Hi Jack,
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1) Why a Datagram is Fragmented:
When a frame is carrying an unlabeled IP datagram, the Frame
Payload is just the IP datagram itself. When a frame is
carrying a labeled IP datagram, the Frame Payload consists of
the label stack entries and the IP datagram.
Now when this frame payload as defined as above exceeds the
conventional layer 2 media MTU then the frame is fragmented.
In case of ethernet this MTU is 1500.
So for example when a unlabelled frame with payload of 1500
bytes is received and the same has to be sent further to
the remote destination by labelling it, then the payload
has to be fragmented.
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2) Why the MPLS MTU command:
Once you receive an unlabelled frame, first the PE router
receives it, labels it and then its put out for forwarding,
when its to be forwarded, it needs to be fragmented.
The problem comes here, when before being forwarded out of
the interface if it gets fragmented, it would create two
fragments or frames.
By conventional fragmentation, the label which is inserted
in the header may not be preserved into the new fragments
created and the frame may be simply discarded as it loses
the forwarding address which was the label.
So to avoid this MPLS MTU command needs to be configured,
so when there is fragmentation, it takes care of putting
in the same label into the fragments created.
Now in IOS even is MPLS MTU command is not configured
it takes the default MTU as the MPLS MTU value.
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3) Solution to your problem:
To aviod configuring the CE devices with MTU 1492,
what you need to take care of is configure all you
core facings links, with an physical MTU of 1508.
So automatically your TCP packets which if total
to 1500 bytes payload with a DF bit set wont need
to be fragmented from PE at one end to other end.
For this your PE <--->P link ethernet media MTU
should be 1508, (if you can configure 1512 or 1516
that would also be great if you plan to increase the
stack size or later provide IPV6 VPN's.)
You P<-->P links ethernet media MTU should also be
the same as set between PE to P. if you have any
SONET/POS links in your backbone then you dont have
to do anything for the MTU.
So the net effect of this would be any TCP sessions
as which are prone to setting the DF bit can be
transparently sent across without send ICMP error message.
HTH-Cheers!
Swaroop
You may also like to see the RFC 3032 about label stack.
