Quick answer: EIGRP uses a composite metric (bandwidth, delay, load, reliability) and converges faster via Diffusing Update Algorithm (DUAL), while OSPF uses cost based on bandwidth and converges via Shortest Path First (SPF). EIGRP is Cisco-proprietary (though partially open), has an administrative distance of 90/170, and scales well in large, hierarchical networks; OSPF is an open standard (AD 110) and is mandatory for multi-vendor environments. On the CCNA exam, know that EIGRP is best for Cisco-only networks needing rapid convergence, while OSPF is required for interoperability and link-state precision.
Why This Comparison Matters for the CCNA
The CCNA exam tests your ability to choose the right routing protocol for a given scenario. EIGRP and OSPF are both interior gateway protocols (IGPs) but operate on fundamentally different principles. EIGRP is a hybrid protocol (distance vector with link-state features), while OSPF is a pure link-state protocol. Misunderstanding these differences is a common trap that costs exam points.
Your goal is to master the mechanics, not just memorize facts. Let’s break down every key dimension side by side.
Metric Calculation: EIGRP’s Composite vs OSPF’s Cost
EIGRP calculates metric using a formula:Metric = [K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * 256
By default, only bandwidth and delay are used (K1=1, K3=1, others=0). The formula favors higher bandwidth and lower delay. EIGRP uses the slowest bandwidth along the path and the cumulative delay.
OSPF uses a simple cost metric:Cost = 10^8 / interface bandwidth (bps)
This is derived from the reference bandwidth (default 100 Mbps). For Fast Ethernet (100 Mbps), cost = 1; for Gigabit Ethernet, cost = 10 (since 10^8 / 10^9 = 0.1, rounded up to 1? No—Cisco rounds to 1, but the actual calculation uses the formula, so cost = 1 for both 100 Mbps and 1 Gbps unless you adjust the reference bandwidth).
Exam trap: OSPF cost is inversely proportional to bandwidth—higher bandwidth = lower cost. But if all links are Fast Ethernet or faster, costs become equal, forcing you to manually set cost or adjust the reference bandwidth. EIGRP’s metric is more granular and avoids this flat-line issue.
| Feature | EIGRP | OSPF |
|---|---|---|
| Metric type | Composite (bandwidth, delay, load, reliability) | Cost (bandwidth-based) |
| Default metric values | Bandwidth and delay only | 10^8 / interface bandwidth |
| Metric granularity | High (values up to 4.29 billion) | Low (integer, often 1–1000) |
| Load balancing | Equal-cost and unequal-cost (variance) | Equal-cost only |
| Tuning complexity | Moderate (K values, variance) | Simple (cost or reference bandwidth) |
Convergence: DUAL vs SPF
EIGRP uses the Diffusing Update Algorithm (DUAL). When a link fails, EIGRP immediately checks its topology table for a feasible successor—a backup route that meets the feasibility condition (reported distance < feasible distance). If a feasible successor exists, convergence is instant (sub-second). If not, DUAL sends queries to neighbors, causing active state and slower convergence.
OSPF uses the Shortest Path First (SPF) algorithm (Dijkstra). Upon a topology change, OSPF runs SPF to recalculate the entire shortest-path tree. This is computationally heavier and takes longer (milliseconds to seconds). OSPF converges slower than EIGRP when feasible successors exist, but faster than EIGRP’s query process in large networks.
Exam trap: The CCNA often asks: “Which protocol converges faster?” The answer is “EIGRP, if a feasible successor exists; otherwise, OSPF may converge faster.” Memorize that EIGRP’s convergence depends on route availability, while OSPF always runs SPF.
Scalability and Network Design
EIGRP scales well in large, flat networks but lacks true hierarchical design. It uses autonomous systems (AS) and can summarize at any interface. EIGRP’s query scope can cause issues in very large networks (1000+ routers) because queries propagate throughout the AS.
OSPF is designed for hierarchical scalability with areas. The backbone area (area 0) connects all other areas. This limits the impact of topology changes to a single area. OSPF supports up to 50 routers per area in practice (Cisco recommends 30–50). OSPF scales better for enterprise and service provider networks.
Exam trap: EIGRP is often called “hybrid” but is classified as an advanced distance vector protocol. OSPF is pure link-state. For the exam, know that OSPF requires area design, while EIGRP does not.
Administrative Distance: Which Protocol Wins?
Administrative distance (AD) is the trustworthiness of the route source. Lower AD = more trusted.
- EIGRP internal routes: AD 90
- EIGRP external routes: AD 170
- OSPF internal and external routes: AD 110
If both protocols learn the same route, EIGRP wins (AD 90 < 110). This is a common exam scenario: “Which route is installed in the routing table?” Answer: EIGRP internal route.
Exam trap: EIGRP external routes (AD 170) are less trusted than OSPF (AD 110). So if a route is redistributed into EIGRP, OSPF will be preferred. Also, EIGRP summary routes have AD 5 (by default), which is even lower.
When Cisco Uses Each Protocol in Real Networks
Cisco recommends EIGRP for:
- Cisco-only environments
- Networks requiring fast convergence and unequal-cost load balancing
- Small-to-medium networks (under 500 routers)
- Sites with limited WAN bandwidth (EIGRP is less chatty)
Cisco recommends OSPF for:
- Multi-vendor environments (open standard)
- Large enterprise or service provider networks requiring hierarchical design
- Networks needing deterministic, predictable path selection
- Environments where link-state detail is required for troubleshooting
Exam trap: The CCNA may present a scenario with a mixed-vendor network. The correct answer is always OSPF (or RIPv2, but OSPF is the modern choice). Never recommend EIGRP for a Juniper or HP network.
Configuration Differences: What You Must Know
EIGRP configuration:
router eigrp 100
network 192.168.1.0 0.0.0.255
No wildcard mask? Actually, EIGRP uses wildcard masks. The network statement includes the interface subnet.
OSPF configuration:
router ospf 1
network 192.168.1.0 0.0.0.255 area 0
OSPF requires an area ID. This is a key difference—EIGRP does not use areas.
Exam trap: EIGRP uses an autonomous system number (ASN) that must match between neighbors. OSPF uses a process ID that is locally significant and does not need to match.
Common Exam Traps and How to Avoid Them
- Metric confusion: EIGRP’s metric is not hop count; it’s a composite. OSPF’s cost is not bandwidth alone—it’s derived from bandwidth.
- Feasible successor: EIGRP does not always converge instantly—only when a feasible successor exists.
- AD precedence: EIGRP internal (90) beats OSPF (110), but EIGRP external (170) loses.
- Open standard: OSPF is always the answer for multi-vendor.
- Areas: OSPF requires area 0; EIGRP does not use areas.
- Load balancing: EIGRP supports unequal-cost (variance), OSPF does not.
- Neighbor discovery: EIGRP uses Hello packets every 5 seconds (default) on LAN; OSPF uses Hello every 10 seconds (broadcast). Both use multicast addresses (224.0.0.10 for EIGRP, 224.0.0.5 and 224.0.0.6 for OSPF).
Takeaway: What to Focus On for the CCNA
Master these three things:
- Metric calculation: Know EIGRP uses bandwidth and delay; OSPF uses cost (10^8 / bandwidth).
- Convergence mechanism: DUAL vs SPF—and when each is faster.
- Design differences: EIGRP is flat, OSPF is hierarchical.
For the exam, you’ll face scenario-based questions where you must pick the protocol based on requirements. Practice with real configurations and troubleshooting. The best way to solidify this knowledge is by working through practice questions that force you to apply these concepts.
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