# Tunnel Interface Parameters

Encapsulating multicast packets in unicast packets is a common method for transmitting multicast packets across a non-multicast-aware network and through IPSec tunnels. NetScreen devices support Generic Routing Encapsulation (GRE) version 1 for encapsulating IP packets in IPv4 unicast packets. On NetScreen devices, you enable GRE encapsulation on tunnel interfaces. You must enable GRE when you transmit multicast packets through an IPSec VPN tunnel between a NetScreen device and a third-party device or router.

### To Configure GRE on a Tunnel Interface

1. Enter the necessary information:

Tunnel Interface Name: The tunnel interface that you are configuring.

Encap: Select GRE to enable GRE encapsulation on the tunnel interface.

Key: Select this option and enter a value to append the key value to outgoing packets. Incoming packets must have this value too.

Local Interface: Select the outgoing interface.

Destination IP: Enter the IP address of the remote endpoint on the tunnel.

Keep-Alive: Select this option to enable the tunnel interface to send keep-alive messages to monitor the status of the connection.

Interval: Specify the interval (in seconds) between keep-alive messages.

Threshold: Specify the number of times the local tunnel interface sends keep-live messages without receiving a reply before it terminates the connection.

2. Click Apply to save the configuration.