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3.2.a Compare routing concepts of EIGRP and OSPF (advanced distance vector vs. link state, load balancing, path selection, path operations, metrics, and area types)

3 min read ENCOR 350-401 v1.2 Updated

Aligned to Cisco's 350-401 ENCOR v1.2 exam topics.

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What this objective tests

The objective says "compare". The exam asks which protocol behaves a certain way. Learn the differences, not the details of one protocol.

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector protocol. Open Shortest Path First (OSPF) is a link-state protocol. This one difference explains most of the comparison.

Side-by-side comparison

TopicEIGRPOSPF
TypeAdvanced distance vectorLink state
AlgorithmDUAL (Diffusing Update Algorithm)Dijkstra SPF
MetricComposite: bandwidth and delay by defaultCost from interface bandwidth
Design areasNone. One autonomous system.Areas. Area 0 is the backbone.
Summarization pointsAny router, on any interfaceABR for inter-area, ASBR for external
Load balancingEqual-cost and unequal-cost (variance)Equal-cost only
Backup routesFeasible successor, ready at onceSPF runs again after a change
Neighbor transportProtocol 88, multicast 224.0.0.10Protocol 89, multicast 224.0.0.5 and 224.0.0.6
Administrative distance90 internal, 170 external110

Metric example: EIGRP

The EIGRP classic metric uses the slowest bandwidth and the sum of delay on the path:

metric = 256 x (10^7 / slowest BW in kbps) + 256 x (total delay in tens of microseconds)

Path with a slowest link of 1,000,000 kbps (1 Gbps) and a total delay of 20 tens of microseconds:

metric = 256 x 10 + 256 x 20 = 2560 + 5120 = 7680

The lowest metric wins. EIGRP also tracks a feasible successor. A neighbor is a feasible successor when its reported distance is less than the current feasible distance. This condition prevents loops.

Metric example: OSPF

OSPF cost is set per interface:

cost = reference bandwidth / interface bandwidth

With a reference bandwidth of 100,000 Mbps and a 1,000 Mbps link:

cost = 100000 / 1000 = 100

A 10 Gbps link costs 10. The router adds the cost of every outgoing interface on the path. The lowest total cost wins.

Note: Set the same reference bandwidth on every router. Example: auto-cost reference-bandwidth 100000. Different values break consistent path selection.

Where each protocol summarizes

  • EIGRP summarizes on any interface with a summary address. There is no area boundary.
  • OSPF summarizes inter-area routes on an Area Border Router (ABR) with area range. External routes summarize on an Autonomous System Boundary Router (ASBR) with summary-address.

For OSPF configuration detail, see OSPFv2 and OSPFv3 simple environments.

Exam traps

  • A feasible successor is not the second-best route. It must pass the feasibility condition.
  • OSPF does equal-cost load balancing only. EIGRP can do unequal-cost with variance.
  • OSPF summarization happens only at area or AS boundaries. EIGRP summarizes anywhere.
  • The default OSPF reference bandwidth makes 1 Gbps and 10 Gbps links look equal.
  • EIGRP stub and OSPF stub areas are different features with the same word.

Pass check

You are ready when you can do these things:

  • Name the algorithm, metric inputs, and load-balancing behavior of each protocol.
  • Compute a simple EIGRP metric and a simple OSPF cost.
  • Name the device types where OSPF can summarize.
  • Explain the feasibility condition in one sentence.

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