Infinity Metric and Route Poisoning Explained

A routing loop occurs when routers running a distance-vector routing protocol advertise and learn incorrect routing information. When routing loops occur, routers fail to deliver data packets to their correct destinations. To prevent routing loops, distance vector routing protocols use four mechanisms: split horizon, infinity metric, route poisoning, and timers.

Split-horizon

The split-horizon feature states that if a router receives a route from a neighboring router, it will not propagate that route back to that neighbor on the same interface. The following image shows an example network.

Router A advertises the network 10.0.0.0/8 out from its S0/0/0 interface. Router B is connected to router A’s S0/0/0 interface. It receives this advertisement and adds the advertised route to its routing table. The manner in which router B advertises this newly learned route depends on whether split-horizon is enabled.

If split-horizon is disabled, router B will advertise this network back to router A. Router A will disregard this update because it already has a better route to reach the network 10.0.0.0/8. However, if the direct connection between Router A and the 10.0.0.0/8 network fails and it receives this update, it will add the route information to its routing table. The received routing update indicates that Router B has an alternative path to reach the network 10.0.0.0/8.

This misunderstanding causes a routing loop in which the actual network is down, but both routers think the network is reachable via each other. In this situation, if either router receives a packet for the destination network 10.0.0.0/8, it will forward it to the other router, and the other router will forward it back to the first router. The packet continues to circulate between the routers. The following image illustrates this scenario.

If split-horizon is enabled, router B will not broadcast the 10.0.0.0/8 network back to router A. The following image demonstrates split-horizon operation.

If the split-horizon feature is enabled and router A’s F0/0 interface fails, router B will not advertise the 10.0.0.0/8 network back to router A. Consequently, both routers recognize that no alternative path exists to reach this network until router A’s F0/0 connection is restored.

Infinity Metric

To maintain routing table stability, routers do not immediately remove unreachable routes. When a route becomes unreachable, the router sets its metric to infinity rather than deleting it. A route with an infinite metric value is treated as deleted and is never used in routing decisions. The infinity metric value is specific to each routing protocol. For example, RIP (Routing Information Protocol) uses 16 as the infinity value. In a RIP network, a route with a metric of 16 is considered unreachable. The following image shows how router A updates the metric for network 10.0.0.0/8 when it becomes unreachable.

Route Poisoning

Route poisoning is a variation of the split-horizon mechanism. If a router receives a route with an infinity metric from a neighboring router, it disregards split-horizon and propagates the route back to the neighboring router on the same interface. When network 10.0.0.0/8 fails, router A sets its metric value to 16 and advertises this route from all of its active interfaces.

Router B receives this routing update and updates the metric for network 10.0.0.0/8 in its routing table. Since this routing update states that the network 10.0.0.0/8 is no longer reachable, router B overrides split-horizon and sends a routing update back to router A.

The route poisoning feature operates in contrast to split-horizon. While a route is active, split-horizon remains in effect. When the route fails, route poisoning overrides split-horizon.

Key Points:-

• When a route fails, routers change the metric value of the failed route to infinity.
• A route with an infinity metric value is considered a deleted route.
• For routing, routers never use a route whose metric value is set to infinity.
• Route poisoning refers to the practice of advertising a failed route back to the source.
• The route poisoning feature applies only to routes with a metric value set to infinity.

Timers

Distance vector routing protocols use various timers to manage route information. For example, RIP, a true distance vector routing protocol, uses four timers. These timers are the following.

Conclusion

Infinity metric and route poisoning are essential mechanisms in distance-vector routing protocols for preventing and eliminating routing loops. By marking failed routes with an infinity metric and propagating this information through route poisoning, routers rapidly inform all network nodes about unreachable destinations. Together with split horizon and timers, these features ensure routing stability and reliable data delivery throughout the network.