EIGRP Explained: Cisco's Advanced Distance-Vector Protocol
EIGRP (Enhanced Interior Gateway Routing Protocol) is Cisco's advanced distance-vector protocol known for very fast convergence. It uses the DUAL algorithm to keep a loop-free backup path (feasible successor) ready, so when the best path fails it switches over almost instantly.
Successors and feasible successors
EIGRP picks the lowest-metric path as the successor (installed in the routing table) and pre-computes a guaranteed loop-free backup called the feasible successor. If the successor dies and a feasible successor exists, failover is immediate — no recalculation needed. This is why EIGRP converges faster than OSPF in many designs.
Metric and adjacency
EIGRP's composite metric uses bandwidth and delay by default (K1 and K3). Neighbours must share matching K values and autonomous-system number to form an adjacency — mismatches are a common troubleshooting cause. A stuck-in-active (SIA) route means query replies didn't return in time. EIGRP is a CCNP ENARSI topic; the fundamentals build on dynamic routing basics.
Frequently asked questions
What algorithm does EIGRP use?
DUAL (Diffusing Update Algorithm), which maintains a loop-free backup path (feasible successor) for near-instant convergence.
What is a feasible successor in EIGRP?
A precomputed, guaranteed loop-free backup route that EIGRP installs immediately if the primary (successor) path fails.
What is the difference between EIGRP and OSPF?
EIGRP is Cisco's advanced distance-vector protocol using DUAL; OSPF is a standards-based link-state protocol. EIGRP often converges faster; OSPF is multi-vendor.
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