What is Routing Metric?

Understanding Routing Metric

A routing metric is the quantitative value a routing protocol uses to determine the best path to a destination network. When multiple routes to the same destination exist, the router compares their metrics and installs the route with the most favorable (usually lowest) value in its routing table, directing how packets are forwarded across the network.

Different routing protocols compute metrics differently. RIP uses a simple hop count, the number of routers between source and destination. OSPF uses cost, derived from interface bandwidth, so higher-speed links are preferred. EIGRP uses a composite metric based on bandwidth and delay (and optionally load and reliability). BGP, an external/path-vector protocol, uses path attributes such as AS-path length, local preference, and MED rather than a single numeric metric. The router runs the protocol's algorithm over these metrics to build a loop-free, efficient forwarding table, and metric values also enable traffic engineering by deliberately steering flows.

Routing metrics matter for security because the path traffic takes determines who can observe or intercept it. Attackers target metrics to redirect data: by injecting false routing advertisements with attractive metrics, an adversary can lure traffic through a system they control for interception or denial of service. Route manipulation underlies attacks like BGP hijacking and rogue OSPF injection. Because routing affects availability and confidentiality, protecting metric integrity through routing authentication (such as OSPF/BGP MD5 or stronger), route filtering, and prefix validation is an important control.

For example, in an OSPF network an engineer wants traffic to prefer a high-speed fiber link over a slower backup. By adjusting the interface cost so the fiber path has a lower total metric, OSPF selects it as the primary route while keeping the backup ready for failover. Conversely, an attacker who injects a spoofed advertisement claiming an artificially low metric for a sensitive prefix could pull that traffic toward themselves, demonstrating why authenticated, filtered routing is essential.