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Troubleshooting Virtual Private Networks by Mark Lewis

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Case Studies

This section contains descriptions and solutions for a number of issues that do not fit easily into the main troubleshooting section.

MPLS MTU Is Too Small in the MPLS VPN Backbone

If large packets are sent across the MPLS backbone with the Don't Fragment (DF) bit set in the IP packet header, and LSR interfaces and Ethernet switches are not configured to support large labeled packets, the packets will be dropped.

NOTE

In this scenario, end hosts will usually quietly reduce the size of the packets they send unless Path MTU Discovery is broken—which usually happens because of a misconfigured router or firewall. Path MTU Discovery is a mechanism that allows hosts to dynamically discover the Maximum Transmission Unit (MTU) of a path ...

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