What are the most common causes of routing loops and how can you prevent them?

2 Inconsistent routing updates

Another common cause of routing loops is inconsistent routing updates. This happens when different routers have different information about the network topology, due to delays, failures, or misconfigurations. For example, if router A learns that network X is reachable via router B, and router B learns that network X is unreachable, router A will still send packets to router B, creating a loop. To prevent this, you should use routing protocols that have mechanisms to detect and avoid loops, such as split horizon, route poisoning, or hold-down timers.

3 Asymmetric routing

A third common cause of routing loops is asymmetric routing. This happens when packets take different paths to and from a destination, due to load balancing, policy routing, or network failures. For example, if router A sends packets to network X via router B, and router B sends packets to network A via router C, router C may forward the packets back to router B, creating a loop. To prevent this, you should use routing protocols that support equal-cost multipath (ECMP) routing, or configure your routers to avoid policy conflicts.

4 Redistribution issues

A fourth common cause of routing loops is redistribution issues. This happens when routers exchange routes between different routing domains, such as autonomous systems (AS) or routing protocols. For example, if router A redistributes routes from AS 1 to AS 2, and router B redistributes routes from AS 2 to AS 1, a loop may occur if the same route is advertised by both routers. To prevent this, you should use route filters, tags, or metrics to control the redistribution process, and avoid mutual redistribution between the same routing domains.

5 Malicious attacks

A fifth common cause of routing loops is malicious attacks. This happens when an attacker injects false or modified routing information into the network, either by compromising a router or by spoofing its identity. For example, if an attacker advertises a route to network X with a lower metric than the legitimate route, other routers may follow the false route and create a loop. To prevent this, you should use security measures such as authentication, encryption, or digital signatures to protect your routing protocols and messages.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?