OSPF Area Boundary Trap
Three /24s arrive in area 0 where one summary route was expected.
Free sample. Three routers, two areas, one misconfiguration that doesn't throw an error and doesn't take an adjacency down. It just quietly doesn't do what it's supposed to.
Delivered by Gumroad, which emails you the download link. Requires Docker and Containerlab. First time? See Getting started (ten to twenty minutes; longer the first time on a machine that has never had WSL2 or a VM).
Topology
- r1: Area 0 only
- r2: interfaces in Area 0 and Area 1
- r3: Area 1 only, with three /24 stub networks
Symptom
The intent of this design is that r3's three subnets are summarized into a single route before they reach Area 0. Check r1:
r1# show ip route ospfO>* 10.3.2.0/24
O>* 10.3.3.0/24
All three subnets are reachable. Nothing is down. Summarization simply isn't happening. Find out why.
What's in the download
- The topology file and a startup config for each of the three routers, with the fault applied as the lab starts.
labctl, the launcher: deploy, a tmux window per router, a shell on any router, reset, andverifyto check your fix.- The scenario, with the investigation questions to work through before anything is explained.
- The reveal: why the protocol behaves this way, the fix, and the same trap on Cisco IOS-XE, kept closed until you choose to open it.
lab.html, all of it as one page that opens in a browser with no network.
Start the lab
Download the zip from the link in your email, get it into ~/labs in your Linux shell the way Run a lab in Getting started shows for your platform, then start it:
cd ~/labs/lab-01-area-boundary-trap
./labctl deployLabs must live under the Linux home (~/labs), not under /mnt/c. Windows-mounted paths carry no Unix file permissions, so labctl arrives without its executable bit and will not run, and they are markedly slower to run from. Copy the zip across rather than unzipping it on the Windows drive.
If ./labctl reports Permission denied, run chmod +x labctl once. Some Windows unzip tools drop the executable bit, and a copy through /mnt/c always does.
The launcher brings the topology up, waits for OSPF to converge, then applies the scenario. When it returns, open lab.html in the lab folder in a browser: it has the investigation questions and, when you have a theory, the reveal. From WSL2, explorer.exe . opens the folder in File Explorer, where you can double-click it. On a Mac the zip is still in the Mac's Downloads folder too: double-click it there and open lab.html from the folder it unzips to. ./labctl tabs then gives you a window per router to work in.
Landed here without the setup done? Getting started installs Docker and Containerlab (and WSL2 on Windows, or a Linux VM on a Mac) in ten to twenty minutes; longer the first time on a machine that has never had WSL2 or a VM.
The full pack has nine more of these (four OSPF, five BGP), each with its own launcher and verifier. Get the pack.