EIGRP: Enhanced Interior Gateway Routing Protocol

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 tracking two networks through the same two neighbors: one has a feasible successor and reroutes instantly the moment its successor link fails, the other doesn't and has to go Active, send a Query, and wait for a Reply — showing exactly what a feasible successor saves you from.

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.

Inspect EIGRP evidence

show ip eigrp neighbors lists every EIGRP neighbor along with the Hold time and uptime — a neighbor that keeps resetting its uptime is dropping and reforming, not staying stable. show ip eigrp topology shows every path EIGRP knows for each destination, not just the installed one: entries marked as the successor and any feasible successors are visible here, along with their Feasible Distance and Reported Distance.

Guided EIGRP metric practice

With default K-values, compute the composite metric for a path with a 1,000,000 kbps (1 Gbps) minimum bandwidth and a cumulative delay of 100 (tens of microseconds): BW term = 10,000,000 / 1,000,000 = 10. Metric = 256 × (10 + 100) = 28,160.

Troubleshoot EIGRP

1. Confirm both routers are configured with the same Autonomous System number — EIGRP neighbors will not form across mismatched AS numbers.
2. Check for a K-value mismatch: routers must agree on which K-values are enabled, or the neighbor relationship will not come up at all.
3. Verify Hello and Hold timers are compatible, and that no access list or firewall rule is silently dropping EIGRP's IP protocol 88 traffic.
4. In show ip eigrp topology, distinguish a missing feasible successor (no backup exists, so a failure forces a Query) from a stable feasible successor already in place.
5. If a route stays Active for an extended period, suspect a Stuck-In-Active (SIA) condition: a neighbor never replied to a Query, often because that neighbor is overloaded or the query propagated too far across a large network.
6. Check interface bandwidth and delay configuration for accuracy — a misconfigured value produces a mathematically valid but practically wrong composite metric, same as with OSPF cost.

Knowledge check and summary

Pro deep dive