EIGRP

Course at a glance

• 39 lessons in 4 units.
• Format: self-paced lessons with Cisco IOS configurations, packet captures, and video.
• Last updated: October 2026.

What is EIGRP?

EIGRP (Enhanced Interior Gateway Routing Protocol) is an interior gateway protocol (IGP) that routes traffic within your own network. EIGRP routers discover each other with hello packets and become neighbors. Neighbors exchange routing information, which each router stores in its topology table. The best path to each destination is called the successor, and that’s the route that ends up in the routing table.

At heart, EIGRP is a distance vector protocol: it only knows what its neighbors tell it, not the complete topology like OSPF. What makes it different is DUAL (Diffusing Update Algorithm). EIGRP compares the advertised distance of each neighbor with the feasible distance of its best path, and any neighbor that passes this feasibility condition becomes a loop-free backup path, the feasible successor. When the successor fails, EIGRP switches to the feasible successor right away. By default, EIGRP only uses bandwidth and delay in its metric, and it is the only IGP that supports unequal cost load balancing.

Who uses EIGRP? It’s popular in enterprise networks that run Cisco routers, especially in hub and spoke WAN and DMVPN networks, where features like stub routing and summarization keep the network stable. EIGRP supports both IPv4 and IPv6.

EIGRP Learning Roadmap

This roadmap follows the course units. Each topic has a short explanation and links to the lessons where we cover it in detail.

EIGRP Fundamentals

We start with how EIGRP selects its best path: the advertised distance, feasible distance, successor, and feasible successor. Once you understand these terms, you can configure EIGRP and read its neighbor and topology table. EIGRP uses five packet types: hello, update, query, reply, and ACK. It uses RTP (Reliable Transport Protocol) to make sure updates, queries, and replies are acknowledged. We also take a look at EIGRP over Frame Relay.

EIGRP Neighbor Adjacency

EIGRP routers send hello packets to find neighbors, form a neighbor adjacency, and exchange their routes with update packets. The AS number has to match and the default hold time is 15 seconds. You can also configure static neighbors that use unicast, and passive interfaces to stop EIGRP from sending hellos where you don’t need them. To prevent rogue routers from joining, we secure EIGRP with MD5 or SHA authentication, which you can also configure per neighbor, and we learn how to troubleshoot neighbors that refuse to form an adjacency.

EIGRP Metrics and Load Balancing

EIGRP calculates a composite metric from bandwidth, delay, load, and reliability, scaled by K values. By default, only K1 and K3 are enabled, so only bandwidth and delay count, and you’ll see what happens when you change the K values. The classic metric can’t tell the difference between interfaces faster than 10 Gigabit; wide metrics fix this with 64-bit values. EIGRP is also the only IGP with unequal cost load balancing: with the variance command, it uses feasible successors with a higher metric too, in proportion to their metric. We finish with how EIGRP selects the best path for external routes.

EIGRP Filtering

Sometimes you don’t want to advertise or accept every route. EIGRP lets you filter routes in both directions with a distribute-list that refers to an access-list, a prefix-list, or a route-map. Prefix-lists are more flexible than access-lists because they match on the prefix length too, and route-maps give you even more options.

EIGRP Queries and Stub Routing

When EIGRP loses a route and has no feasible successor, the route goes active and the router sends queries to its neighbors. In a large network, these queries can travel far, and when a router doesn’t get all its replies in time, the route becomes stuck in active (SIA). The DUAL finite state machine explains what happens during this process. Stub routing tells neighbors not to query a router, which is ideal for branch routers in hub and spoke networks, and a leak map lets a stub router advertise some routes anyway. With graceful shutdown, a router tells its neighbors that it’s going down, so they don’t have to wait for the hold time to expire.

EIGRP Summarization and Default Routes

Summarization keeps routing tables small and limits how far queries travel. In EIGRP, you can configure a summary on any interface. The router then installs a route for the summary that points to null0 to prevent routing loops. A leak map lets you advertise specific networks next to the summary. We also look at auto-summary and the different ways to advertise a default route.

Advanced EIGRP Features

Since IOS 15, we can configure EIGRP in named mode, where all configuration lives under the router process and address families for IPv4 and IPv6. Each router also needs a unique router ID, which EIGRP uses to prevent loops with external routes. EIGRP OTP lets you run EIGRP between routers over a service provider network without the provider’s involvement. Add path support helps spoke routers in DMVPN networks, and LFA fast reroute installs a backup path so EIGRP switches over in less than 50 ms. The ip bandwidth-percent command limits how much bandwidth EIGRP may use on an interface. We finish with troubleshooting route advertisements.

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