Reading a routing table: Linux, Cisco IOS and Junos output
For network technicians, systems administrators and MSP engineers who know subnetting and the idea of a default gateway (see Subnets and gateways, and Switching and routing) and now need to read real routing tables with confidence. You'll pick apart route entries in Linux ip route, Cisco IOS show ip route and Junos show route output, choose the route a device uses by applying longest match before preference and metric, confirm the decision with ip route get, interpret connected, local, default, blackhole, unreachable, multipath and policy-routing entries, and trace a failing path in both directions. Grounded in RFC 1812, RFC 4632, the Linux ip-route and ip-rule manuals, Cisco's route selection article and Juniper's show route reference. Run commands only on devices you're authorised to manage; most are read-only, but route changes go through your change process. Ends with a supervisor-graded path investigation.
- Level
- Intermediate
- Length
- About 95 minutes
- Contents
- 5 lessons · final exam
- Status
- Published · updated 10 Oct 2026
Skills you'll practise
- Identify the parts of a routing table entry (prefix, source protocol, administrative distance or preference, metric, next hop and interface) in Linux, Cisco IOS and Junos output
- Choose the route a router uses for a destination by applying longest match first, then administrative distance and metric
- Use ip route get and per-destination lookups to confirm the route and source address a device will actually use
- Interpret special routes: connected and local, default, blackhole or discard, unreachable and prohibit, multipath, and policy routing tables
- Diagnose a reachability problem from the routing tables on both ends, including a missing return route
Course outline
- 1.Identify the parts of a routing table entry in Linux, Cisco IOS and Junos outputLesson · 16 min
- 2.Choose the route a router uses: longest match first, then administrative distance and metricLesson · 16 min
- 3.Use ip route get and per-destination lookups to confirm the route and source addressLesson · 14 min
- 4.Interpret special routes: connected, local, default, blackhole, unreachable, multipath and policy tablesLesson · 14 min
- 5.Diagnose a reachability problem from the routing tables on both endsLesson · 15 min
- 6.Reading a routing table: Linux, Cisco IOS and Junos output: knowledge checkKnowledge check · 16 questions
- 7.Reading a routing table: Linux, Cisco IOS and Junos output: practical exerciseKnowledge check · 1 question
- 8.Final exam10 questions · passing it completes the course, so people who already know the material can test out
Sources it draws on
The lessons and questions are written from these references, so learners can go back to the original.
- RFC 1812: Requirements for IP Version 4 Routers (section 5.2.4.3 next-hop selection pruning rules; 5.2.4.4 administrative preference)
- RFC 4632: Classless Inter-domain Routing (longest-match forwarding; aggregates discard unmatched traffic)
- Linux man page ip-route(8): route types, proto, scope, src, metric, multipath next hops, ip route show and ip route get
- Linux man page ip-rule(8): routing policy database, rule priorities and the local, main and default tables
- Cisco: Route selection in Cisco routers (administrative distance, metrics, longest prefix in forwarding)
- Juniper Networks: show route (Junos CLI reference; active route markers, [protocol/preference], selected next hop, Discard)
See it with your own jobs and topics
Tell us about your team and we'll walk you through setup, from choosing jobs to your first skills check.