[c-nsp] Suspect MTU Issues

[Keegan Holley]

Most switches (not specifically familiar with the ME series) will compensate
for dot1q tags by transparently allowing packets of MTU+dot1q tag size.
 This is not usually true for mpls (again not directly familiar with the ME
series)  that being said the mpls header is exactly 4 bytes which seems to
be your ceiling so I'd say you were right.

2011/7/12 Leigh Harrison <lharrison at convergencegroup.co.uk>

> 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
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