Chapter 3 · CCNA 200-301 Study Guide · Based on the CCNA 200-301 v2.0 exam blueprint (effective February 2027)

Chapter 3: IP Connectivity

Getting packets from one network to another is the core job of a router, and the heart of the CCNA exam. This chapter covers how routers learn routes, how to read the routing table, static routing, OSPF, first-hop redundancy, and applying subnetting to real designs. Work every calculation and exercise — Cisco tests this chapter heavily, and it is all configuration you will do on the job.

3.1 How Routers Learn Routes

A router only forwards a packet if it knows a path to the packet's destination network. It learns those paths from three sources, listed here from most trusted to least trusted.

Connected routes appear automatically when you address an interface and bring it up — no extra configuration, and the most trusted routes the router knows.

Static routes are typed in by hand with the ip route command. Simple and predictable, but they never adapt when the network changes.

Dynamic routes come from routing protocols (OSPF, EIGRP, RIP, BGP) that let routers share routes and reconverge automatically after failures — essential at scale.

3.2 Reading the Routing Table

The routing table is the router's decision map. Display it with show ip route. Here is a typical output:

Gateway of last resort is 203.0.113.1 to network 0.0.0.0

S*    0.0.0.0/0 [1/0] via 203.0.113.1
      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        10.1.1.0/24 is directly connected, GigabitEthernet0/0
L        10.1.1.1/32 is directly connected, GigabitEthernet0/0
S     192.168.20.0/24 [1/0] via 192.168.10.2
O     192.168.20.128/25 [110/3] via 192.168.10.2, 00:14:22, GigabitEthernet0/1

The letter at the start of each line is the route code, telling you how the router learned the route:

The numbers in brackets are [administrative distance/metric]. Administrative distance (AD) measures how trustworthy the source is; metric measures how good this particular path is. The exam expects you to know these AD values cold:

The router's selection process for each destination runs in this exact order:

1. Longest prefix match wins. The route whose subnet mask matches the most bits of the destination address is used, regardless of AD or metric.
2. Lowest administrative distance wins (only compared among routes with the same prefix length).
3. Lowest metric wins (only compared among routes from the same source).

3.3 Static Routing

Static routes are configured with the ip route command:

R1(config)# ip route 192.168.20.0 255.255.255.0 192.168.10.2

The three parts are the destination network, its subnet mask, and the next hop — either the next-hop IP address or the local exit interface. On a point-to-point serial link you can safely point at the exit interface (ip route 192.168.20.0 255.255.255.0 Serial0/0/0), but on a multi-access network like Ethernet you should use the next-hop IP address; otherwise the router must ARP for every destination address, which hurts performance.

When the next hop is an IP address, the router performs a recursive lookup: it searches the routing table a second time to find how to reach the next-hop address itself. If the next hop is unreachable, the static route is not installed.

A default route — the "gateway of last resort" — matches everything not matched by a more specific route. It uses the quad-zero address:

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1

A floating static route is a backup path that only enters the table when the primary route disappears — create it by raising the static route's AD above the primary's:

R1(config)# ip route 10.0.0.0 255.0.0.0 172.16.1.2 120

With AD 120, this loses to OSPF's 110 under normal conditions and floats unused. If the OSPF route fails, the floating static installs and traffic keeps flowing.

3.4 OSPF Concepts

Open Shortest Path First (OSPF) is the dynamic routing protocol Cisco expects you to know best. OSPFv2 carries IPv4 routes (OSPFv3 handles IPv6). It is a link-state protocol: every router builds a complete map of the network and independently calculates the shortest path to each destination using the Dijkstra algorithm. Its metric is cost, and the lowest-cost path wins.

Areas. OSPF splits large networks into areas so no router must map the entire topology. Every design has a backbone area, Area 0, and all other areas must connect to it. A router with interfaces in multiple areas is an Area Border Router (ABR). All routers in Area 0 only = single-area OSPF; anything more = multi-area.

Router ID (RID). Each OSPF router needs a unique 32-bit ID, chosen as: (1) the address from the router-id command, (2) the highest IP among up loopback interfaces, (3) the highest IP among active physical interfaces. Duplicate RIDs break adjacency, so set them manually.

Neighbor adjacency. OSPF routers become neighbors by exchanging Hello packets on multicast 224.0.0.5. To reach the Full adjacency state, neighbors must agree on: the area ID, the Hello and Dead timers, the subnet they share, authentication settings, the stub-area flag, and the MTU. The adjacency progresses through these states:

Hello and Dead timers. On broadcast and point-to-point networks, Hellos go out every 10 seconds and a neighbor is declared dead after 40 seconds of silence (the Dead interval defaults to four times the Hello). On NBMA networks the defaults are 30 and 120 seconds. Mismatched timers prevent adjacency.

DR and BDR election. On multi-access networks (broadcast, NBMA), OSPF elects a Designated Router (DR) and Backup (BDR) so routers form full adjacencies only with the DR/BDR instead of every neighbor. Highest OSPF priority (0–255, default 1) wins; ties break on highest router ID. Priority 0 = never DR/BDR. The election is not preemptive: a higher-priority newcomer does not displace an elected DR. Updates go to the DR/BDR on 224.0.0.6. Point-to-point links have no DR/BDR.

Cost calculation. Interface cost = reference bandwidth ÷ interface bandwidth. The default reference bandwidth is 100 Mbps (100,000,000 bps), which you can raise with auto-cost reference-bandwidth (value in Mbps) so fast modern links get distinct costs.

3.5 Configuring Single-Area OSPFv2

Basic OSPF configuration needs a process ID (locally significant — it does not have to match between neighbors) and network statements that use wildcard masks to select which interfaces participate:

R1(config)# router ospf 1
R1(config-router)# router-id 1.1.1.1
R1(config-router)# network 192.168.10.0 0.0.0.255 area 0
R1(config-router)# network 10.0.0.1 0.0.0.0 area 0
R1(config-router)# passive-interface GigabitEthernet0/1
R1(config-router)# auto-cost reference-bandwidth 1000

Notes: the wildcard mask is the inverse of a subnet mask (0s = must match, 1s = ignore), so 0.0.0.0 selects one exact address. passive-interface stops Hellos on LANs with no OSPF neighbor. Verify with show ip ospf neighbor (Full state) and show ip ospf interface (timers, DR/BDR, cost).

3.6 First-Hop Redundancy Protocols

Hosts send off-network traffic to their default gateway — but one gateway router is a single point of failure. First-hop redundancy protocols (FHRPs) let two or more routers share a virtual IP address that hosts use as their gateway; if the active router fails, a standby takes over the virtual IP within seconds, and hosts never change their configuration.

HSRP elects one Active router to own the virtual IP and MAC; the Standby takes over if Hellos stop. VRRP works the same way but is vendor-neutral. GLBP differs: the Active Virtual Gateway answers ARP requests with a different virtual MAC per forwarder, spreading client traffic across up to four routers while still providing failover.

A typical HSRP configuration on a Layer 3 switch:

SW1(config)# interface Vlan10
SW1(config-if)# ip address 192.168.10.2 255.255.255.0
SW1(config-if)# standby 10 ip 192.168.10.1
SW1(config-if)# standby 10 priority 110
SW1(config-if)# standby 10 preempt

The group number (10) must match on both routers, the virtual IP (192.168.10.1) is the hosts' gateway, and preempt lets the higher-priority router reclaim the Active role after recovering.

3.7 Applying Subnetting to Network Design

Subnetting is not just an exam exercise — it drives real designs. Two rules of thumb cover most cases:

Point-to-point links get /30s. A serial or routed point-to-point link has exactly two endpoints, so a /30 (2 usable host addresses, 4 addresses total) is the perfect fit with zero waste. (A /31 is also valid per RFC 3021, but /30 remains the CCNA convention and the safe exam answer.)

Size VLANs for hosts plus growth. Count the hosts the VLAN needs today, add room for growth (20–30% is typical), then pick the smallest subnet that fits using 2^n − 2 ≥ required hosts.

End-of-Chapter Test