Network

For network engineers, it is essential to be familiar with and master routing troubleshooting ideas and techniques.

However, while it is necessary, many novice network engineers still lack a clear understanding or are not very familiar with these skills, causing them a lot of trouble.

For senior network engineers, while they have a good grasp of the general troubleshooting ideas and can generally handle issues, there is still room to enrich their troubleshooting techniques.

Just like how everyone has used Excel to some extent, but only those who can handle large amounts of data more efficiently are the real ace players.

So today, let’s talk about routing troubleshooting. Through several typical routing fault troubleshooting scenarios, we’ll analyze them step by step.

01 What If the External Interface of the Router is Down?

Sometimes, you may encounter situations where the external network interface of the router fails to work properly. This can prevent the entire network from accessing the internet or external services. This issue is usually caused by physical connection problems, configuration errors, or security policy restrictions.

01 Solutions

1. Check Physical Connections
   • Ensure all cables are securely plugged into the correct ports.
   • Check for any visible physical damage, such as damaged cables or ports.
   • If using modular interface cards, ensure they are correctly installed and functioning.
2. Verify Configuration Settings
   • Use the command show ip interface brief to check the status and IP address configuration of the interface.
   • Ensure the interface is in the up/up state (both physical and link layers are active).
   • Confirm the interface has the correct IP address and subnet mask configured.
   • Check if the interface is correctly associated with the corresponding VLAN or other logical groups.
3. Troubleshoot Firewall Rules
   • Review ACLs (Access Control Lists) or other security policies on the router to ensure they do not block external network traffic.
   • If there are additional security devices (such as firewalls) in front of the router, also check their configurations.
   • Use the command show access-lists or similar to view ACL rules and ensure they allow the required traffic.
4. Check Advanced Settings
   • Confirm if NAT (Network Address Translation) is enabled and if the NAT configuration is correct.
   • If using dynamic routing protocols, check if they are correctly configured and advertising the correct routes.
   • Ensure no advanced features, such as load balancing or multipath routing, are affecting the external interface.

02 Oh No, the Routing Table is Incorrect Again

Sometimes, you may find incorrect or missing entries in the router’s routing table, which can prevent packets from being forwarded correctly to their destination. This issue usually arises from configuration errors, routing protocol failures, or manual input errors.

01 Solutions

1. View the Routing Table
   • Use the command show ip route or show ip bgp (for BGP) to check the current routing table.
   • Pay attention to the presence of default routes, directly connected routes, and static routes.
   • Confirm the existence of correct routing entries to the destination.
2. Manually Add or Remove Routing Entries
   • If necessary routing entries are missing, try manually adding static routes:
     ip route <destination-network> <subnet-mask> <next-hop>

   • If there are unnecessary entries, use the no ip route command to remove them:
     no ip route <destination-network> <subnet-mask> <next-hop>

3. Use Routing Protocols to Update Routing Information
   • Confirm if dynamic routing protocols (such as OSPF, EIGRP, or BGP) are correctly configured.
   • Check if neighbor relationships are successfully established.
   • Use the command show ip ospf neighbor (for OSPF) or similar to view neighbor status.
   • If there are issues with the routing protocol, try reconfiguring or adjusting relevant parameters.
   • Use the command debug ip ospf (for OSPF) or similar to debug routing protocol interactions.
4. Analyze Routing Protocol Status and Message Logs
   • Use the command show ip protocols to view the current enabled routing protocols and their status.
   • Review the message logs of the routing protocol to identify any errors or warning messages.
   • If errors are found, make corresponding configuration changes based on the error prompts.

03 What Now! Cross-Device Routing Failure

When packets cannot be correctly routed between two or more devices, cross-device routing failure may occur. This can be caused by configuration errors, physical connection problems, or improper routing protocol configuration.

01 Solutions

1. Check Inter-Device Connection Status
   • Confirm all physical connections are normal, including cables, interfaces, and power.
   • Use the ping command to test connectivity between devices.
   • Use the traceroute or tracert tool to trace the path of packets and identify any issues in the path.
2. Confirm Routing Protocols are Correctly Configured
   • Check if the correct dynamic routing protocols, such as OSPF, EIGRP, or BGP, are enabled.
   • Confirm if neighbor relationships are established.
   • Use the command show ip ospf neighbor (for OSPF) or similar to check neighbor status.
   • Confirm if routing advertisements and receptions are correctly configured.
3. Use Traceroute/Tracert Tools to Trace the Path
   • Run traceroute (on Linux/Unix systems) or tracert (on Windows systems) to determine all intermediate nodes packets pass through before reaching the destination.
   • Check if any nodes are unreachable or have abnormal response times.
   • Locate the node where the problem occurs based on the results of traceroute or tracert.
4. Analyze Routing Table and Routing Protocol Status
   • Use the command show ip route to view the routing table and confirm if there are valid routing entries to the destination.
   • Use the command show ip protocols to view the status of routing protocols and ensure they are running normally.
   • If there are issues with the routing table or routing protocols, follow the guidance in previous sections to make corresponding adjustments.
5. Check Firewall and ACL Rules
   • Confirm if there are firewall or ACL rules blocking normal packet transmission.
   • Use the command show access-lists to view access control lists.
   • If necessary, adjust firewall or ACL rules to allow the required packets to pass.

04 Tough, Dynamic Routing Protocol Failure

Dynamic routing protocols such as OSPF, EIGRP, or BGP may encounter various issues during operation, which can lead to inaccurate routing tables, unstable neighbor relationships, or failed routing updates.

01 Solutions

1. Check Protocol Configuration
   • Use the command show running-config or show configuration to view the configuration file and ensure the protocol configuration is correct.
   • Confirm if the correct protocol is enabled and if the network range and neighbors are correctly specified.
   • Use the command show ip protocols or similar to view an overview of the protocol information.
2. Analyze Neighbor Relationships
   • Use the command show ip ospf neighbor (for OSPF) or similar to view neighbor status.
   • Confirm if neighbors are in the FULL state (for OSPF) or have established an ESTABLISHED state (for BGP).
   • If there are issues with neighbor relationships, check if authentication information, Hello intervals, and Dead intervals are consistent.
3. View Protocol Status and Message Logs
   • Use the command show ip ospf database (for OSPF) or show ip bgp (for BGP) to view the status of the routing database.
   • Use the command debug ip ospf (for OSPF) or similar to enable debugging information and view detailed records of protocol interactions.
   • Analyze if there are any errors or warning messages based on the debugging information and make corresponding configuration adjustments.
4. Adjust Routing Priority and Metric Values
   • If the routing selection is not ideal, adjust the priority or metric values to optimize routing selection.
   • For OSPF, use the command ip ospf cost to adjust the cost of links.
   • For EIGRP, use the command metric to adjust metric values.
   • For BGP, use attributes such as local-preference or as-path to influence routing selection.
5. Restart Protocol Processes
   • If the above methods do not solve the problem, try restarting the routing protocol process.
   • Use the command clear ip ospf process (for OSPF) or similar to reset the protocol status.
   • After restarting, observe if neighbor relationships can be re-established and if the routing table returns to normal.

05 Routing Loop Again? Really Frustrating

A routing loop occurs when packets circulate indefinitely between two or more routers. This usually happens due to improper static routing configuration or incorrect dynamic routing protocol configuration.

01 Solutions

1. Enable Loop Prevention Mechanisms
   • For OSPF, the SPF (Shortest Path First) algorithm itself has loop prevention features.
   • For RIP, enable Split Horizon and Poison Reverse features to avoid loops.
   • For BGP, automatically detect duplicate AS numbers in the AS_PATH to prevent loops.
2. Set Maximum Hop Count (TTL)
   • In static routing configuration, set the maximum hop count or TTL (Time to Live) value to limit the packet’s survival time in the network.
   • Each time a packet passes through a router, the TTL value decreases by 1, and when the TTL value reaches 0, the packet is discarded, thus avoiding infinite loops.
3. Use Static Routing or Policy Routing
   • When using static routing, ensure the entries in the routing table do not form a cyclic path.
   • Use policy-based routing (Policy-Based Routing) to specify the forwarding path for specific traffic, avoiding routing loops.
4. Check Routing Protocol Configuration
   • Confirm if dynamic routing protocols such as OSPF, EIGRP, or BGP are correctly configured.
   • Use the command show ip protocols to view the status of routing protocols.
   • Confirm if the correct features to prevent loops, such as OSPF’s SPF algorithm, are enabled.
5. Adjust Routing Metric Values and Priorities
   • If using RIP or EIGRP, adjust metric values to ensure reasonable routing selection.
   • For BGP, adjust local preference (Local Preference) or AS_PATH attributes to optimize routing selection.

06 Traffic Anomalies, Tiring

Network traffic anomalies, such as packet loss or increased latency, are usually caused by network congestion, device failures, configuration errors, or malicious attacks. These issues affect the overall network performance and user experience.

01 Solutions

1. Monitor Network Traffic
   • Use network monitoring tools such as SNMP (Simple Network Management Protocol), NetFlow, or IPFIX to monitor network traffic.
   • Analyze traffic trend graphs to identify abnormal traffic patterns or burst traffic.
   • Use real-time monitoring tools to track the current network status.
2. Optimize Traffic Using QoS Techniques
   • Configure QoS (Quality of Service) policies to assign priorities to different traffic types.
   • Use traffic shaping and traffic policing to control bandwidth usage.
   • Configure queue management mechanisms such as WFQ (Weighted Fair Queuing) or CBWFQ (Class-based Weighted Fair Queuing) to ensure critical applications’ bandwidth needs are met.
3. Adjust Bandwidth Allocation
   • If network congestion is due to insufficient bandwidth, consider increasing bandwidth capacity.
   • Adjust link aggregation or load balancing strategies to optimize bandwidth utilization.
   • For multi-link environments, use ECMP (Equal-Cost Multi-Path) to balance traffic.
4. Troubleshoot Network Failures
   • Use the ping and traceroute tools to detect network connectivity and path issues.
   • Check device CPU and memory utilization to determine if there is an overload situation.
   • Check for any device failures or configuration errors.
5. Defend Against Malicious Attacks
   • Use firewalls and IPS (Intrusion Prevention System) to filter malicious traffic.
   • Implement DDoS (Distributed Denial of Service) protection measures such as traffic scrubbing services.
   • Configure ACLs (Access Control Lists) to limit traffic from suspicious sources.