CCNA 200-301 Cheat Sheet: 3.0 IP Connectivity (25%)

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May 29, 2025

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After reviewing the physical and data-link layers of the network in previous sections, we now move up to a core area of networking: IP Connectivity. This domain is where we start making real routing decisions—how packets find their way across networks, how protocols like OSPF work, and how routers avoid downtime through redundancy.

This section represents the largest weight on the CCNA exam at 25%, and rightly so—it’s essential for both certification and daily networking tasks.

3.1 Routing Table Interpretation

The routing table is the heart of how a router makes forwarding decisions. Every route in the table tells the router where to send traffic for a given network.

Key components you should understand:

Routing Codes: Letters that indicate the route type (e.g., C for connected, S for static, O for OSPF).
Next Hop: The IP address of the next device to which the packet should be sent.
Metric: A value used to choose the best route when multiple paths exist. Lower is better.
Administrative Distance (AD): The trust level of the route source. Lower AD means more preferred.
Default Route: A catch-all route (usually 0.0.0.0/0) used when no other specific route matches.

Interpreting routing tables accurately is critical for troubleshooting and validating routing behavior.

3.2 Routing Decision Process

When a router receives a packet, it goes through a decision-making process to choose the best path:

Longest Prefix Match (LPM): The route with the most specific (longest) match to the destination IP is chosen. For example, /30 takes priority over /24 or /16.
Administrative Distance: If there are multiple routes to the same destination with different sources (e.g., static vs OSPF), the route with the lower AD is preferred.
Metric Evaluation: If routes have the same prefix and AD, the metric (e.g., hop count, bandwidth) is compared to find the most efficient path.

Understanding this hierarchy helps explain why certain routes are preferred and why others may be ignored.

3.3 IPv4 and IPv6 Static Routing

Static routing involves manually adding routes to the routing table. It’s simple, secure, and useful in small or controlled environments.

Types of static routes:

Default Route: Directs traffic destined for unknown networks (e.g., ip route 0.0.0.0 0.0.0.0 192.168.1.1).
Network Route: Routes for specific subnets (e.g., ip route 192.168.2.0 255.255.255.0 10.0.0.2).
Host Route: Most specific; used for a single IP address (e.g., /32 mask).
Floating Static Route: A backup route with a higher AD than a dynamic route; activates if the primary fails.

IPv6 static routes follow similar logic but use ipv6 route and typically /64 or /128 prefixes.

3.4 OSPFv2 Configuration (Single Area)

Open Shortest Path First (OSPF) is a dynamic link-state routing protocol widely used in enterprise networks.

Key configuration topics for the CCNA exam:

Single-Area OSPF: All routers share the same area (usually Area 0, the backbone).
Router ID (RID): A unique 32-bit identifier for each OSPF router; either manually configured or taken from the highest IP address.
Neighbor Adjacency: Routers must agree on certain parameters (hello/dead intervals, area ID, authentication) to form a neighbor relationship.
DR/BDR (Designated Router / Backup Designated Router): Used in multi-access networks (like Ethernet) to reduce OSPF traffic. The DR handles all LSAs to limit overhead.

Understanding how to configure, verify, and troubleshoot OSPF is essential, especially in real-world scalable networks.

3.5 First Hop Redundancy Protocols (FHRP)

In most networks, the default gateway is a single IP address. But what happens if that gateway device fails? That’s where First Hop Redundancy Protocols come in.

While configuration isn’t required at the CCNA level, understanding the concept is:

HSRP (Hot Standby Router Protocol): Cisco proprietary. Uses an active/standby model.
VRRP (Virtual Router Redundancy Protocol): Open standard, similar to HSRP, with one master router.
GLBP (Gateway Load Balancing Protocol): Cisco proprietary. Offers redundancy and load balancing.

These protocols ensure hosts always have an available default gateway by using a virtual IP that moves between routers as needed.

Final Thoughts

The IP Connectivity section is where everything starts coming together. You’re not just configuring individual devices—you’re building a connected, redundant, and intelligent network. This is a domain where lab practice is essential. Whether it’s interpreting routing tables or deploying OSPF in a lab environment, real configuration experience will make these topics stick.

Up next, we’ll dive into 4.0 IP Services, where we cover NAT, NTP, DHCP, SNMP, and other critical services that make networks more dynamic and functional.