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Configuring Path MTU Checks for VPN Routing Instances

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 router. 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.

For more information about the path MTU check, see the Junos OS Administration Library for Routing Devices.

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

Enabling Path MTU Checks for a VPN Routing Instance

To enable path checks on the outgoing interface for unicast traffic routed on a VRF or virtual-router routing instance, include the vrf-mtu-check statement at the [edit chassis] hierarchy level:

Assigning an IP Address to the VPN Routing Instance

To ensure that the path MTU check functions properly, at least one IP address must be associated with each VRF or virtual-router routing instance. If an IP address is not associated with the routing instance, ICMP reply messages cannot be sent.

Typically, the VRF or virtual-router routing instance IP address is drawn from among the IP addresses associated with interfaces configured for that routing instance. If none of the interfaces associated with a VRF or virtual-router routing instance is configured with an IP address, you need to explicitly configure a logical loopback interface with an IP address. This interface must then be associated with the routing instance. See Configuring Logical Units on the Loopback Interface for Routing Instances in Layer 3 VPNs for details.