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New Member

MTU and Fastrthernet interface

I want to implement MPLS L3 VPN service on a network using 7206 and 3640 routers that have FE interfaces

But it seems that MTU is not configurabel on FE interfaces

When i want to chaneg MTU settign i get this mwssage

"Interface FastEthernet0/0 does not support user settable mtu."

What it really mean ?

Am i able to use FE for MPLS and support 2 MPLS label ?


Re: MTU and Fastrthernet interface

Yes you can very well configure MPLS switching with a 2 label stack on an FE interface.

You can define the Edge/CE offered MTU + 4 bytes per label i:e in your case 1508 as the MPLS MTU. This will enable the frame to be fragmented but still the fragments retain the original label mapping.

Caveat: If you edges have a DF bit set at any time then the process fails as there cannot be any fragmentation, this is when you will need interfaces which are Core Class, supporting larger configurable MTU's.



New Member

Re: MTU and Fastrthernet interface

suppose that CE is conected to PE via FE

PE is also connected to MPLS cloude usign FE

MTU on FE is 1500

If i use "MPLS MTU 1500" command on PE interface that is connected to MPLS cloud and CE send a packet with MTU size larger than 1500 , will PE defragment this packet (If DF is not set) prior to transmission out of the outbound intarface ? and all the packets entering MPLS cloud will have 1500 MTU with 2 label added ?

Re: MTU and Fastrthernet interface


some remarks. Whether an interface supports labeled frames with an mtu larger than 1500 depends also on the hardware. So in your case it might mean it wont work.

Second, labeled packets are never fragmented, they are discarded. Only IP packets can be fragmented. Hence the need to adjust the MTU to accomodate the additional overhead.

Be aware that the MTU must only be adjusted on interfaces where labels are used. On PE-CE interfaces only IP traffic is received and thus only 1500 Bytes are sufficient.

Hope this helps! Please rate all posts.

Regards, Martin

Re: MTU and Fastrthernet interface


A small note to add, the previous reply was in reference to the RFC 3032, section 3.2//

3.2. Maximum Initially Labeled IP Datagram Size

Every LSR which is capable of

a) receiving an unlabeled IP datagram,

b) adding a label stack to the datagram, and

c) forwarding the resulting labeled packet,

SHOULD support a configuration parameter known as the "Maximum

Initially Labeled IP Datagram Size", which can be set to a non-

negative value.

If this configuration parameter is set to zero, it has no effect.

If it is set to a positive value, it is used in the following way.


a) an unlabeled IP datagram is received, and

b) that datagram does not have the DF bit set in its IP header,


c) that datagram needs to be labeled before being forwarded, and

d) the size of the datagram (before labeling) exceeds the value of

the parameter,


a) the datagram must be broken into fragments, each of whose size

is no greater than the value of the parameter, and

b) each fragment must be labeled and then forwarded.

For example, if this configuration parameter is set to a value of

1488, then any unlabeled IP datagram containing more than 1488 bytes

will be fragmented before being labeled. Each fragment will be

capable of being carried on a 1500-byte data link, without further

fragmentation, even if as many as three labels are pushed onto its

label stack.

In other words, setting this parameter to a non-zero value allows one

to eliminate all fragmentation of Previously Labeled IP Datagrams,

but it may cause some unnecessary fragmentation of Initially Labeled

IP Datagrams.

Note that the setting of this parameter does not affect the

processing of IP datagrams that have the DF bit set; hence the result

of Path MTU discovery is unaffected by the setting of this parameter.



New Member

Re: MTU and Fastrthernet interface

Hi Pooria

I faced the same problem. PE (3640 router) is connected to MPLS cloud with FE interface. Some applications working behind 3640 router needs to not fragmented. So I can try yo write "ip mtu 1600", however it can only adjust to 1500.

How did you solve this problem?



New Member

Re: MTU and Fastrthernet interface

Try tag-switching mtu 1508

New Member

Re: MTU and Fastrthernet interface

Use the "mpls MTU 15xx" under the interface configuration

New Member

Re: MTU and Fastrthernet interface

I would think that if the maximum physical interface mtu supported in your case is 1500, then you'd want to set the ip mtu to some multiple of 4 less than that, to allow for 2 or more mpls labels on top of the IP packet.

So wouldn't it be more correct, under the FastEthernet interface, to do something like so?

interface FastEthernet1/0

mtu 1500

mpls mtu 1500

ip mtu 1488

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