What are some common causes and symptoms of routing loops and how do you diagnose them?

1 Common causes

Routing loops can be caused by a variety of factors, such as misconfigured static routes that do not reflect the current topology of the network, inconsistent routing protocols or parameters that lead to incompatible routing updates, network failures or changes that prevent normal convergence, and human errors or malicious attacks that introduce false information.

2 Common symptoms

Routing loops can have many detrimental effects on your network, including high utilization and congestion from repeated packet transmission, increased latency and packet loss from the expiration of the TTL field or router buffer overflow, unpredictable or incorrect routing behavior from inconsistent or unstable routing tables, and reduced security and visibility from potential packet spoofing or hijacking.

3 Diagnosis tools

To diagnose and troubleshoot routing loops, you need to use some tools and commands that can help you identify and isolate the source and extent of the problem. For example, ping and traceroute can test the connectivity and trace the path of packets between two hosts or routers. Show ip route and show ip protocols can display the routing table and the routing protocol configuration of a router. Additionally, debug ip routing and debug ip packet can provide detailed information about the routing process and the packet forwarding decisions of a router. Lastly, show ip bgp and show ip bgp summary can show the Border Gateway Protocol (BGP) routing information and the status of BGP peers.

4 Prevention techniques

To prevent or minimize the occurrence of routing loops, you should use dynamic routing protocols that have loop prevention mechanisms, such as split horizon, poison reverse, or route tagging. Route filtering and summarization can also reduce the size and complexity of routing tables and updates. Additionally, you should use route redistribution carefully and selectively to avoid creating conflicting or duplicate routes between different routing domains or protocols. Lastly, route authentication and verification can protect your routing information from unauthorized or malicious modifications.

5 Recovery steps

To recover from routing loops, you need to identify and correct the cause of the loop, whether it is a configuration error, a network failure, or a security breach. Additionally, you should clear or reset the routing tables and protocols of the affected routers to remove any invalid or outdated routes. To ensure normal operation and stability, verify and monitor your network's connectivity and performance with ping, traceroute, or other tools. Lastly, document and report the incident and the actions taken to prevent future occurrences.