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, which makes it 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 bandwidth, 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.

Figure 5 - OSPF route type order vs EIGRP lowest feasible distance

OSPF: Route Type First

OSPF compares route type before cost: intra-area (O), then inter-area (O IA), then external E1, then E2.
An intra-area route with a high cost still beats an inter-area route with a lower cost.

Cost decides only within the same type.

EIGRP: Lowest Feasible Distance

EIGRP installs the path with the lowest Feasible Distance as the Successor.
There is no route type. The lowest total metric wins.

On R1, let's look at how each protocol picks the route, starting with EIGRP.

R1# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(1)/ID(1.1.1.1)
P 10.0.23.0/24, 1 successors, FD is 3072
        via 10.13.1.3 (3072/2816), GigabitEthernet0/1
        via 10.12.1.2 (3328/2816), GigabitEthernet0/2

The path via 10.13.1.3 wins with an FD of 3072.
The second path stays as a backup.

Now look at how OSPF handles the same route.

R1# show ip route 10.0.23.0
Routing entry for 10.0.23.0/24
  Known via "ospf 1", distance 110, metric 11, type intra area
  * 10.0.11.2, from 3.3.3.3, via GigabitEthernet0/0
      Route metric is 11, traffic share count is 1

Here OSPF installed an intra-area route with a metric of 11, even though a lower-metric inter-area path existed.
The route type decided first; the metric only breaks ties within the same type.

OSPF and EIGRP both load-balance, but not over the same paths.

Figure 6 - OSPF equal-cost (ECMP) vs EIGRP unequal-cost (variance)

OSPF: Equal-Cost Only

OSPF load-balances only across paths that have an identical cost (ECMP).

EIGRP: Equal and Unequal-Cost

EIGRP does both equal-cost and unequal-cost load balancing through the variance command.
OSPF cannot do unequal-cost natively.

Here is how you turn it on at R1:

R1# configure terminal
R1(config)# router eigrp 1
R1(config-router)# variance 2
R1(config-router)# end

With variance 2, EIGRP also uses the 3328 backup path alongside the best one.
OSPF would leave that slower path unused.

When a link goes down, OSPF and EIGRP converge in different ways.

Figure 7 - OSPF SPF recalculation vs EIGRP instant feasible successor

OSPF: SPF Recalculation

A failed link floods a new LSA through the area.
Every router in that area runs SPF again, even routers whose own paths were never affected.

EIGRP: DUAL and Feasible Successor

If a Feasible Successor already exists, the router switches to it instantly.
The route stays Passive, and no other router gets involved.
With no Feasible Successor, the route goes Active and DUAL sends queries to its neighbors.
Summarization limits how far those queries travel.

OSPF splits a large network into areas. EIGRP keeps it flat.

Figure 8 - OSPF areas with ABR and ASBR vs flat EIGRP

OSPF: Areas, ABR, and ASBR

OSPF organizes a large network into areas around a central backbone called Area 0.

• An ABR (Area Border Router) joins another area to the backbone.

• An ASBR (Autonomous System Boundary Router) connects OSPF to an external routing domain.

EIGRP: Flat, No Areas

EIGRP has no areas and no backbone. Its domain is flat.
It controls routing information with summarization and stub routers instead.

When a router learns the same route from both protocols, administrative distance decides which one wins.

Figure 9 - Administrative distance: EIGRP 90 beats OSPF 110

OSPF: 110

OSPF uses an administrative distance of 110 for all of its routes.

EIGRP: 90 Internal / 170 External

EIGRP internal routes have an AD of 90, lower than OSPF, so EIGRP wins by default.
EIGRP external routes sit at 170 and lose to OSPF.

Confirm it yourself on a router that learns the same prefix from both protocols.

R1# show ip route 192.168.50.0
Routing entry for 192.168.50.0/24
  Known via "eigrp 1", distance 90, metric 3072, type internal
  * 10.0.12.2, from 10.0.12.2, via GigabitEthernet0/0

The router chose EIGRP.
The OSPF copy of this route, at distance 110, never made it into the table.