EIGRP Cheat Sheet
EIGRP Ultimate Cheat Sheet
Configuration, Commands, Troubleshooting & Security
An original, step-by-step reference from protocol fundamentals and DUAL route selection to realistic IOS XE labs, named mode, HMAC authentication and incident diagnosis.
Published by MAG212 CYBER · Technical field guide · October 2026
EIGRP at a glance
EIGRP is an advanced distance-vector interior gateway protocol that uses the Diffusing Update Algorithm (DUAL) to converge and maintain loop-free paths. It is documented in informational RFC 7868; it is not an IETF Standards Track protocol.
Typical hello/hold timers: 5/15 seconds on most fast links and 60/180 seconds on some low-speed NBMA links; platform and interface settings matter.
Packet types, tables & neighbors
| Packet | Purpose |
|---|---|
| Hello | Neighbor discovery and maintenance |
| Update | Advertise topology information |
| Query | Ask neighbors about an unavailable route |
| Reply | Respond to route query |
| Acknowledgment | Reliable Transport Protocol acknowledgment (often an empty Hello) |
Neighbor table: EIGRP peers and reachability. Topology table: learned paths and DUAL state. Routing table: selected paths installed for forwarding. EIGRP uses RTP for ordered and reliable exchanges as needed.
DUAL: FD, RD, successor and feasible successor
Reported Distance (RD) is the metric advertised by a neighbor. Total distance via that neighbor is the neighbor RD plus the local link metric (conceptually; composite metrics depend on bandwidth/delay). Successor is the selected next hop. Feasible successor is a loop-free backup that meets the feasibility condition: neighbor RD < current feasible distance (FD).
Worked feasibility example (illustrative metric units)
| Via | Neighbor RD | Total metric | Result |
|---|---|---|---|
| R2 | 15,000 | 25,000 | Successor |
| R3 | 20,000 | 31,000 | Feasible successor: 20,000 < 25,000 |
| R4 | 27,000 | 30,000 | Not feasible under this FD |
Important nuance: RFC 7868 defines FD as the least-known metric since the last transition from active to passive; it is not necessarily identical to the currently displayed successor’s total metric. A non-feasible path may still be useful after DUAL performs a diffusing computation.
Legacy metric & wide metrics
Classic EIGRP uses bandwidth and cumulative delay by default, with K1=1, K2=0, K3=1, K4=0, K5=0. A simplified classic default expression is 256 × (10⁷ / minimum bandwidth in kbps + sum of delays in tens of microseconds). Named mode can use wide metrics, which support higher-speed links and different scale factors. Always inspect metric calculations on the actual software release.
Warning: Do not modify K-values casually. Mismatches can prevent neighbor relationships or produce inconsistent route selection.
Three-router original lab topology
R1 is the core; R2 advertises 192.0.2.0/24; R3 advertises 198.51.100.0/24. These are documentation networks, not public production addressing. R1–R2 uses 10.10.12.0/30, R1–R3 uses 10.10.13.0/30, and R2–R3 uses 10.10.23.0/30.
Detailed topology: interfaces and failover
Each transit subnet and interface is labeled. The R2–R3 link provides redundant connectivity; an immediate DUAL backup requires the feasibility condition to be met.
IP allocation: R1 = .1 on both links to R2/R3; R2 = 10.10.12.2, 10.10.23.1; R3 = 10.10.13.2, 10.10.23.2. Modify interface numbering to match your hardware or lab simulator.
Classic Cisco IOS XE configuration
These examples assume Gi0/0 and Gi0/1 are the routed uplinks and a Loopback0 or Gi0/2 LAN interface is already addressed. Configure IP interfaces first, then EIGRP:
R1 — core
interface GigabitEthernet0/0 description TO-R2 ip address 10.10.12.1 255.255.255.252 no shutdown interface GigabitEthernet0/1 description TO-R3 ip address 10.10.13.1 255.255.255.252 no shutdown interface Loopback0 ip address 10.100.1.1 255.255.255.255 router eigrp 100 eigrp router-id 10.100.1.1 network 10.10.12.0 0.0.0.3 network 10.10.13.0 0.0.0.3 network 10.100.1.1 0.0.0.0 passive-interface Loopback0
R2 — distribution
interface GigabitEthernet0/0 ip address 10.10.12.2 255.255.255.252 no shutdown interface GigabitEthernet0/1 ip address 10.10.23.1 255.255.255.252 no shutdown interface GigabitEthernet0/2 ip address 192.0.2.1 255.255.255.0 no shutdown router eigrp 100 network 10.10.12.0 0.0.0.3 network 10.10.23.0 0.0.0.3 network 192.0.2.0 0.0.0.255 passive-interface GigabitEthernet0/2
R3 — distribution
interface GigabitEthernet0/0 ip address 10.10.13.2 255.255.255.252 no shutdown interface GigabitEthernet0/1 ip address 10.10.23.2 255.255.255.252 no shutdown interface GigabitEthernet0/2 ip address 198.51.100.1 255.255.255.0 no shutdown router eigrp 100 network 10.10.13.0 0.0.0.3 network 10.10.23.0 0.0.0.3 network 198.51.100.0 0.0.0.255 passive-interface GigabitEthernet0/2
Expected neighbor count: R1 sees R2 and R3; R2 sees R1 and R3; R3 sees R1 and R2. Verify with show ip eigrp neighbors.
Modern named mode configuration
Named mode uses an EIGRP process name and IPv4 address family. The example below is for R1; adjust networks and router IDs on R2/R3. Some platforms require explicit no shutdown on the address family.
router eigrp MAG212 address-family ipv4 unicast autonomous-system 100 eigrp router-id 10.100.1.1 network 10.10.12.0 0.0.0.3 network 10.10.13.0 0.0.0.3 network 10.100.1.1 0.0.0.0 af-interface Loopback0 passive-interface exit-af-interface no shutdown exit-address-family
Do not run classic and named EIGRP configurations for the same routes without an intentional migration design. Use one method per lab.
EIGRP authentication & hardening
Prefer authenticated adjacencies on supported platforms. Example, in named EIGRP mode on the R1–R2 link (configure the same shared secret on both routers):
router eigrp MAG212 address-family ipv4 unicast autonomous-system 100 af-interface GigabitEthernet0/0 authentication mode hmac-sha-256 0 LAB-REPLACE-WITH-STRONG-SECRET exit-af-interface exit-address-family
This is an educational example. Do not put real secrets in public configuration repositories; confirm platform support and secret-handling capabilities. Classic mode supports key chains with MD5, but MD5 is legacy. Security baseline: passive access-facing interfaces, filter redistributed routes, limit EIGRP to authorized links, use infrastructure ACLs, monitor neighbor changes and minimize routing privileges.
Summarization, stubs & unequal-cost load sharing
EIGRP stub reduces unnecessary queries toward a spoke or branch. Summarization can reduce table size and query scope, but only summarize networks that truly exist. Variance permits unequal-cost load balancing among eligible loop-free feasible paths, subject to configuration and platform limits.
! Classic mode: branch router (example) router eigrp 100 eigrp stub connected summary ! Different design: optionally tune variance after analyzing path metrics router eigrp 100 variance 2
Split horizon: leave it enabled unless a reviewed topology (such as some hub-and-spoke overlays) specifically requires an exception.
Verification command reference
| Command | Question it answers |
|---|---|
show ip eigrp neighbors | Are peers established and stable? |
show ip eigrp topology | What successor/feasible paths exist? |
show ip eigrp topology all-links | What alternative paths were learned? |
show ip route eigrp | Which EIGRP routes made the RIB? |
show ip protocols | Process, AS, networks, passive interfaces, timers? |
show ip eigrp interfaces detail | Interface EIGRP details? |
show ip eigrp traffic | Are queries/replies/updates increasing? |
show logging | include DUAL|EIGRP | Recent adjacency and DUAL events? |
show running-config | section router eigrp | What EIGRP configuration is active? |
Verify command syntax on your exact IOS XE version. Avoid production debugging without a maintenance plan.
Troubleshooting decision tree
1 · No neighbor? Check link state, ping, IP/mask, EIGRP AS, EIGRP-enabled interfaces, passive-interface status, authentication, K-values, and filtering of IP protocol 88.
2 · Neighbor up, route missing? Check network statements, topology table, distribute lists, summarization, route redistribution, and AD of competing protocols.
3 · Route Active / SIA? Check query propagation, unstable or slow links, CPU pressure, missing replies, and query scope; consider stub and summarization design.
4 · Unexpected path? Examine minimum bandwidth, cumulative delay, feasible condition, successors, variance, and administrative distance.
5 · Flapping neighbor? Investigate packet loss, interface errors, MTU-related exchange behavior, timer/hold expiration, authentication mismatches and device resource load.
Real troubleshooting scenarios
Scenario A — EIGRP neighbors never appear
R1 has router eigrp 100; R2 has router eigrp 200. Diagnosis: mismatched AS numbers. Fix: place intended peers in the same EIGRP AS, confirm addresses/network statements, then verify adjacency.
Scenario B — LAN network not advertised
R2 has the LAN configured but EIGRP enables only WAN network statements. Diagnosis: LAN interface not matched by EIGRP. Fix: add network 192.0.2.0 0.0.0.255 and make Gi0/2 passive; passive means advertise connected subnet without sending hellos.
Scenario C — backup route not shown as feasible successor
The alternative RD is greater than or equal to the stored FD. Diagnosis: failed feasibility condition, not necessarily a broken path. Fix: validate actual metrics and allow DUAL to recompute during failures; do not force metric changes solely to fill a backup slot.
Exam-style Q&A with explanations
No. It uses IP protocol number 88, not a TCP or UDP port.
Q2. Which route wins: internal EIGRP AD 90 or OSPF AD 110?For identical prefix lengths and default administrative distances, internal EIGRP generally wins. Different prefixes use longest-prefix match first.
Q3. Is a neighbor’s RD of 30,000 feasible if stored FD is 25,000?No. The feasibility condition is strictly RD < FD.
No. It prevents neighbor formation/hellos on that interface while its matched connected network can still be advertised.
Q5. Which EIGRP route code appears in the routing table?D identifies internal EIGRP and D EX external EIGRP in common IOS/IOS XE outputs.
The router has lost a usable successor and has no immediately usable feasible successor, so DUAL requests information from neighbors.
Q7. What should you do before changing K-values?Check platform behavior and ensure all intended peers have consistent metric settings; avoid changing K-values as a shortcut for traffic engineering.
Q8. Is EIGRP an IETF Standards Track protocol?No. RFC 7868 is informational.
Deployment checklist & references
- Plan unique addresses and documentation-grade lab IPs.
- Enable EIGRP only on intended networks and keep LAN ports passive.
- Confirm adjacencies, topology and installed routes before traffic tests.
- Require approved authentication on participating links where supported.
- Limit route redistribution and summarize only where safe.
- Collect baseline neighbor counts, CPU, route table and events.
- Test controlled failover and restoration during a lab/maintenance window.
Primary references: RFC 7868 · Cisco Named Mode Guide · Cisco IOS XE EIGRP Configuration · Cisco EIGRP Authentication Command Reference.
This is independent original educational material for MAG212 CYBER. Command availability and syntax vary by software version and platform. Lab metrics and Q&A scenarios are illustrative, not observed production telemetry.
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