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EIGRP
Cisco's advanced distance-vector protocol: neighbours share distances, with a backup path worked out in advance.
A distance-vector router only knows what its neighbours tell it: "I can reach this network at this cost". RIP is the simple kind, counting hops and refreshing every 30 seconds. EIGRP (Cisco's, published as RFC 7868) keeps the idea but sends updates only when something changes, weighs bandwidth and delay instead of hops, and works out a loop-free backup before it is needed. OSPF is the other family, link state: every router holds the whole map and computes paths itself. Administrative distance: 90 for internal EIGRP routes, 170 for external (redistributed) ones.
Neighbours. Routers send hellos to 224.0.0.10 (IP protocol 88), every 5 seconds on most links; the hold
time, three hellos (15 seconds), is how long a neighbour may stay silent before it is declared down. Two routers
become neighbours only on a shared subnet with the same AS number (router eigrp 100 on both) and the same
K-values; a mismatch in either and the hellos are ignored.
Metric. The composite metric is built from the K-values. With the defaults (K1 = K3 = 1, K2 = K4 = K5 = 0) only bandwidth and delay count: 256 × (10⁷ / the slowest bandwidth on the path in kbit/s + the sum of the delays in tens of microseconds). Load and reliability are off by default, and best left off: they change with traffic.
DUAL. For each network a router keeps, per neighbour, the reported distance (RD, or advertised distance: the neighbour's own metric to it) and the total through that neighbour. The lowest total is the feasible distance (FD), and the neighbour giving it is the successor, installed in the routing table. Another neighbour is a feasible successor if its RD is lower than the FD: the feasibility condition, which proves its path does not loop back through this router. When the successor fails, a feasible successor takes over at once, with no messages; with none, the route goes active and the router queries its neighbours and waits for their replies.
Netlab teaches EIGRP by reading, here and in the quizzes: the open-source implementation, FRR's eigrpd, does not carry out DUAL's failover correctly, so a lab would show behaviour a Cisco router never would. The RIP lab shows the distance vector it improves on, and the OSPF lab the link-state alternative.