INGIniousThis lab will allow you to:
- Design: choose OSPFv3 interface costs that produce a specific set of best paths.
- Diagnose: use vtysh to inspect the OSPFv3 routing table and verify that SPF selects the expected next-hop interfaces.
In this lab, you will need to verify and enforce the following intradomain routing property:
- From every router, the next-hop interface used to reach each destination loopback must match the reference table given below.
In this lab, all OSPFv3 adjacencies are correct by construction. You should not encounter any neighbor stuck below the Full state. There is nothing to debug at the adjacency level. The only thing you have to configure is the OSPFv3 interface cost on each link, so that SPF computes the expected best paths.
Lab Presentation
This image represents the topology used by the lab. All adjacencies will form correctly as soon as the lab is started; your job is to assign costs.
Reference: Expected Best Paths
The following table indicates, for each (source, destination) pair, the interface of the source router used in the SPF best path. This is the reference used by the automatic correction.
| Source \ Dest | R1 lo | R2 lo | R3 lo | R4 lo | R5 lo |
|---|---|---|---|---|---|
| R1 | local | eth0 | eth0 | eth0 | eth1 |
| R2 | eth0 | local | eth0 | eth0 | eth0 |
| R3 | eth0 | eth0 | local | eth1 | eth1 |
| R4 | eth1 | eth0 | eth0 | local | eth1 |
| R5 | eth0 | eth1 | eth0 | eth0 | local |
Routing Policy
The most counter-intuitive entry of the reference table is R2 → R5, which is forced to traverse R3 and R4 instead of taking the direct link R2-R5. The operator's rationale is the following:
- The direct R2–R5 link is a low-bandwidth fallback line. R2 must not prefer it for transit traffic, but only as a backup.
- R2's traffic to R5 must exit via R2's interface toward R3, and be forwarded along the R3 → R4 backbone, even though this path is one hop longer.
- Note that R2 has a direct link to R1. However, R1's uplink is reserved for R1's own egress traffic: any cost assignment that would route R2 → R5 traffic through R1 is therefore invalid, even if it satisfies the interface constraint.
Your cost assignment must be consistent with this intent for every entry of the table, not only for the R2 → R5 case. [1]
📥 Download the Base Lab
To begin the exercise, you need to download the archive. It contains the base configuration with errors to fix.
Diagnostic Tools
vtysh is the unified command-line interface of FRRouting.
Useful commands for validating SPF results:
# Access vtysh root@r1:/# vtysh r1# # Show running configuration (router-id, interfaces, costs) show running-config # Show the OSPFv3 routing table (SPF result, with next-hop interface) show ipv6 ospf6 route # Inspect interface-level OSPF state (notably the configured cost) show ipv6 ospf6 interface <ifname> # Confirm that adjacencies are healthy (sanity check, should all be Full) show ipv6 ospf6 neighbor
show ipv6 ospf6 route.
That field, not the absolute cost, is what the correction compares against the reference table.Recommended Approach
Explore the topology
Examine the
lab.conffile to understand connectivity and map interfaces to links.Connect to containers
cd ./variant_X kathara lstart kathara connect [node_name] kathara wipe
Derive a cost assignment that produces the expected best paths
Apply the costs in the FRRouting configuration files of the relevant routers
Validate SPF paths against the reference
On each router, run
show ipv6 ospf6 routeand check that every destination is reached via the interface specified in the reference table.📌 Important Rules:
- ❌ Do not modify
lab.conf - ❌ Do not create new files
- ✅ Only adjust existing configuration files (interface cost statements)
- ❌ Do not modify
Validate your configuration locally
cd ./variant kathara_lab_checker --config correction_student.yaml --no-cache --lab . or python3 -m kathara_lab_checker --config correction_student.yaml --no-cache --lab .
Submit your solution
Compress the configured
variant_Xfolder and upload it.
- Kathara: https://github.com/KatharaFramework/Kathara/wiki
- FRRouting Documentation (OSPFv3): https://docs.frrouting.org/en/latest/ospf6d.html
- RFC 5340 (OSPF for IPv6): https://datatracker.ietf.org/doc/rfc5340/
| [1] | Finding cost values that satisfy all 10 entries by hand is feasible but it don't scale well with large network. The problem can be modelled as a constraint satisfaction problem (CSP) and solved in milliseconds with a tool such as pycsp3. See the Constraint Programming course for the underlying theory. |