EIGRP: Enhanced Interior Gateway Routing Protocol

Learning objective

Explain how EIGRP's DUAL algorithm finds loop-free successors and feasible successors, and how its composite metric is calculated per RFC 7868.

What EIGRP is

EIGRP (Enhanced Interior Gateway Routing Protocol) was a Cisco-proprietary protocol for most of its life; Cisco published its operation as an informational standard in RFC 7868 (2016). EIGRP is often called an "advanced distance-vector" or hybrid protocol: like RIP, it only learns routes from directly connected neighbors and never builds a full topology map the way OSPF does, but its DUAL algorithm gives it link-state-like fast, loop-free convergence. EIGRP runs directly over IP (IP protocol 88) and uses five packet types: Hello, Update, Query, Reply, and Ack.

The composite metric

EIGRP's full composite metric formula is Metric = 256 × ((K1×BW + (K2×BW)/(256−load) + K3×Delay) × (K5/(reliability+K4))). With EIGRP's default K-values (K1 = K3 = 1, K2 = K4 = K5 = 0), this collapses to Metric = 256 × (BW + Delay), where BW = 10,000,000 ÷ minimum bandwidth (kbps) along the path (using the slowest link, not the sum), and Delay is the cumulative interface delay in tens of microseconds, summed across every link in the path.

DUAL and loop-free paths

DUAL (Diffusing Update Algorithm) is what makes EIGRP fast without sacrificing loop-freedom. For each destination, a router tracks its neighbors' Reported Distance (RD) — the metric each neighbor reports for reaching that destination — and computes its own Feasible Distance (FD), the lowest total metric to that destination through any neighbor. The neighbor that produces the FD becomes the successor — the currently installed best path. Any other neighbor whose Reported Distance is strictly less than the FD satisfies the feasibility condition and qualifies as a feasible successor: a pre-verified, loop-free backup path that can be installed immediately, with no risk that traffic would loop back through the router being replaced.

EIGRP packet types

Hello packets (sent unreliably to multicast 224.0.0.10) discover and maintain neighbor relationships. Update packets (sent reliably) carry route information. When a router loses its successor and has no feasible successor available, it goes active on that route and sends Query packets asking its neighbors whether they have a path; each neighbor must respond with a Reply. Ack packets confirm reliable delivery of Updates, Queries, and Replies.

Interactive EIGRP DUAL

Step through R1 computing its feasible distance via two neighbors, qualifying one as a feasible successor, and instantly rerouting to it the moment its successor link fails — with no query needed.

EIGRP vs. classic distance-vector

Unlike RIP, EIGRP does not send its full routing table on a fixed timer. Bounded, incremental Updates are sent only when something changes, and each router maintains a topology table listing every advertised path to each destination — not just the single best one — which is exactly what makes instant feasible-successor reroutes possible. This combination of distance-vector's simplicity (no full topology database) with link-state-like convergence speed is why EIGRP occupies a middle ground between RIP and OSPF.