EIGRP Fundamentals: Cisco's Advanced Distance-Vector Protocol

Where EIGRP Fits Among Routing Protocols

OSPF, discussed earlier in this series, is a link-state protocol that builds a complete topology map. EIGRP (Enhanced Interior Gateway Routing Protocol) takes a different approach, classified as an Advanced Distance-Vector protocol — it does not build a full topology map like OSPF, but it overcomes the slow convergence and loop-avoidance weaknesses of simpler distance-vector protocols through a sophisticated algorithm covered later in this article.

EIGRP's Composite Metric

Unlike OSPF's single cost value based on bandwidth, discussed earlier in this series, EIGRP calculates its metric from multiple interface characteristics combined into a single composite value.

EIGRP metric components (by default, only
bandwidth and delay are actually used in the
calculation, though the protocol supports all four):

Bandwidth: the slowest link's bandwidth
  along the entire path
Delay: the cumulative delay across every
  link along the path
Reliability: link reliability (not used by default)
Load: current link utilization (not used by default)

Simplified default formula:
Metric = 256 × (10,000,000 / min bandwidth) +
         256 × (cumulative delay / 10)

Because the metric incorporates cumulative delay across the entire path rather than just a single hop's characteristics, EIGRP can make more nuanced path selections than a protocol relying purely on hop count or a simpler per-hop cost.

Configuring Basic EIGRP

Router(config)# router eigrp 100
Router(config-router)# network 192.168.1.0 0.0.0.255
Router(config-router)# network 10.0.0.0 0.0.0.3
Router(config-router)# no auto-summary

-- "100" is the Autonomous System Number, and
-- unlike OSPF's process ID, this value MUST
-- match between routers that need to become
-- EIGRP neighbors -- a common source of failed
-- neighbor formation when mismatched

The no auto-summary command disables EIGRP's legacy behavior of automatically summarizing routes at classful network boundaries, discussed earlier in this series regarding classful addressing — a behavior that causes serious routing problems in modern discontiguous networks and should almost always be disabled.

The DUAL Algorithm: Guaranteeing Loop-Free Paths

EIGRP's core innovation is the DUAL (Diffusing Update Algorithm), which allows a router to instantly switch to a backup path without any risk of creating a routing loop, and without needing the complete topology awareness that link-state protocols require to make this same guarantee.

Key DUAL terminology:

Feasible Distance (FD): this router's own best
  (lowest) metric to reach a destination

Reported Distance (RD): a neighbor's own metric
  to reach that same destination, as reported
  by that neighbor

Successor: the neighbor providing the best path
  (lowest FD) — this is what actually appears
  in the routing table

Feasible Successor (FS): a backup neighbor whose
  Reported Distance is less than the current
  Feasible Distance -- this specific condition
  is what mathematically guarantees this backup
  path cannot possibly loop back through this router

The feasibility condition — a neighbor's reported distance must be strictly less than the local router's feasible distance — is the mathematical guarantee that makes DUAL's fast convergence safe: if a neighbor's own reported cost to reach the destination is already smaller than this router's total cost, that neighbor cannot possibly be routing back through this router to reach the destination, ruling out a loop entirely.

Why a Feasible Successor Enables Near-Instant Convergence

Without a feasible successor available:
  If the successor path fails, EIGRP must query
  neighbors to find a new loop-free path,
  a process that takes measurable time

With a feasible successor already identified:
  If the successor path fails, EIGRP immediately
  promotes the feasible successor to be the new
  successor, with essentially zero convergence
  delay, since the loop-free guarantee was already
  mathematically established in advance

This is the core reason EIGRP is prized for extremely fast convergence in properly designed topologies — when a feasible successor exists for every important destination, EIGRP can react to a link failure essentially instantaneously, without needing to query other routers or recompute anything.

Forming EIGRP Neighbor Relationships

EIGRP neighbor requirements (similar in spirit
to OSPF's requirements, discussed earlier in
this series, but with protocol-specific values):

- Matching Autonomous System Number
- Matching K-values (the weighting constants
  used in the composite metric formula)
- Same subnet
- No duplicate router IDs

Verifying EIGRP Neighbors and the Topology Table

Router# show ip eigrp neighbors

H   Address         Interface   Hold Uptime   SRTT
0   10.0.0.2        Gi0/1        13  00:15:42   1

Router# show ip eigrp topology

P 192.168.2.0/24, 1 successors, FD is 3072
        via 10.0.0.2 (3072/2816), GigabitEthernet0/1
        via 10.0.1.2 (3840/2816), GigabitEthernet0/2

The topology table output reveals the underlying DUAL data directly: the first listed path (via 10.0.0.2) is the successor, actually installed in the routing table, while the second path is a feasible successor — its reported distance of 2816 is less than the successor's feasible distance of 3072, satisfying the feasibility condition and making it immediately available as a backup without any recomputation needed.

Verifying the Routing Table

Router# show ip route eigrp

D    192.168.2.0/24 [90/3072] via 10.0.0.2, GigabitEthernet0/1

EIGRP routes appear with the code D (for the "Diffusing" in DUAL's name) and an administrative distance of 90 by default — a value lower than OSPF's 110, meaning that if both protocols somehow learned a route to the exact same destination, EIGRP's route would be preferred and installed in the routing table.

Why EIGRP Remains Relevant Alongside OSPF

Though EIGRP was originally Cisco-proprietary and OSPF's open-standard status, discussed earlier in this series, made OSPF the more common choice in mixed-vendor environments, EIGRP remains widely deployed in Cisco-only enterprise networks specifically because of its combination of configuration simplicity and genuinely fast convergence in well-designed topologies. Understanding the feasible successor concept and how to read the topology table is essential not only for day-to-day EIGRP operation, but for the more advanced EIGRP topics — such as unequal-cost load balancing and route summarization — covered later in this series.