Configuring Path MTU Discovery on a GRE Tunnel

Path MTU discovery (PMTUD) is enabled by default and has several configurable options.

PMTUD is enabled by default on tunnel interfaces. In this task, PMTUD is assumed to be disabled and a step shows how to re-enable PMTUD. Options such as changing the PMTUD age timer, manually clear the tunnel PMTUD, and view the PMTUD configuration are also explained using the following steps.

Before configuring this task, review the “Configuring IP GRE tunnels” task to view the required steps for creating GRE tunnels. In this task, it is assumed that you have already configured the GRE tunnel. Full tunnel configuration is shown in the example at the end of this task.

Enter global configuration mode.
```
device# configure terminal
```

Enter interface configuration mode and specify an Ethernet interface.
```
device(config)# interface ethernet 1/3/1
```

Assign an IP address to Ethernet port 1/3/1.

device(config-if-e10000-1/3/1)# ip address 10.0.8.108 255.255.255.0

Return to global configuration mode to configure the GRE tunnel interface.
```
device(config-if-e10000-1/3/1)# exit
```

Create a tunnel interface.
```
device(config)# interface tunnel 1
```

(Optional) Re-enable PMTUD for the tunnel interface.
```
device(config-tnif-1)# no tunnel path-mtu-discovery disable
```
To disable PMTUD, use the tunnel path-mtu-discovery disable command.

(Optional) Change the age timer for PMTUD.
```
device(config-tnif-1)# tunnel path-mtu-discovery age-timer 20
```
In this example, the device waits for 20 minutes before resetting the path MTU to its original value. By default, when PMTUD is enabled on a tunnel interface, the path MTU is reset to its original value every 10 minutes. If desired, you can change the reset time (default age timer) to a value of up to 30 minutes.
comm

(Optional) Reset a dynamically-configured PMTUD on tunnel interface 1.
```
device(config-tnif-1)# clear ip tunnel pmtud 1
```

Display PMTUD information using the

show interfaces
                     						tunnel

command.

show interfaces tunnel 1

Tunnel1 is up, line protocol is up
Hardware is Tunnel
Tunnel source 10.0.8.108
Tunnel destination is 131.108.5.2
Tunnel mode gre ip
Internet address is 10.10.3.1/24, MTU 1476 bytes, encapsulation GRE
Keepalive is not Enabled
Path MTU Discovery: Enabled, MTU is 1428 bytes, age-timer: 20 minutes
Path MTU will expire in 0 minutes 50 secs

In this example, GRE tunnel configuration and PMTUD aging timer information is displayed.

The following example configures a GRE tunnel and changes the PMTUD aging timer to 20.

device# configure terminal
device(config)# interface ethernet 1/3/1
device(config-if-e1000-1/3/1)# ip address 10.0.8.108/24
device(config)# exit
device(config)# interface tunnel 1 
device(config-tnif-1)# tunnel source 10.0.8.108
device(config-tnif-1)# tunnel destination 131.108.5.2
device(config-tnif-1)# tunnel mode gre ip
device(config-tnif-1)# tunnel path-mtu-discovery age-timer 20
device(config-tnif-1)# tunnel loopback 4/1
device(config-tnif-1)# ip address 10.10.3.1/24
device(config-tnif-1)# exit
device(config)# ip route 131.108.5.0/24 10.0.8.1
device(config)# ip route 10.10.2.0/24 tunnel 1

IP Addressing

Configuring Path MTU Discovery on a GRE Tunnel