Lesson 35 · Routing

Static routes

A router only knows the networks plugged into it. Watch an administrator type static routes into three routers, one at a time, until a ping from LAN A reaches LAN C and the reply finds its way back. Then add a default route to the ISP, break a cable, and see why static routes need a floating backup.

10 min12 steps5 questionsBeginner

Fig 35 · Interactive

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5 quick questions

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01A brand-new router has three working ports with addresses. Which routes does it have before anyone types anything?

A router adds a connected route for every working port by itself. Anything further away has to be configured or learned.

02Router 1 has 192.168.3.0/24 via 10.0.12.2. What does “via 10.0.12.2” tell it?

A static route is a destination network plus a next hop. The next hop must be on a network the router is directly connected to.

03A ping reaches Laptop C, but Laptop A never gets the reply. What is the most likely mistake?

Routes work in one direction. Every router on the way back also needs a route to the sender's network, or the reply is dropped.

04Router 1 has 192.168.3.0/24 via 10.0.12.2 and 0.0.0.0/0 via 203.0.113.1. Where does a packet for 192.168.3.10 go?

Both routes match, but the router picks the most specific one (longest prefix). The default route is only used when nothing more specific fits.

05Why does a floating static route use administrative distance 200 instead of 1?

Lower distance wins. With 200 the backup loses to the normal static route and is only installed when the main route goes away, for example when its port goes down.

Key terms

Glossary

Static route: A route an administrator writes by hand: a destination network plus a next hop. It never changes by itself.
Connected route: A route a router adds automatically for each network plugged into one of its working ports.
Next hop: The neighbouring router a packet is handed to on its way to a destination network.
Return route: The route back toward the sender's network. Without it, replies are dropped.
Default route: 0.0.0.0/0: matches every address and is used only when no more specific route fits. Often points to the ISP.
Administrative distance: A trust score for routes to the same destination (0 to 255). Lower wins; connected 0 and static 1 on Cisco routers.
Floating static route: A backup static route with a higher administrative distance, used only when the main route disappears.
Dynamic routing: Protocols such as OSPF and BGP that let routers tell each other about networks and failures, so routes adapt on their own.

Fine print

Notes and sources

LANs and router links use RFC 1918 private addresses; the ISP link (203.0.113.0/24) and the website (198.51.100.7) use RFC 5737 documentation addresses.
RFC 1812 §5.2.4.4 describes this "administrative preference": a number from 0 to 255, lower wins, 255 means never use. The values here (connected 0, static 1) are Cisco IOS defaults; other vendors use other numbers (Juniper: static preference 5).
For clarity the figure only adds the routes needed between LAN A and LAN C. LAN B would need routes on router 1 and router 3 too, written the same way.
LAN A uses private addresses, so reaching the website also needs NAT at router 1, which is a later lesson. Router 2 and router 3 have no default route here.
A floating static route only takes over when the router itself sees the main route fail, for example when its own port goes down. A failure further away stays invisible to it.

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