3.2 Layer 3
📘CCNP Encore (350-401-ENCORE-v1.1)
This section focuses on understanding the differences between EIGRP and OSPF, two common routing protocols in Layer 3 networking. The exam expects you to know:
- Their routing types (distance vector vs. link-state)
- Metrics and path selection
- OSPF area types and basic design principles
1. Routing Types: Distance Vector vs. Link-State
Routing protocols are classified into two main types:
| Feature | EIGRP | OSPF |
|---|---|---|
| Routing Type | Advanced Distance Vector | Link-State |
| How It Works | Shares routing tables with neighbors. Each router knows the best path to a network using metrics. | Routers build a full map of the network topology and calculate the shortest path using the SPF algorithm. |
| Convergence | Fast, uses DUAL algorithm to quickly find backup paths | Fast, but slightly slower in large networks because it recalculates SPF tree |
| Routing Updates | Sends partial updates only when changes occur | Sends LSAs (Link-State Advertisements) to all routers in the area, flooding the network |
Key Point for Exam:
- EIGRP = faster in smaller environments, less resource-intensive
- OSPF = scalable for large enterprise networks
2. Metrics and Path Selection
Both protocols use metrics to select the best path to a destination, but they calculate them differently.
EIGRP Metrics
- Uses bandwidth, delay, reliability, load, and MTU (MTU is ignored for path selection)
- Formula is complex but simplified in exams: lower total metric = better path
- Example in IT: If you have multiple WAN links to a branch office, EIGRP will choose the path with higher bandwidth and lower delay.
OSPF Metrics
- Uses cost, which is based on interface bandwidth
- Default formula:
Cost = 100,000,000 / bandwidth in bps - Lower cost = preferred path
- Example: OSPF prefers a 1 Gbps link (cost 100) over a 100 Mbps link (cost 1000)
Key Exam Difference:
- EIGRP uses composite metrics (multiple factors)
- OSPF uses bandwidth-based cost
3. OSPF Area Types
OSPF is area-based to optimize large networks. You don’t need to memorize every command, but understand the concepts:
| Area Type | Description | Exam Tip |
|---|---|---|
| Backbone (Area 0) | Core of OSPF network. All areas connect here. | Must always exist |
| Standard/Normal Area | Stores full routing info. Can connect multiple routers | Common default |
| Stub Area | Blocks external routes (reduces routing table size) | Only one exit point (default route) |
| Totally Stubby Area (TSA) | Blocks external and inter-area routes, only default route | Cisco-specific |
| Not-So-Stubby Area (NSSA) | Allows importing external routes but still limited | Useful for connecting external networks via OSPF |
Key Exam Tip:
- You should know which area types reduce routing table size and which allow external routes.
4. Summarized Comparison
| Feature | EIGRP | OSPF |
|---|---|---|
| Type | Advanced Distance Vector | Link-State |
| Metric | Bandwidth, Delay, Reliability, Load | Cost (Bandwidth-based) |
| Algorithm | DUAL | SPF (Shortest Path First) |
| Convergence | Very fast, uses feasible successors | Fast, but recalculates SPF tree |
| Updates | Partial (triggered only when topology changes) | Full LSAs flooded within the area |
| Scalability | Medium | High (supports large networks with areas) |
| Design Complexity | Simple | Complex (requires area planning) |
| Common Use in IT | Smaller to medium enterprise WAN | Large enterprise LAN/WAN |
5. Exam Focus Points
- Know the type: EIGRP = distance vector, OSPF = link-state
- Know metrics: EIGRP = composite, OSPF = cost
- Understand areas: Backbone (0), Stub, Totally Stubby, NSSA
- Remember convergence differences: EIGRP has backup paths pre-calculated (feasible successors), OSPF recalculates SPF tree
- Know update methods: Partial vs. full LSAs
6. Simple IT Example for Understanding
- EIGRP: Imagine 3 routers in an office WAN. Each router tells neighbors only about changes (faster, smaller messages). Router calculates backup paths in case a link goes down.
- OSPF: Imagine 10 routers in a corporate network. Each router has a complete map of the network, so if a link fails, it recalculates the best path for all routes (more data, more planning).
✅ Tips to Remember for Exam:
- Distance Vector = EIGRP → Think table-based, neighbor updates
- Link-State = OSPF → Think network map, SPF tree
- EIGRP metrics = multiple factors
- OSPF metrics = bandwidth-based cost
- OSPF areas reduce routing table and improve scalability
