OSPF vs EIGRP: which one for CCNA and real US networking work

The CCNA 200-301 blueprint tests OSPF in depth; EIGRP is not an exam objective, but it still runs in many US networks. The interview question US recruiters ask (“if you had to pick one for a greenfield network, which and why?”) expects a real opinion. Here’s the honest read on when each wins and how to answer.

The short version

• OSPF: open standard (RFC 2328 / RFC 5340 for v3). Link-state. Runs everywhere. Slower to converge without tuning, more predictable at scale.
• EIGRP: Cisco proprietary (now RFC 7868 informational). Advanced distance-vector using DUAL. Blazing fast convergence with Feasible Successors. Only makes sense on Cisco-only networks.

If your enterprise is Cisco-only and stays that way: EIGRP is faster to converge and simpler to run. If you have any non-Cisco routers (Juniper, Arista, Palo Alto, cloud vendors): OSPF is the only option.

The CCNA exam scope for each

OSPF on the 200-301. Expect deep questions on:

• Single-area configuration (router ospf 1, network <ip> <wildcard> area <n>)
• Router-ID selection (manual → loopback → highest active interface IP)
• Neighbor states (DOWN → INIT → 2WAY → EXSTART → EXCHANGE → LOADING → FULL)
• DR / BDR election on broadcast networks
• Cost calculation (10^8 / bandwidth default)
• Hello / Dead timer defaults (10 / 40 on broadcast, 30 / 120 on non-broadcast)
• LSA basics (Type 1 and Type 2 within a single area)

EIGRP on the 200-301. Not an exam objective. You only need its administrative distance (90 internal, 170 external) for route-selection questions. The rest is CCNP ENARSI and on-the-job knowledge:

• Configuration (router eigrp 100, network, wildcard masks)
• K-values (default: K1=1, K3=1, others 0): must match between neighbors
• Metric formula (bandwidth + delay in default)
• Successor + Feasible Successor + Feasibility Condition
• Neighbor formation (Hello / Hold, must match Kvalue + AS number)
• AD 90 internal, 170 external

Study OSPF for the exam; learn EIGRP for the job.

Where each actually lives in US enterprise networks

OSPF is used at:

• ISPs (backbone routing, OSPFv3 for IPv6)
• Multi-vendor enterprises: Fortune 500 with Cisco + Juniper + Arista mix
• Government / DoD contractors (open standard is a requirement in many RFPs)
• Any greenfield network built after ~2018 where cloud connectivity is a factor

EIGRP is used at:

• Legacy Cisco-only enterprises (banks, healthcare, universities with 20+ year Cisco relationships)
• Cisco SD-WAN underlays
• Cisco DMVPN networks (EIGRP over the tunnel)
• Retail chains with Cisco Meraki + traditional Cisco IOS mixed

In practice: any US networking role you interview for will use ONE of them for the interior gateway. Ask in the interview which. It’s a smart question, and the answer tells you a lot about how the network was designed.

Head-to-head technical comparison

When OSPF wins

• Multi-vendor network: the only sensible choice.
• Very large network needing hierarchy: areas + LSAs give you structural containment.
• Predictable, well-documented behavior: everyone in the industry knows OSPF.
• Cloud interconnect: AWS Direct Connect, Azure ExpressRoute and Google Cloud VPN use BGP for dynamic routing, so your IGP choice stays inside your network. OSPF redistributes into BGP cleanly on any vendor.

When EIGRP wins

• All-Cisco environment: simpler config, faster convergence out of the box.
• DMVPN / Cisco SD-WAN: Cisco optimized EIGRP for their overlay technologies.
• Uneven bandwidth links: unequal-cost load balancing via variance is genuinely useful (OSPF has no equivalent).
• Fast pre-computed backup paths: Feasible Successor logic. When your Successor dies, DUAL installs the FS in milliseconds without recomputation.

The interview question: how to answer

“You’re designing an interior routing protocol for a new enterprise network. OSPF or EIGRP: pick and defend.”

Wrong answer: “OSPF because it’s the standard.”

Better answer: “Depends on the environment. For a greenfield, multi-vendor, or cloud-connected enterprise, OSPF, because it’s open and integrates with everything downstream. For a well-established Cisco-only enterprise with existing DMVPN or SD-WAN, EIGRP, because it converges faster natively and I can use unequal-cost load balancing. In both cases I’d run BFD on top so my sub-second convergence numbers actually mean sub-second.”

That answer signals: you understand the trade-off, you know the ecosystem, and you think about convergence tuning.

Common exam traps

1. Confusing OSPF and EIGRP AD. OSPF = 110. EIGRP internal = 90 (better). EIGRP external = 170 (worse than OSPF). If BOTH are running on the same router, EIGRP internal wins by default.

2. Metric calculation on the exam. OSPF cost = 10^8 / bandwidth. Default reference bandwidth is 100 Mbps, so anything at or above 100 Mbps is cost 1 (bad in modern networks). Change with auto-cost reference-bandwidth 100000 (100 Gbps ref) to distinguish 100M/1G/10G/100G as cost 1000/100/10/1.

3. EIGRP K-values MUST match. If two EIGRP routers have different K-values, no adjacency. Same for OSPF area IDs.

4. DR/BDR only on broadcast networks. OSPF on point-to-point doesn’t elect a DR. The exam loves this.

5. EIGRP uses ports? No: direct IP protocol 88. OSPF uses direct IP protocol 89. Neither uses TCP or UDP.

Cheat strip

What to actually do

If you’re studying CCNA: deep-dive OSPF; for EIGRP, know its administrative distance. EIGRP itself is not on the exam, so learn the rest for the job.

If you’re job hunting: be ready to name which is running at their environment and give a coherent one-minute answer on why you’d pick each. Recruiters trip up unprepared candidates on this.

If you’re setting up a lab: run BOTH. Configure the same 4-router topology first with OSPF, then flush and configure with EIGRP. Watch the neighbor states, look at the routing tables, kill a link and time the convergence. That drill is worth 10 read-throughs of a textbook.