Technical Documentation

Configuring the JUNOS Software to Enable MTU Path Check for a Routing Instance on M Series Routers

By default, the maximum transmission unit (MTU) check for routing instance is disabled on M Series routers (except the M320), and enabled for all T Series and J Series routers.

Note: The MTU check is automatically present for interfaces belonging to the main router.

On M Series routers (except the M320 router) you can configure MTU path checks on the outgoing interface for unicast traffic routed on a virtual private network (VPN) routing and forwarding (VRF) routing instance. When you enable MTU check, the router sends an Internet Control Message Protocol (ICMP) message when the size of a unicast packet traversing a VRF routing instance or virtual-router routing instance has exceeded the MTU size and when an IP packet is set to "do not fragment". The ICMP message uses the routing instance local address as its source address.

For an MTU check to work in a routing instance, you must 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, complete the following tasks:

  1. Enabling MTU Check for a Routing Instance
  2. Assigning an IP Address to an Interface in the Routing Instance

Enabling MTU Check for a Routing Instance

To enable MTU check for a routing instance, include the vrf-mtu-check statement at the [edit chassis] hierarchy level:

[edit chassis]vrf-mtu-check;

Assigning an IP Address to an Interface in the Routing Instance

To assign an IP address to an interface in the VRF or virtual-router routing instance, configure the local address for that routing instance. A local address is any IP address derived from an interface that is assigned to the routing instance.

To assign an interface to a routing instance, include the interface statement at the [edit routing-instances routing-instance-name] hierarchy level:

[edit routing-instances routing-instance-name]interface interface-name;

To configure an IP address for a loopback interface, include the address statement at the [edit interfaces interface-name unit logical-unit-number family inet] hierarchy level:

[edit interfaces interface-name unit logical-unit-number family inet]address address;

Note: If you are assigning Internet Protocol Security (IPsec) or generic routing encapsulation (GRE) tunnel interfaces without IP addresses in the routing instance, include a loopback interface to the routing instance. To do this, include the lo0.n option at the [edit routing-instances routing-instance-name interface] hierarchy level. n cannot be 0, because lo0.0 is reserved for the main router (and not appropriate for use with routing instances). Also, an IP address must be assigned to this loopback interface in order to work. To set an IP address for a loopback interface, include the address statement at the [edit interfaces lo0 unit logical-unit-number family inet] hierarchy level.


Published: 2010-04-26