Counts ICMP Destination Unreachable messages sent by the device, telling senders their packet couldn't be delivered. On a firewall, a rising icmpOutDestUnreachs often directly reflects reject-style policy actions or routing failures being reported back to senders; a much bigger increase than usual can mean either a rule change started actively rejecting more traffic, or a downstream route just went dark. Example: icmpOutDestUnreachs spiking right after ipOutNoRoutes spikes confirms the gateway is now actively telling senders their traffic can't be routed, consistent with an upstream outage rather than a policy change.