[j-nsp] MTU signalling on l2circuit

[sthaug at nethelp.no]

> If I follow, the circuit should come up, and operate fine, as long as
> the lesser of the two MTUs is not exceeded.

Nope, they won't come up, he'll get MM (MTU Mismatch).

> It is easy to have MTU issues with Ethernet, as the default 1500 byte
> MTU is easily exceeded when the CPE generate a 1500 byte frame due to
> the added MPLS/VPN encap. It would be better if you can raise the MTU to
> be larger than the CPE can generate and the same on both ends.

The problem here is often not the CPE, but the MTU for the Ethernet ports
at both ends of the Martini tunnel.

Fortunately, the problem is solved in 7.0, where you can set MTU for each
l2circuit (not under the interface configuration but under protocols
l2circuit interface ...).

We have *lots* of Martini tunnels, and 7.0 made our life much simpler.
Our last router at 6.4-20050107.0 is being upgraded in a few days.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no