For network engineers, mastering the principles and techniques of Network Routing Troubleshooting is essential. Many new network professionals still struggle with understanding these troubleshooting methods, leading to significant challenges in diagnosing and resolving routing issues effectively.

For experienced network engineers, if they have mastered the general troubleshooting ideas, they will generally have no problems in handling the problem. However, they can still improve their troubleshooting skills.

01 What should I do if the router’s external network port is closed?

Sometimes, you may encounter a situation where the external interface of your router does not work properly. This may cause the entire network to be unable to access the Internet or external services. This situation is usually caused by physical connection problems, configuration errors, or security policy restrictions.

01 Solution

1. Check the physical connection

• Make sure all cables are securely plugged into the correct ports.
• Check for any visible physical damage, such as broken cables or ports.
• If you are using modular interface cards, make sure they are properly installed and not faulty.

2. Verify configuration settings

• Run the show ip interface brief command to view the interface status and IP address configuration.
• Check that the interface is in up/up state (that is, both the physical layer and the link layer are active).
• Check whether the correct IP address and subnet mask are configured on the interface.
• Check whether the interfaces are correctly associated with the corresponding VLANs or other logical groupings.

3. Check firewall rules

• Review the ACL (access control list) or other security policies on your router to make sure they are not blocking external traffic.
• If there are additional security devices (such as firewalls) in front of the router, you need to check their configuration as well.
• Use the show access-lists or similar command to view the ACL rules and ensure that they allow the desired traffic.

4. Check Advanced Settings

• Verify that NAT (Network Address Translation) is enabled and check that NAT is configured correctly.
• If dynamic routing protocols are used, check that they are configured correctly and are advertising the correct routes.
• Check whether any advanced features that may affect the external network port are enabled, such as load balancing or multipath routing.

02 Oops , the routing table is incorrect again

Sometimes, it is found that there are wrong or missing entries in the routing table of the router, which may prevent the data packets from being correctly forwarded to the destination. This problem usually stems from misconfiguration, routing protocol failure, or manual input errors.

01 Solution

1. Check the routing table

• Use the command show ip route or show ip bgp (for BGP) to check the current routing table.
• Pay attention to check the existence of default routes, directly connected routes, and static routes.
• Verify that the correct routing entry exists for the destination.

2. Manually add or delete routing entries

• If the necessary routing entries are missing, you can try adding static routes manually.
• ip route <destination-network> <subnet-mask> <next-hop>​​
• If unnecessary entries exist, you can delete them using the no ip route command.
• no ip route <destination-network> <subnet-mask> <next-hop>

3. Update routing information using routing protocols

• Verify that the dynamic routing protocol (such as OSPF, EIGRP, or BGP) is configured correctly.
• Check whether the neighbor relationship is established successfully.
• Use show ip ospf neighbor (for OSPF) or similar command to view neighbor status.
• If there is a problem with the routing protocol, try reconfiguring or adjusting related parameters.
• Use debug ip ospf (for OSPF) or similar commands to debug routing protocol interactions.

4. Analyze routing protocol status and message logs

• Use show ip protocols to view the currently enabled routing protocols and their status.
• Check the routing protocol’s message log to determine if there are any errors or warnings.
• If errors are found, make corresponding configuration changes based on the error prompts.

03 What to do! Cross-device routing failed

Cross-device routing failures occur when packets cannot be properly routed between two or more devices. This can be due to misconfiguration, physical connectivity issues, or misconfigured routing protocols.

01 Solution

1. Check the connection status between devices

• Verify that all physical connections are functioning properly, including cables, connectors, and power supplies.
• Use the ping command to test connectivity between devices.
• Use the traceroute or tracert tools to trace the path of the packets to identify any problems in the path.

2. Confirm whether the routing protocol is configured correctly

• Check that the correct dynamic routing protocol is enabled, such as OSPF, EIGRP, or BGP.
• Check whether the neighbor relationship has been established.
• Use the show ip ospf neighbor (for OSPF) or similar command to check the neighbor status.
• Verify that route advertisement and reception are configured correctly.

3. Use traceroute/tracert tools to trace the path

• Run traceroute (on Linux/Unix systems) or tracert (on Windows systems) to determine all the intermediate nodes that the packet passes through before reaching its destination.
• Check if any nodes appear to be unreachable or have abnormal response times.
• Based on the traceroute or tracert results, locate the node where the problem occurs.

4. Analyze routing tables and routing protocol status

• Use the show ip route command to view the routing table and confirm whether there is a valid route entry pointing to the destination.
• Use the show ip protocols command to view the status of the routing protocols and ensure that they are running properly.
• If you find problems with the routing table or routing protocol, refer to the instructions in the previous section to make corresponding adjustments.

5. Check firewall and ACL rules

• Check whether there are firewalls or ACL rules that block the normal transmission of data packets.
• Run the show access-lists command to view the access control list.
• If necessary, adjust firewall or ACL rules to allow the required packets to pass.

04 Difficulty : Dynamic routing protocol failure

Dynamic routing protocols such as OSPF, EIGRP or BGP may encounter various problems during operation, which may lead to inaccurate routing tables, unstable neighbor relationships or route update failures.

01 Solution

1. Check the protocol configuration

• Run the show running-config or show configuration command to view the configuration file and ensure that the protocol configuration is correct.
• Verify that the correct protocols are enabled and that the network range and neighbors are correctly specified.
• Use the show ip protocols or similar command to view an overview of the protocols.

2. Analyze neighbor relationships

• Use show ip ospf neighbor (for OSPF) or similar command to view neighbor status.
• Verify that the neighbor is in the FULL state (for OSPF) or the ESTABLISHED state (for BGP).
• If there is a problem with the neighbor relationship, check whether the configurations such as authentication information, Hello time interval, and Dead time interval are consistent.

3. View protocol status and message logs

• Use show ip ospf database (for OSPF) or show ip bgp (for BGP) to view the status of the routing database.
• Enable debugging information using debug ip ospf (for OSPF) or similar commands to see a detailed record of the protocol interactions.
• Based on the debugging information, analyze whether there are any errors or warning messages, and make corresponding configuration adjustments accordingly.

4. Adjust route priority and metric

• If you find that the route selection is not ideal, you can adjust the priority or metric to optimize the route selection.
• For OSPF, you can use the ip ospf cost command to adjust the cost of the link.
• For EIGRP, the metric value can be adjusted using the metric command.
• For BGP, you can use attributes such as local-preference or as-path to influence route selection.

5. Restart the protocol process

• If the above methods cannot solve the problem, you can try restarting the routing protocol process.
• Use clear ip ospf process (for OSPF) or similar commands to reset the protocol state.
• After the restart, observe whether the neighbor relationship can be reestablished and whether the routing table returns to normal.

05 Routing loops again? Really broken

A routing loop is a phenomenon where a packet loops endlessly between two or more routers. This usually occurs when static routing is improperly configured or when dynamic routing protocols are not configured correctly.

01 Solution

1. Enable loop prevention mechanism

• For the OSPF protocol, the SPF (Shortest Path First) algorithm itself has the function of preventing loops.
• For the RIP protocol, you can enable the Split Horizon and Poison Reverse features to avoid loops.
• For BGP, duplicate AS numbers in AS_PATH are automatically detected to prevent loops.

2. Set the maximum number of hops (TTL)

• In static routing configuration, you can set the maximum hop count or TTL (Time to Live) value to limit the lifetime of a data packet in the network.
• Each time a data packet passes through a router, the TTL value is reduced by 1 until the TTL value reaches 0, at which point the data packet is discarded, thus avoiding an infinite loop.

3. Use static routing or policy routing

• When using static routing, ensure that the entries in the routing table do not form loops.
• Policy-Based Routing can be used to specify the forwarding path for specific traffic to avoid routing loops.

4. Check routing protocol configuration

• Verify that dynamic routing protocols, such as OSPF, EIGRP, or BGP, are configured correctly.
• Use the show ip protocols command to view the status of routing protocols.
• Verify that the correct features are enabled to prevent loops, such as the SPF algorithm of OSPF.

5. Adjust routing metrics and priorities

• If you are using RIP or EIGRP, you can adjust the metric to ensure proper routing.
• For BGP, you can adjust the Local Preference or AS_PATH attribute to optimize route selection.

06 Abnormal traffic, very tired

Network traffic anomalies, such as packet loss or increased latency, are usually caused by network congestion, equipment failure, configuration errors, or malicious attacks . Such problems can affect the overall performance of the network and user experience.

01 Solution

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 unusual traffic patterns or traffic bursts.
• Use real-time monitoring tools to track current network conditions.

2. Use QoS technology to optimize traffic

• Configure QoS (Quality of Service) policies to assign priorities to different traffic types.
• Use traffic shaping and traffic policing to control bandwidth usage.
• Configure a queue management mechanism, such as Weighted Fair Queuing (WFQ) or Class-based Weighted Fair Queuing (CBWFQ), to ensure that the bandwidth requirements of critical applications are met.

3. Adjust bandwidth allocation

• If network congestion is caused by insufficient bandwidth, consider increasing bandwidth capacity.
• Adjust link aggregation or load balancing policies to optimize bandwidth utilization.
• In a multi-link environment, you can use ECMP (Equal-Cost Multi-Path) to balance traffic.

4. Troubleshoot network problems

• Use ping and traceroute tools to detect network connectivity and path problems.
• Check the CPU and memory utilization of the device to determine if there is an overload condition.
• Check for any device failures or configuration errors.

5. Defense 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 cleaning services.
• Configure ACL (Access Control List) to restrict traffic from suspicious sources.