Routing: Static Routes and Introduction to OSPF

Routing: Static Routes and Introduction to OSPF

Why Routers Need Routing Information

A router forwards packets based on its routing table — a database of known networks and the paths to reach them. When a packet arrives, the router finds the best matching entry and sends the packet out the correct interface toward the destination. Without routing information for a remote network, the router drops the packet and the destination is unreachable. You build that routing information in two main ways: configuring routes by hand (static routing) or running a routing protocol that lets routers exchange information automatically (dynamic routing).

Static Routes

A static route is a manually entered instruction that tells a router: "To reach network X, send packets out interface Y, or toward next-hop address Z." Static routes are simple, predictable, and use no extra bandwidth for protocol messages. They are appropriate for small networks, stub networks with a single path, or situations where you want strict control.

IPv4 static route syntax on Cisco IOS:

```

ip route <destination-network> <subnet-mask> <next-hop-IP | exit-interface>

```

Example — R1 needs to reach the 10.0.2.0/24 network behind R2:

```

R1(config)# ip route 10.0.2.0 255.255.255.0 10.0.1.2

```

Here `10.0.1.2` is the IP address of R2's interface on the link shared with R1.

For a three-router linear topology (R1 — R2 — R3), each router needs static entries for every network it cannot see directly. R1 and R3 each need two static routes; R2 needs routes to the networks on the far sides of R1 and R3. A common mistake is forgetting both directions: pings travel out and back, so every router along the path must know how to reach both the source and destination networks.

Verify static routes with:

```

show ip route

```

Static entries appear with the code S in the routing table. Connected networks (interfaces already configured and up) appear with code C.

Administrative Distance

When a router learns about the same destination from more than one source, it uses administrative distance (AD) to choose which entry to trust. Lower AD wins. Common values:

• Connected interface: 0
• Static route: 1
• OSPF: 110
• RIP: 120

Because static routes have AD 1, they override OSPF routes for the same destination unless you remove them first. In today's lab you will remove static routes before enabling OSPF so the protocol can populate the table cleanly.

Longest-Prefix Match

If the routing table has overlapping entries — for example, 10.0.0.0/8 and 10.0.2.0/24 — the router forwards using the most specific (longest) prefix that matches the destination address. A /24 is more specific than a /8, so it wins even if the /8 has a lower AD. This rule applies regardless of which routing source supplied the entry.

OSPF Fundamentals

Open Shortest Path First (OSPF) is a link-state, interior gateway routing protocol defined in RFC 2328. Link-state means each router builds a complete map of the network topology (the link-state database, LSDB), then runs the Dijkstra Shortest Path First (SPF) algorithm to calculate the best path to every destination. The resulting routes are installed in the routing table.

Key OSPF concepts for this lab:

• Router ID: A 32-bit identifier for the router, written in dotted-decimal. Cisco IOS selects the highest loopback address, or the highest active physical interface address, unless configured manually.
• Area: A logical grouping of routers. All routers in this lab belong to Area 0, the backbone area. Single-area OSPF is the starting point for CCNA study.
• Adjacency / Neighbor: Two routers that have exchanged OSPF Hello packets and agreed on parameters (Hello/Dead timers, area ID, authentication, stub flags) become neighbors and share topology information.
• Network statement: Tells OSPF which interfaces to activate the protocol on and which area they belong to.

Enabling OSPF (Cisco IOS):

```

Router(config)# router ospf 1

Router(config-router)# network <network-address> <wildcard-mask> area 0

```

The process ID (1 here) is locally significant — it does not need to match on neighboring routers. The wildcard mask is the inverse of the subnet mask: a /24 subnet mask of 255.255.255.0 becomes wildcard 0.0.0.255.

Verification commands:

```

show ip ospf neighbor ! confirms adjacency formed

show ip route ! OSPF routes appear as O entries

show ip ospf database ! shows the link-state database

```

Safety and Lab Conduct

Physical lab work with live Cisco routers involves low-voltage DC equipment and console cable connections. Follow these practices:

• Do not force connectors. RJ-45 console connectors and USB adapters insert smoothly; forcing damages ports.
• Inspect cables before use. Frayed or damaged cables should be set aside and reported to the instructor.
• Power down routers before swapping interface modules if any module-capable chassis is used. Treat powered chassis as energized equipment.
• Keep liquids away from all equipment.
• When working in a rack or equipment room, be aware of sharp metal edges — use cut-resistant gloves if handling sheet-metal blanks or installing equipment in a rack.
• In Packet Tracer, none of the physical hazards apply, but save your file frequently to prevent losing work.

Putting It Together

In this lab you will first build connectivity using static routes, confirm full reachability, then convert the same topology to OSPF and confirm that the routing table shows O entries. Finally, you will analyze a mixed routing table and answer questions about how the router selects paths. These skills appear on both the CompTIA Network+ and Cisco CCNA exams and are foundational to every networking role.