Summarization: Invalid Block Boundary
A summary that advertises a range nobody intended, blackholing one network and swallowing another. Work out the real block boundary, live, right below.
Symptom
A summary route was configured to replace four specific prefixes, but it covers the wrong range and blackholes traffic. Check where the block actually starts.
Your mission
Inspect, recover, then verify
Format
Cisco IOS CLI
Investigate before you configure.
Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.
Observe the symptom and link state.
Diagnose by comparing the protocol evidence.
Verify the expected device state and confirm the original symptom is resolved.
Console access: R1 · R2
This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.
Need a hint?
Reveal the root cause when you're ready.
Try the investigation first, then use this as your escape hatch, not a dead end.
Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
Four contiguous /24 networks were replaced with a single summary advertisement to reduce table size. Some of the original prefixes are now unreachable from the rest of the network, while addresses that were never part of the block are being attracted toward the summarizing router.
What you'll practice
- Calculate the correct summary prefix for a set of networks
- Explain why a summary must start on a valid block boundary
- Recognise the blackhole symptom of an over-broad summary
- Apply the corrected summary and verify all original prefixes remain reachable
1. Calculate the correct summary prefix for a set of networks
Compare the networks in binary and count the leading bits they share. That count is the summary prefix length, and the shared bits are the summary address.
2. Explain why a summary must start on a valid block boundary
A /22 block can only begin at a multiple of 4 in the third octet. Starting elsewhere means the mask silently rounds the address down, so the advertised range is not the one intended.
3. Recognise the blackhole symptom of an over-broad summary
A summary attracts every address it covers, including ones the router cannot reach. Recognising that pattern separates a summarization fault from a routing protocol fault.
4. Apply the corrected summary and verify all original prefixes remain reachable
You will confirm every component network still resolves, which is the only way to prove a summary is both correct and complete.
The topology
A distribution router advertises several downstream LAN prefixes toward the core, replacing them with a single summary route in the advertisement.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
show ip routeshow ip route 10.1.4.0 255.255.252.0show ip protocolsTopology diagram
Fact-checked references
The commands and behaviour in this lab were checked against these primary references.
Frequently asked
Why does IOS accept a summary that starts on the wrong boundary?
The mask is applied to the address you supply, so an invalid start address is simply rounded down to the block boundary rather than rejected. The configuration looks accepted while advertising a different range.
Should the summarizing router also have a null route?
It is good practice. A route to null0 for the summary prevents traffic for the uncovered portion from looping if a more specific route is also learned from elsewhere.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.