[j-nsp] Layer 2 VPNs and MTU

[Dermot Williams]

Hi,

Hope some of you can give me some pointers on the above issue. We're in
the process of deploying a significant change to our architecture. Some
of this target architecture will rely on the use of Layer 2 VPNs to
deliver subscribers to a Redback. Unfortunately we're hitting some
roadblocks in the form of MTU and Path MTU Discovery.

The most obvious solution to our problem is to increase the MTU across
our network but unfortunately that isn't an option because many of the
links that we use for backhaul don't support jumbo frames. Asking our
subscribers to decrease their MTU is a non-runner as well for customer
support reasons.

We've reduced the MTU on the next-hop interfaces for our subscribers and
this has alleviated some of the issues but some sites
(http://www.microsoft.com/) don't seem to support PMTUD properly, so
they are still problematic. I'm hoping and assuming that others have
come across similar issues - PPPoE, for example, should present the same
challenge - so I'd appreciate if someone could give me a heads up on how
they circumvented this problem - if at all.

TIA

Dermot Williams
Senior Network Engineer
Irish Broadband Internet Services Ltd.