfaces try to reduce MTU on the L3 interface.

We actually do this way and MPLS VPN services are fine.

We have L2 trunks with MTU 9216 and L3 interfaces with MTU 1520 and is fine.

We use IS-IS and we reduced the clns mtu to 1497 in order to avoid to see input error as giants in some access layer switches.


Hope to help

Giuseppe


So...if I'm understanding this correctly..


- the Layer2 switch connection could have a MTU of 1546 on one interface and an MTU of 9216 on the other interface and be okay.


- if the switch has L3 interfaces I will just need to confirm a matching MTU on the other side . This could be achieved with 'ip mtu xxxx'.


Correct?

Giuseppe Larosa Fri, 10/10/2008 - 12:14

Hello Jeff,

I agree if L3 MTU match on both sides your setup should be fine


You could try to use mtu command under SVI L3 interfaces because this applies to MPLS too, ip mtu applies only to IPv4

OR

you set both ip mtu and mpls mtu on L3 interfaces.

with mpls_mtu -ip_mtu = mpls stack depth


Hope to help

Giuseppe


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