EIGRP occupies the middle ground between RIP's simplicity and OSPF's complexity. It is still distance-vector — routers advertise distances and directions rather than a full topology map — but it fixes everything that made RIP unusable at scale: updates are sent only on change, the metric understands bandwidth and delay, and the DUAL algorithm guarantees loop freedom without counting to infinity. Originally Cisco-proprietary, EIGRP was published as informational RFC 7868 in 2016 and appears in multi-vendor networks, though its home remains Cisco-heavy enterprises.
EIGRP runs directly over IP as protocol number 88, using multicast 224.0.0.10 for discovery and updates, with unicast retransmissions to specific neighbours. Reliable delivery is EIGRP's own job — the Reliable Transport Protocol (RTP) sequences and acknowledges the packet types that matter.
Neighbour adjacency: the contract
Two routers become EIGRP neighbours only if their hellos agree on three things: the autonomous system number, the K-values (metric weights), and a shared primary subnet. Authentication, if configured, must also pass. Hellos go out every 5 seconds on LAN and fast WAN interfaces (60 seconds on slow multipoint links), with a hold time of three times the hello — a neighbour silent past its hold time is declared down and every route through it is re-evaluated immediately. This explicit adjacency is the first big break from RIP, which simply shouted into the void every 30 seconds.
R1# show ip eigrp neighborsEIGRP-IPv4 Neighbors for AS(100)H Address Interface Hold Uptime SRTT RTO Q Seq1 10.1.13.3 Gi0/1 13 01:22:46 1 100 0 420 10.1.12.2 Gi0/0 11 01:24:02 2 100 0 57
The three tables
- Neighbour table — every adjacent router, its interface, hold timer and RTP state. Built from hellos.
- Topology table — every route learned from every neighbour, with two distances per path: the neighbour's own metric to the destination (reported distance) and that plus the cost to reach the neighbour (computed distance). Successors and feasible successors live here.
- Routing table — only the best (successor) routes, installed with administrative distance 90 for internal routes and 170 for external ones.
The five packet types
- Hello — neighbour discovery and keepalive; unacknowledged.
- Update — routing information; sent reliably, multicast for topology changes, unicast to bring a new neighbour up to date.
- Query — 'my path to X died; do you have one?'; sent reliably when a route is lost and no feasible successor exists.
- Reply — the reliable, unicast answer to a query.
- ACK — a hello with no data, acknowledging reliable packets.
Baseline configuration
Modern named-mode configuration keeps all EIGRP settings — including per-interface ones — under a single hierarchy, and is the mode worth learning:
R1(config)# router eigrp CORPR1(config-router)# address-family ipv4 unicast autonomous-system 100R1(config-router-af)# network 10.0.0.0 0.255.255.255R1(config-router-af)# af-interface gi0/2R1(config-router-af-interface)# passive-interface
Once adjacencies are up and the topology table is populated, EIGRP goes quiet: no periodic updates, just hellos. The next three chapters cover what happens inside that topology table — how the metric is computed, how DUAL picks loop-free paths, and how queries are contained in a well-designed network.