OSPF vs EIGRP

You have already met OSPF and EIGRP on their own.
Before we compare them in detail, let's refresh the essentials.

What OSPF Brings

OSPF is the open, standards-based link-state protocol.
Every router in an area builds the same view of the topology, then works out its own shortest paths from that shared map.

Figure 1 - OSPF and EIGRP at a glance

What EIGRP Brings

EIGRP is Cisco's advanced distance-vector protocol.
It skips the full map and learns the network from its direct neighbors.
DUAL then keeps every path it picks loop-free.

Both are interior gateway protocols (IGPs): they fill your routing table inside one organization.
But almost everything else about them is different.

The rest of this lesson walks through those differences, one at a time.

Almost every difference in this lesson traces back to one decision: how each protocol learns the network in the first place.

Figure 2 - OSPF builds a full map (LSDB), EIGRP learns from neighbors

OSPF: Link-State

OSPF is a link-state protocol.
Every router in an area advertises its own links.
They all end up with an identical LSDB, and each one runs SPF on that shared map.

EIGRP: Advanced Distance-Vector

EIGRP is an advanced distance-vector protocol.
It never builds that map.
Each router only knows the distances its neighbors advertise.
DUAL then keeps each chosen path loop-free.

OSPF and EIGRP differ first on the hardware you can run them on.

Figure 3 - OSPF open standard (RFC 2328) vs EIGRP Cisco origin (RFC 7868)

OSPF: Open Standard

OSPF is an open standard defined in RFC 2328.
It runs on any vendor's hardware, the default choice in multi-vendor environments.

EIGRP: Cisco, Now Open

EIGRP started as Cisco-proprietary.

Cisco released it as an open standard, RFC 7868, in 2016.
But adoption outside Cisco hardware stayed rare.
In practice you still run it on all-Cisco networks.

OSPF and EIGRP measure the cost of a path in different ways.

Figure 4 - OSPF cost vs EIGRP composite metric

OSPF: Cost

OSPF uses one value called cost, taken straight from interface bandwidth: the faster the link, the lower the cost.
By default OSPF uses a 100 Mbps reference, so every link at 100 Mbps or faster gets a cost of 1.

A 1 Gbps link then looks no better than a 100 Mbps one.
You fix this with auto-cost reference-bandwidth so faster links get a lower cost again.

On R1, OSPF shows the cost on each interface, and a 1 Gbps link comes out at cost 1:

R1# show ip ospf interface g0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.0.11.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1

EIGRP: Composite Metric

EIGRP uses a composite metric from K-values, with only K1 (bandwidth) and K3 (delay) active by default.
It combines the slowest bandwidth on the path with the total delay.
That gives a richer picture than a single cost.

On R1, the same route shows what the metric is built from, bandwidth and delay:

R1# show ip eigrp topology 10.0.23.0/24
EIGRP-IPv4 Topology Entry for 10.0.23.0/24, FD is 3072
  Composite metric is (3072/2816)
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 20 microseconds

OSPF and EIGRP do not choose their best route the same way.