EIGRP

Cisco advanced distance-vector running the DUAL algorithm. AD 90 (internal) / 170 (external), IP protocol 88.

// metric (default K1=K3=1)

256 × ( bandwidth + delay )

bw = 10⁷ ÷ min link bandwidth (Kbps) · delay = Σ interface delays ÷ 10

// packet types

1	Update	Carries route information
3	Query	Ask neighbors about a lost route
4	Reply	Answer a query
5	Hello	Discover & maintain neighbors
8	Acknowledge	Confirm reliable delivery

// the three tables

Neighbor	Adjacent EIGRP routers
Topology	All learned routes with FD / RD
Routing	Best routes (successors) installed

// terminology

Feasible Distance (FD)	Neighbor's distance + cost to reach it
Reported Distance (RD)	Metric a neighbor advertises
Successor	Lowest-FD next hop (best path)
Feasible Successor	Loop-free backup (RD < FD)
Stuck In Active (SIA)	Query unanswered → adjacency reset

// facts

AD: internal 90 · external 170 · summary 5
Multicast 224.0.0.10 · IP proto 88 · transport RTP
Hello / Hold 5 / 15 (fast) or 60 / 180 (slow links)
Metric (K1=K3=1) = 256 × (bandwidth + delay)
DUAL guarantees a loop-free topology

configuration · cisco ios

router eigrp 100
 network 192.168.0.0 0.0.0.255
 network 10.0.0.0
 no auto-summary
 passive-interface default
!
interface GigabitEthernet0/0
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp 100 KC
 ip summary-address eigrp 100 192.168.0.0 255.255.0.0

verify

show ip eigrp neighbors
show ip eigrp topology
show ip eigrp interfaces
show ip route eigrp