[c-nsp] Suspect MTU Issues

[Leigh Harrison]

All,

I have inherited a network which I feel is having MTU issues, but I cannot be sure and would like a second opinion.

There is a legacy layer 2 network which has had an mpls network built over it.  A link between two of the data centres is a dark fibre between two Cisco 3750E switches running on the ten gig ports with a variety of vlans passing over a dot1q trunk.  Both of these switches are pretty much out of the box and have a standard system mtu of 1500.

Connected to these 3750E switches are a set of Cisco ME3600X switches that are talking mpls to each other via an SVI, over a vlan on the trunk between the 3750E switches.  Sending packets of 1496 bytes over the mpls works just fine, but when I push 1497, it doesn't work - hence my thoughts of an MTU issue.

Have we fallen into a silly trap or is there a straightforward solution?  Is this an MTU problem or am I clutching at straws?

Thanks in advance,

Leigh