What is EIGRP?

Enhanced Interior Gateway Routing Protocol (EIGRP) is classified as an advanced distance-vector routing protocol.
However, it incorporates characteristics commonly associated with link-state protocols.

As you may already know, RIP is one of the simplest distance-vector protocols.
EIGRP was designed to improve the limitations of RIP.

To understand why EIGRP is more powerful, let’s compare both.

EIGRP vs RIP

Imagine a topology where links between routers do not have the same bandwidth.
If you run RIP in this network, routing decisions are based only on hop count.

Figure 1 – RIP Path Selection Based on Hop Count

That means:

• RIP does not evaluate bandwidth.

• RIP does not evaluate delay.

• RIP does not evaluate link performance.

It only counts how many routers are in the path.
So in this scenario, R1 will choose the direct path through R3 to reach 10.0.23.0/24, simply because it has fewer hops.

This is the main limitation of RIP:
It assumes that fewer hops always means better performance.

But in real networks, that is not necessarily true.

How EIGRP Improves Path Selection

Now, this is where EIGRP changes the game.

Unlike RIP, EIGRP does not just count routers.
It evaluates the quality of the entire path before making a decision.

By default, EIGRP considers:

• The minimum bandwidth along the path

• The total delay across the path

Figure 2 – EIGRP Path Selection Based on Link Quality

This means that when you run EIGRP in the same topology, R1 will not automatically choose the path with fewer hops.
Instead, it will calculate which path offers better overall performance.

In this case, R1 selects the path:
R1 → R2 → R3 → 10.0.23.0/24

because it provides better link quality, even if it has more hops.

To understand how EIGRP operates, you need to look at how it exchanges information and processes routes internally.

EIGRP uses IP protocol number 88 to exchange routing information.
When possible, it sends control messages to the multicast address 224.0.0.10.

Figure 3 - EIGRP Routers Forming Neighbor Relationships

Routers running EIGRP form neighbor relationships and exchange routing information dynamically.

How EIGRP Evaluates Path Quality

You already know that EIGRP does not rely on hop count alone.
Instead, EIGRP evaluates link quality using its composite metric.

By default, this metric is calculated using:

• Minimum bandwidth along the path

• Total delay across the path

In this topology, you can see the metric values associated with each link between routers.

Figure 4 – EIGRP Metric Example

Each router then independently calculates the best path to a destination.

The Three Core Tables of EIGRP

To understand how EIGRP works, you need to understand its three main tables.

EIGRP does not immediately install routes into the routing table.
It processes information step by step using these three tables.

Figure 5 – EIGRP Internal Tables

Neighbor Table

This table lists all directly connected EIGRP neighbors.
These neighbor relationships are formed and maintained using Hello packets.
If a router is not in this table, no routing information will be exchanged with it.

Topology Table

This table contains all routes learned from neighbors.
EIGRP stores every possible path to a destination here before choosing the best one.

Routing Table

This table contains only the best routes selected by EIGRP.
These are the routes actually used to forward traffic.

One of the key strengths of EIGRP is that it does not calculate only a single path.
EIGRP uses the DUAL (Diffusing Update Algorithm) to calculate loop-free primary and backup paths.

When multiple paths to the same destination exist, EIGRP:

• Selects the best path based on its metric

• May keep a loop-free backup path ready

The Primary Path – Successor Route

In this topology, R1 can reach 10.0.23.0/24 through different paths.

For the path:
R1 → R3 → R4 → 10.0.23.0/24

Figure 6 – Successor Route Selection

The total metric is:
256 + 256 + 2560 = 3072

Because this path has the lowest cumulative metric, EIGRP selects it as the best path.
This best path is called the Successor Route.

The Successor Route is the route installed in the routing table and used to forward traffic.

The Backup Path – Feasible Successor Route

Now consider the alternative path:
R1 → R2 → R4 → 10.0.23.0/24

Figure 7 – Feasible Successor

Its cumulative metric is:
512 + 256 + 2560 = 3584

Since this metric is higher, this path is not selected as the primary route.
A path becomes a Feasible Successor only if it satisfies a specific loop-free condition (covered in a later lesson).

When a backup path meets those conditions, it is called a Feasible Successor Route.

Why This Is Important

If the Successor Route becomes unavailable, EIGRP does not need to recompute everything immediately.

Figure 8 – Primary and Backup Paths

If a Feasible Successor Route exists, it can replace the primary path instantly.
This mechanism significantly improves convergence speed.
Because backup paths are pre-calculated, EIGRP can react to failures almost immediately.

To run EIGRP, you must configure an Autonomous System (AS) number.

This AS number defines the routing domain for that specific EIGRP process.
Routers that share the same EIGRP AS number belong to the same routing domain.

Figure 9 – EIGRP Autonomous System (AS 1)

Only routers configured with the same EIGRP AS number can:

• Form neighbor relationships

• Exchange routing information

If two routers use different EIGRP AS numbers, they will not become neighbors and will not share routes.

Figure 10 – EIGRP Autonomous System Boundary

Each EIGRP AS operates as an independent routing domain.