Quick review: Routing – CCNA

What you really need to know

Routing is the process that allows IP packets to travel between different networks. If switching mainly works inside a LAN and inside a VLAN, routing connects networks, subnets, VLANs and different sites.

For CCNA, you need to understand how a router or Layer 3 switch chooses the best path, how the routing table works, when to use static routes, default routes and dynamic protocols such as OSPF.

The central point is this: a Layer 3 device forwards packets based on the destination IP address and the best route present in the routing table.

Key concepts

• Router: device that forwards packets between different networks.
• Routing table: table that contains known routes.
• Next hop: next device to which the packet is sent.
• Exit interface: interface through which the packet leaves.
• Static route: route configured manually.
• Default route: route used when there is no more specific route.
• Dynamic routing: automatic learning of routes through protocols.
• OSPF: link-state dynamic routing protocol used in CCNA.
• Metric: value used by a protocol to choose the best path.
• Administrative distance: trustworthiness of a routing source.
• Longest prefix match: selection of the most specific route.
• Loopback interface: logical interface often used for identification and testing.

Differences not to confuse

Concept	Main meaning
Routing	Forwarding between different networks
Switching	Layer 2 forwarding inside the LAN/VLAN
Static route	Manually inserted route
Dynamic route	Route learned from a protocol
Default route	Route used when there is no specific route
Next hop	Next router in the path
Exit interface	Outgoing interface
Metric	Path cost for a protocol
Administrative distance	Reliability of the route source
Longest prefix match	Preference for the most specific route

How routing works

When a router receives an IP packet, it reads the destination IP address and checks the routing table.

The router looks for the best route to reach that destination. If it finds a match, it forwards the packet toward the next hop or through the correct exit interface.

If it finds no route and there is no default route, the packet is discarded.

For CCNA, you need to remember that routing works at Layer 3 and is based on IP addresses, not MAC addresses like Layer 2 switching.

Routing table

The routing table contains the networks known by the device.

Routes can come from:

• directly connected networks;
• static routes;
• default routes;
• dynamic routing protocols;
• routes learned through other mechanisms.

A route usually indicates:

• destination network;
• subnet mask or prefix length;
• next hop;
• exit interface;
• route source;
• metric;
• administrative distance.

In Cisco commands, different routes may appear with codes such as:

• C = connected;
• L = local;
• S = static;
• O = OSPF;
• R = RIP, although today it is less central;
• D = EIGRP, more relevant in Cisco environments but less central in the current CCNA than OSPF.

Directly connected and local routes

A connected route appears when an interface has an IP address configured and is active.

A local route represents the specific IP address of the router interface, often as a /32 route for IPv4.

Conceptual example:

• interface: 192.168.10.1/24;
• connected route: 192.168.10.0/24;
• local route: 192.168.10.1/32.

For CCNA, you need to know how to interpret these elements in the routing table.

Longest prefix match

Longest prefix match is a fundamental rule: if multiple routes match the destination, the most specific route is chosen.

Example:

• route 192.168.0.0/16;
• route 192.168.10.0/24;
• destination 192.168.10.50.

The router chooses 192.168.10.0/24 because it is more specific.

This also applies when a default route exists. The default route is used only when there is no more specific route.

Static route

A static route is manually configured by the administrator.

It is useful when:

• the network is small;
• the path is simple and stable;
• you want to control traffic exactly;
• a route to a specific network is needed;
• a backup route is needed.

Conceptual example:

• command: ip route 192.168.20.0 255.255.255.0 10.0.0.2;
• meaning: to reach network 192.168.20.0/24, send traffic to next hop 10.0.0.2.

Static routes are simple, but they do not automatically adapt to network changes.

Default route

The default route is the route used when there is no more specific route in the table.

In IPv4, it is often represented as 0.0.0.0/0.

Conceptual example:

• command: ip route 0.0.0.0 0.0.0.0 10.0.0.1;
• meaning: send traffic toward unknown networks to next hop 10.0.0.1.

The default route is widely used to send traffic toward the Internet or an upstream router.

In CCNA quizzes, if a router needs to reach unknown networks and has only one path outward, the correct answer is often a default route.

Floating static route

A floating static route is a static route configured with a higher administrative distance, so it is used only if the primary route is unavailable.

Example:

• primary route learned from OSPF;
• backup static route with higher AD;
• if OSPF disappears, the static route enters the table.

This mechanism is useful for backup and redundancy.

Dynamic routing

Dynamic routing uses protocols that allow routers to exchange information about networks.

Advantages:

• adapts to changes;
• reduces manual configuration;
• scales better in medium and large networks;
• can choose alternative paths.

Disadvantages:

• requires correct configuration;
• can introduce complexity;
• consumes resources;
• if misconfigured, it can create routing problems.

For CCNA, the most important dynamic protocol to know is single-area OSPF.

OSPF

OSPF, Open Shortest Path First, is a link-state dynamic routing protocol.

In CCNA, you especially need to know:

• area concept;
• single-area OSPF;
• neighbor adjacency;
• router ID;
• OSPF cost;
• advertised networks;
• DR and BDR on multiaccess networks;
• show ip ospf neighbor command;
• show ip route ospf command.

OSPF builds a view of the topology and calculates the best path using cost.

OSPF router ID

The router ID identifies an OSPF router.

OSPF chooses the router ID in this order:

• manually configured router ID;
• highest IP address of an active loopback;
• highest IP address of an active physical interface.

Best practice: manually configure the router ID or use stable loopbacks.

The router ID is important because it identifies the router in OSPF adjacencies.

Neighbor adjacency

To exchange OSPF information, routers must become neighbors.

They must have compatible parameters, such as:

• same area;
• same subnet on the link;
• compatible hello/dead timers;
• compatible authentication, if used;
• compatible MTU in many cases;
• compatible network type.

If two routers do not become neighbors, they do not correctly exchange OSPF routes.

DR and BDR

On multiaccess networks, such as Ethernet, OSPF can elect a DR and a BDR.

• DR = Designated Router;
• BDR = Backup Designated Router.

They are used to reduce the number of full adjacencies between routers on the same network.

The election depends on:

• OSPF priority;
• router ID.

The router with the highest priority wins. In case of a tie, the highest router ID wins.

OSPF metric

OSPF uses cost as its metric.

Cost is related to interface bandwidth. Paths with a lower total cost are preferred.

Important concept:

• lower cost = preferred path.

For CCNA, you do not need to become mathematical, but you must understand that OSPF chooses based on cost, not simply on hop count.

Administrative distance

Administrative distance indicates how trustworthy a routing source is.

If two different sources propose a route to the same network with the same prefix length, the router chooses the one with the lower administrative distance.

Common values to remember:

Source	Administrative distance
Connected	0
Static	1
OSPF	110

Example: if there is a static route and an OSPF route for the same network and same prefix, the static route wins because AD 1 is lower than 110.

Metric vs administrative distance

This is a very important difference.

Administrative distance chooses between different routing sources.

Metric chooses between paths of the same protocol.

Example:

• static route vs OSPF = administrative distance matters;
• two OSPF paths to the same network = OSPF metric matters.

Many quiz mistakes come from confusing these two concepts.

Routing troubleshooting

When a network is not reachable, you need to reason in an orderly way.

Typical checks:

• correct IP addresses and subnet masks;
• interfaces up/up;
• correct gateway on hosts;
• route present in the table;
• next hop reachable;
• ACL or firewall not blocking traffic;
• correct OSPF neighbors;
• default route present if needed;
• return traffic correctly configured.

Useful commands:

• show ip route;
• show ip interface brief;
• ping;
• traceroute;
• show ip protocols;
• show ip ospf neighbor;
• show running-config.

Common quiz mistakes

• Confusing routing and switching.
• Thinking that a different VLAN communicates without routing.
• Forgetting the return route.
• Thinking that the default route always wins.
• Forgetting longest prefix match.
• Confusing metric and administrative distance.
• Thinking that OSPF always chooses based on hop count.
• Forgetting that OSPF requires neighbor adjacency.
• Confusing router ID and interface IP address.
• Thinking that a static route updates automatically.
• Forgetting that connected route and local route are different.

Mini exam scenario

A router has these routes:

• 10.0.0.0/8 toward Router A;
• 10.1.1.0/24 toward Router B;
• 0.0.0.0/0 toward the Internet.

It must forward a packet to 10.1.1.50. The chosen route will be 10.1.1.0/24, because it is the most specific route according to longest prefix match.

Mini checklist before the quiz

Before starting the quiz, you should be able to explain:

• what a router does;
• how a routing table works;
• what next hop means;
• what exit interface means;
• what connected routes and local routes are;
• what a static route does;
• what a default route is used for;
• what floating static route means;
• what OSPF does;
• how the OSPF router ID is chosen;
• what DR and BDR are used for;
• the difference between metric and administrative distance;
• why longest prefix match is fundamental.