What is a routing loop?

A routing loop is a networking issue that occurs when a packet is continuously circulated through a series of routers, unable to reach its intended destination. This can happen due to errors or misconfigurations in the routing tables of the routers involved. The packet keeps traversing the same set of routers indefinitely, creating a loop and consuming network resources without making progress towards its destination.

Routing loops can lead to degraded network performance, increased latency, and potential network instability. They are usually the result of incorrect or inconsistent routing information within the network. Common causes of routing loops include:

  1. Inconsistent Routing Information: If routers in a network have different and conflicting information about the best path to reach a destination, it can lead to loops.
  2. Routing Table Update Delays: When routing information changes, it takes some time for routers to update their routing tables. During this convergence period, if routers still forward packets based on outdated information, loops may occur.
  3. Split Horizon: This is a technique used to prevent routing loops in distance-vector routing protocols. In split horizon, a router does not advertise routes back to the same interface from which it received the updates. If split horizon is not implemented correctly, it can contribute to routing loops.
  4. Count to Infinity Problem: In distance-vector routing protocols, such as RIP (Routing Information Protocol), if there is a failure in the network, routers may take some time to update their routing tables, leading to the "count to infinity" problem where routers keep incrementing the metric for a failed route.

To prevent and mitigate routing loops, various routing protocols and techniques have been developed, including link-state routing protocols like OSPF (Open Shortest Path First) and EIGRP (Enhanced Interior Gateway Routing Protocol), which use more sophisticated algorithms to prevent looping and converge more quickly.