EIGRP (Enhanced Interior Gateway Routing Protocol) combines the best of distance vector and link state. Cisco developed it, but it's now an open standard.
EIGRP characteristics:
- Uses composite metric (bandwidth, delay, and optionally reliability and load)
- Administrative distance: (internal), (external)
- Fast convergence through DUAL algorithm
- Supports VLSM and route summarization
- Partial updates (only changes, not full tables)
EIGRP maintains backup routes (feasible successors) that can be used immediately if the primary path fails. This enables sub-second convergence.