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New Member

MTU setting on FastEthernet


Is it possibel to set MTU on Fastethernet ?

I was trying to change MTU on 7206 and i got this error

config-if)#ip mtu ?

<68-1500> MTU (bytes)

config-if)#mtu 1510

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


Re: MTU setting on FastEthernet

The message means that the MTU cannot be configured for the datalink for the specific hardware in question.

If you are trying to set the MTU to support MPLS label stack,

1) If you have not configured MPLS MTU, then any 1500 bytes packet would be fragmented at 1492 and the resulting fragments would be assigned the labels. As the MPLS MTU defaults to the datalink MTU value.

2) Most ethernet hardware is supposed to switch 4 to 8 bytes more than the conventional 1500 bytes frame size. If this support is there in the hardware being used by you then, you can configure MPLS MTU to avoid fragmentation at 1492, and transmit the 1500 bytes packet. Hence supporting the dual label stack.



For further information on the MTU, as a reading reference, you can refer to RFC 3032 complete Section "3".

New Member

Re: MTU setting on FastEthernet

Thank you . The RFC was so useful

And know i want to aks you

1. MPLS MTU command , should be on the Edge-LSR interface connected to MPLS clou or on the interface connected to CE ?

2.On section 3.4 of the RFC it says that :

4. If the IP datagram has the "Don't Fragment" bit set in its IP


a. the datagram MUST NOT be forwarded

b. Create an ICMP Destination Unreachable Message:

i. set its Code field [3] to "Fragmentation Required and DF


ii. set its Next-Hop MTU field [4] to the difference between

the Effective Maximum Frame Payload Size and the value

of N

c. If possible, transmit the ICMP Destination Unreachable

Message to the source of the of the discarded datagram.

How can i do this

If the source who set DF is client , it will be easy , but what if a webserver is setting DF ? and also do not support path Mtu discover ?


New Member

Re: MTU setting on FastEthernet

Please note the range available. Your "1510" exceeds it.