EIGRP (Enhanced Interior Gateway Routing Protocol)

1. Core concepts

Type: Advanced distance vector (Cisco proprietary). Often called a hybrid — fast convergence like link-state, but exchanges routing info like distance vector.

Algorithm: DUAL (Diffusing Update Algorithm) — loop-free path calculation with backup routes ready before failure.

Metric: Composite (lower is better). By default only bandwidth and delay are used (K1 and K3). Reliability, load, and MTU are off by default.

AS number: Routers must share the same autonomous system number to become neighbors (router eigrp 100).

Updates: Multicast to 224.0.0.10. Sends partial triggered updates, not full periodic table dumps like RIP.

Admin distance: 90 (internal), 170 (external/redistributed).

2. DUAL — Successor and Feasible Successor

Term	Meaning
Successor	Best route to a destination (installed in routing table)
Feasible Distance (FD)	Metric of the successor route
Feasible Successor (FS)	Backup route stored in topology table
Reported Distance (RD)	Metric a neighbor reports to reach the destination
Feasibility Condition	`RD < FD` of current successor — guarantees loop-free backup

If the successor fails and a feasible successor exists, EIGRP switches instantly (local computation). No query flood.

If no FS exists, DUAL goes active and queries neighbors (active computation).

Three tables:

Table	Command
Neighbor	`show ip eigrp neighbors`
Topology	`show ip eigrp topology`
Routing	`show ip route eigrp`

3. Cisco IOS Configuration

Router(config)# router eigrp 100
Router(config-router)# eigrp router-id 1.1.1.1
Router(config-router)# network 192.168.1.0 0.0.0.255
Router(config-router)# network 10.0.0.0 0.255.255.255
Router(config-router)# no auto-summary

network uses a wildcard mask. no auto-summary prevents summarizing to classful boundaries (important on older IOS; modern IOS disables auto-summary by default).

Passive interface

Router(config-router)# passive-interface GigabitEthernet0/0

4. Verification

Router# show ip eigrp neighbors
Router# show ip eigrp topology
Router# show ip eigrp topology 10.1.1.0/24
Router# show ip route eigrp
Router# show ip protocols

In show ip eigrp topology, look for:

P — route in passive state (stable)
A — active (DUAL is querying, convergence in progress)
Entry marked successor and optional feasible successor

5. Debugging

Router# debug ip eigrp
Router# debug eigrp packets

What to look for:

Hello/Update packets — neighbor formation and route exchange.
Query/Reply — active state when no feasible successor exists.
Metric changes — bandwidth/delay affecting composite metric.

Turn off when done:

Router# undebug all

Or disable EIGRP debug only:

Router# undebug ip eigrp
Router# undebug eigrp packets

Without debug — show ip eigrp neighbors and show ip eigrp topology all-links for FS visibility.