Routing Failure Signatures Database: Route Tables, Gateways, Prefixes, and Path Clues

Fast symptom-to-evidence table

Use the table to choose the next artifact, not to skip diagnosis. Scroll horizontally on smaller screens.

Routing decision rule

For a destination, list all matching routes and choose the longest prefix first. Then evaluate the selected next hop/interface and whether it is reachable. Finally consider the return path. Metrics matter among otherwise applicable candidates according to the routing implementation, but a less-specific route does not beat a more-specific prefix merely because its metric looks smaller.

Local subnet works, remote networks fail

Primary interpretation: Default gateway/route candidate. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: host route table and gateway reachability. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: Local success does not test routing beyond subnet. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

One remote prefix fails, others work

Primary interpretation: Specific route/policy/path. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: route table longest-prefix match. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: Broad default route may still be healthy. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

Default route missing

Primary interpretation: Unknown remote destinations have no fallback. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: 0.0.0.0/0 presence. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: Connected routes can still work. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

More-specific route sends traffic wrong way

Primary interpretation: Longest-prefix selection. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: competing prefixes/metrics. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: More-specific prefix wins before ordinary metric comparison. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

Next-hop route exists but next hop unreachable

Primary interpretation: Recursive/adjacency problem. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: connected route, ARP, interface. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: A route line alone does not prove forwarding. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

Forward path works, replies never return

Primary interpretation: Return routing/policy. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: destination-side route, firewall state. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: End-to-end communication needs a return path. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

Traceroute stops after gateway

Primary interpretation: Upstream path/filtering candidate. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: later-hop/destination behavior. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: One silent hop does not prove forwarding stops there. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

Two routes same prefix differ metric

Primary interpretation: Metric/administrative selection. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: route source and metric. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: Do not confuse metric with prefix length. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

Wrong client mask bypasses gateway

Primary interpretation: Host classification error. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: on-link route and ARP. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: May look like routing failure but originates at endpoint addressing. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

VPN remote subnet unreachable

Primary interpretation: Tunnel route/overlap/policy. The observation should be treated as a boundary clue rather than a complete diagnosis. Capture the condition exactly as observed and compare it with the intended network design.

Evidence to collect: VPN route, local prefix, selectors. Use those artifacts to confirm whether the suspected mechanism actually exists. A useful next test changes one uncertainty at a time and preserves any checkpoint that has already succeeded.

Important limit: Check overlap before blaming Internet path. This limit matters because many networking symptoms are shared by more than one failure mechanism. A correct answer explains both the failed behavior and the controls that still work.

Turn three database records into a troubleshooting sequence

Step 1: Observe Local subnet works, remote networks fail. Before naming a cause, write the supported boundary: Default gateway/route candidate. Collect host route table and gateway reachability. Then explicitly respect this constraint: Local success does not test routing beyond subnet.

Step 2: Observe One remote prefix fails, others work. Before naming a cause, write the supported boundary: Specific route/policy/path. Collect route table longest-prefix match. Then explicitly respect this constraint: Broad default route may still be healthy.

Step 3: Observe Default route missing. Before naming a cause, write the supported boundary: Unknown remote destinations have no fallback. Collect 0.0.0.0/0 presence. Then explicitly respect this constraint: Connected routes can still work.

In the Routing Failure Signatures Database context, treat this observation as evidence that must be tested against the page's specific failure boundary. Record the observed value and expected value, compare a known-good control, and choose the next artifact that can disprove the leading hypothesis. After any correction, repeat the original failed operation and confirm the related network requirement still behaves as designed.

How to study this database for AP Networking

In the Routing Failure Signatures Database context, treat this observation as evidence that must be tested against the page's specific failure boundary. Record the observed value and expected value, compare a known-good control, and choose the next artifact that can disprove the leading hypothesis. After any correction, repeat the original failed operation and confirm the related network requirement still behaves as designed.

In the Routing Failure Signatures Database context, treat this observation as evidence that must be tested against the page's specific failure boundary. Record the observed value and expected value, compare a known-good control, and choose the next artifact that can disprove the leading hypothesis. After any correction, repeat the original failed operation and confirm the related network requirement still behaves as designed.

Advanced interpretation notes

For route-table practice, use a destination such as 10.40.8.25 and compare 0.0.0.0/0, 10.40.0.0/16, and 10.40.8.0/24. The /24 is the longest matching prefix and therefore the relevant candidate before considering less-specific matches. Then inspect its next hop. A configured route whose next hop cannot be resolved/reached may still fail operationally. For asymmetric failures, inspect the destination side's route back to the source. Stateful firewalls can also care about path symmetry, so forward-path success is not the entire conversation.

Evidence drill: observe “Next-hop route exists but next hop unreachable.” The first boundary to evaluate is Recursive/adjacency problem. Inspect connected route, ARP, interface. The critical reasoning constraint is: A route line alone does not prove forwarding. Write one alternative explanation and identify a control test that would distinguish it before making a configuration change.

Verification drill: begin from “Two routes same prefix differ metric.” After correcting the supported mechanism (Metric/administrative selection), repeat the original failed operation, capture route source and metric, and confirm the result no longer matches the failure signature. Also preserve this caution during validation: Do not confuse metric with prefix length.

Reason through: VPN remote subnet unreachable

Start with the observed clue “VPN remote subnet unreachable.” Its strongest initial boundary is Tunnel route/overlap/policy. Collect VPN route, local prefix, selectors before changing configuration. The interpretation must preserve this limitation: Check overlap before blaming Internet path. Now compare a known-good control that shares the same upstream dependencies. Record the exact values, timestamps, and expected state. If the control succeeds, narrow toward the components unique to the failed path. If it fails in the same way, widen scope to the shared dependency. After a supported correction, repeat the original test and the control, then verify that required access works and intended security or segmentation remains intact.

For the Routing Failure Signatures Database reference, the goal is not to memorize a cause beside a symptom. The goal is to select evidence that can falsify the current hypothesis. Explain what result would make Tunnel route/overlap/policy less likely, and where you would move next in the packet or service dependency chain. That explicit alternative is what separates evidence-based troubleshooting from confirmation bias.

Related evidence resources

Artifact Analysis · Troubleshooting Guide · CLI Tools · Troubleshooting Steps · Evidence Decision Lab