Configuring a Path MTU Check for VPNs

By default, the maximum transmission unit (MTU) check for VPN routing instances is disabled on M-series routers (except the M320 router) and enabled for the M320, T-series, and J-series routers. On M-series routers, you can configure path MTU checks on the outgoing interfaces for unicast traffic routed on VRF routing instances and on virtual-router routing instances.

When you enable an MTU check, the routing platform sends an Internet Control Message Protocol (ICMP) message when a packet traversing the routing instance exceeds the MTU size and has the do-not-fragment bit set. The ICMP message uses the VRF local address as its source address.

For an MTU check to work in a routing instance, you must both include the vrf-mtu-check statement at the [edit chassis] hierarchy level and assign at least one interface containing an IP address to the routing instance.

To configure path MTU checks, do the tasks described in the following sections:

• Enabling Path MTU Checks for a VPN Routing Instance
• Assigning an IP Address to the VPN Routing Instance

For more information on the Path MTU check, see the JUNOS System Basics Configuration Guide.