Hybrid protocol. DUAL algorithm. Near-instant convergence on Cisco networks.
Your Cisco-only network needs a routing protocol that converges faster than OSPF.
EIGRP — Cisco's advanced distance-vector hybrid — keeps a backup path in its pocket.
When a link fails, it switches to the pre-calculated feasible successor in milliseconds,
no SPF recalculation required.
Enhanced Interior Gateway Routing Protocol — Cisco proprietary since 1992, partly open since 2013.
Distance-Vector Foundation
Like RIP, EIGRP shares routing information with directly connected neighbors.
Unlike RIP, it does not send full table updates on a timer — only triggered
incremental updates when something changes.
Link-State Enhancements
EIGRP maintains a full topology table (not just the best routes), uses
a sophisticated metric, and forms explicit neighbor relationships with
Hello packets — behaviors borrowed from link-state protocols like OSPF.
Why "Hybrid"
EIGRP combines the simplicity of distance-vector (neighbors share metrics,
not full topology maps) with the speed and stability of link-state (topology
awareness, event-driven updates). This is why it is called "advanced
distance-vector" or "hybrid."
Key Numbers
Protocol: IP 88 (not TCP/UDP) | Admin Distance: 90 (internal), 170 (external)
| Multicast: 224.0.0.10 | Cisco-proprietary (though RFC 7868 opened a limited version)
Your Cisco-only network means EIGRP is a valid choice. If you had mixed vendors,
you would need OSPF instead — EIGRP will not form neighbors with non-Cisco gear
in the standard proprietary implementation.
Slide 3 of 8
Composite Metric — K Values
EIGRP calculates a single metric from multiple interface parameters. By default, only two actually matter.
Metric = [K1 x Bandwidth + (K2 x Bandwidth)/(256-Load) + K3 x Delay] x [K5/(Reliability + K4)]
K1 — Bandwidth
Default K1 = 1. Uses slowest bandwidth along the path. Higher bandwidth = lower metric = preferred.
K2 — Load
Default K2 = 0 (disabled). Interface utilization 0-255. Rarely used — causes metric instability under load.
K3 — Delay
Default K3 = 1. Sum of delays along the path in tens of microseconds. Both bandwidth and delay are used by default.
K4 / K5 — Reliability
Default K4 = K5 = 0 (disabled). Link reliability 0-255. Disabled by default — causes instability when enabled.
Default Behavior (K1 + K3 only)
With K2, K4, K5 at zero, the formula simplifies to:
Metric = 256 x (10^7/Bandwidth_kbps + Delay_us/10).
Bandwidth and delay. Everything else is noise unless tuned.
K Values Must Match
All EIGRP routers in the same autonomous system must have identical K values.
Mismatch = no neighbor relationship. This is one of the first things to check
when EIGRP neighbors won't form.
Slide 4 of 8
DUAL — Diffusing Update Algorithm
DUAL guarantees loop-free paths and enables near-instant convergence via pre-calculated backups.
Feasible Distance (FD)
The total metric from THIS router to the destination, via the best known path. This is what goes in the routing table.
Reported Distance (RD)
The metric a NEIGHBOR reports for reaching the destination. Also called Advertised Distance (AD). This is the neighbor's FD — not yours.
Successor
The neighbor with the lowest FD to the destination.
This is the active, currently-used path. It goes in the routing table.
If this path fails and no feasible successor exists, DUAL enters
Active state and queries neighbors.
Feasible Successor
A backup neighbor whose RD is less than the current FD
(the Feasibility Condition). DUAL guarantees this path is loop-free.
When the successor fails, the feasible successor is promoted instantly —
no queries, no recalculation. This is EIGRP's convergence advantage.
Active State (no FS available)
If no feasible successor exists, EIGRP goes Active and sends Query
packets to all neighbors. Neighbors respond with their best metric.
DUAL recalculates. Convergence is slower but still loop-free.
Stuck in Active (SIA) = neighbor never replied within 3 minutes.
Feasibility Condition: A neighbor qualifies as a Feasible Successor only if its
RD < your current FD. This mathematical guarantee is what makes EIGRP loop-free
without needing a full topology map like OSPF.
Slide 5 of 8
Three Tables — How EIGRP Stores Information
EIGRP maintains three separate data structures. Know what each one contains.
Neighbor Table
Lists all directly connected EIGRP neighbors and their state.
Built via Hello packets. Shows hold time, uptime, and the queue count.
If a neighbor disappears here, all routes through it are immediately
removed from the topology table.
Command: show ip eigrp neighbors
Topology Table
Lists all destinations learned from all neighbors, with FD and RD values.
Contains the successor AND all feasible successors for each destination.
This is the "complete picture" — more than what ends up in the routing table.
Command: show ip eigrp topology
Routing Table
Contains only the best path (successor) to each destination — the result
DUAL selected from the topology table. EIGRP routes appear as "D" in the
routing table. External routes appear as "D EX."
Command: show ip route eigrp
Why three tables matter for troubleshooting: If a route is in the topology table
but not the routing table, DUAL preferred a different path or the Feasibility Condition was not met.
If it is not in the topology table at all, the neighbor is not advertising it — check the neighbor table first.
Link to a branch office fails. EIGRP checks the topology table — there is a feasible successor
(RD < FD). It is promoted to successor instantly. The routing table updates in under a second.
No queries, no reconvergence delay. The network never stopped forwarding traffic.
Slide 6 of 8
Convergence & EIGRP vs OSPF
EIGRP's speed advantage is real — but only when a feasible successor is available.
EIGRP — With Feasible Successor
When the active path fails and a feasible successor exists in the topology table,
EIGRP switches paths without querying any neighbor.
The routing table updates in under 1 second. This is EIGRP's primary
convergence advantage over OSPF.
OSPF — Always Runs SPF
When any link changes, OSPF floods an LSA and every affected router runs
Dijkstra SPF. This is CPU-intensive and takes seconds to tens of seconds
depending on network size. Faster than RIP, but not as instant as EIGRP with a FS in hand.
Feature
EIGRP
OSPF
Type
Advanced distance-vector (hybrid)
Link-state
Algorithm
DUAL
Dijkstra SPF
Metric
Composite (BW + delay by default)
Cost (bandwidth only)
Convergence
Sub-second (with FS)
Seconds
Admin distance
90 (internal) / 170 (external)
110
Vendor support
Cisco proprietary
Open standard (RFC 2328)
Topology table
Yes — all paths stored
Yes — full LSDB
Best use
Cisco-only networks
Mixed-vendor enterprise
Slide 7 of 8
EIGRP Configuration — Cisco IOS
Simpler to configure than OSPF. Autonomous System number must match all neighbors.
Basic Setup
! EIGRP AS 100 — must match all neighborsrouter eigrp100! Advertise directly connected networksnetwork10.1.0.0 0.0.0.255network10.0.12.0 0.0.0.3! Disable auto-summarization (IOS 12 default)no auto-summary! Silence EIGRP on user-facing portspassive-interfaceGigabitEthernet0/1
Verification
! Neighbors — check state and hold timeshow ip eigrp neighbors! All paths including feasible successorsshow ip eigrp topology! Only best paths in routing tableshow ip route eigrp! EIGRP process summary — K values, ASshow ip eigrpinterfaces
Common failure: AS number mismatch. EIGRP will not form a neighbor relationship
if the AS numbers don't match. Unlike OSPF process IDs (which are local), EIGRP AS numbers
are significant — they must be identical on both ends of every neighbor link.
Named EIGRP mode (IOS 15.0+) is the modern approach:
router eigrp MYORG
followed by address-family ipv4 autonomous-system 100.
Supports both IPv4 and IPv6 in a single process.
Slide 8 of 8 | N10-009 Obj 1.1
EIGRP in Your Cisco Network
Your Cisco-only network needed faster convergence than OSPF. EIGRP delivered it: when the branch link
failed, EIGRP promoted the feasible successor from its topology table
without querying a single neighbor. The routing table updated in under a second.
No SPF calculation. No waiting. That sub-second recovery is EIGRP's entire value proposition on
a Cisco network — at the cost of vendor lock-in.
6 Facts to Carry Out of This Presentation
1EIGRP is Cisco proprietary. Protocol IP 88. Admin distance: 90 internal, 170 external.
2Composite metric uses bandwidth and delay by default (K1=1, K3=1, others=0). K values must match all neighbors.
3DUAL algorithm: FD = your best metric. RD = neighbor's reported metric. FS requires RD < FD.