# Monitoring the status of NetLinks

To monitor the status of the links, define the probe IP addresses in the NetLink properties.

The Security Engine sends ICMP messages to make sure that a link is still available. Only NetLinks that are used in an Outbound Multi-Link element are probed. Status monitoring is not available for NetLinks that are only used in Routing.

Probing is always recommended, and it is mandatory with the following features:

- The ratio-based load-balancing method
- Fail over to standby NetLinks
- Inbound traffic balancing with dynamic DNS updates

Make sure the Probe IP Addresses that you select produce a reliable measurement of the link performance. For example, probing the IP address of an ADSL router usually succeeds even if the ISP network is unreachable. Probing the default gateway provided to you by the ISP might succeed even if the ISP is not able to forward traffic anywhere outside the ISP’s own network. You can add several alternative probe IP addresses to avoid probing failures caused by a probed host going down.