EIGRP — Enhanced Interior Gateway Routing Protocol

Overview

EIGRP (Enhanced Interior Gateway Routing Protocol) was developed by Cisco in 1992 as a proprietary protocol. It was opened as an informational RFC in 2013 and standardised in RFC 7868 (2016), though it remains most commonly deployed in Cisco environments.

EIGRP is classified as an advanced distance-vector protocol — sometimes called a “hybrid” protocol. It shares characteristics with distance-vector protocols (each router knows only its neighbours’ information, not the full topology) but adds features that make it behave more like a link-state protocol:

• Partial, bounded updates: Only changed routes are sent, only to affected neighbours (not periodic full table dumps like RIP)
• DUAL algorithm: Guarantees loop-free paths at all times
• Rapid convergence: Pre-computed backup paths activate instantly on primary failure
• Multiple metrics: Bandwidth, delay, reliability, load, and MTU (in practice, only bandwidth and delay are used)

IP Protocol 88 (not TCP or UDP — EIGRP has its own transport layer built in, called RTP — Reliable Transport Protocol, not to be confused with the audio RTP).

EIGRP Metric — The Composite Formula

EIGRP calculates a composite metric from up to five components. In default configuration, only bandwidth and delay are used:

Metric = 256 × (K1 × BW + (K2 × BW)/(256 - Load) + K3 × Delay) × K5/(Reliability + K4)

With default K-values (K1=1, K2=0, K3=1, K4=0, K5=0), this simplifies to:

Metric = 256 × (10^7 / min_bandwidth_kbps + sum_of_delays_in_tens_of_microseconds)

Bandwidth: The lowest bandwidth link on the path (bottleneck). Expressed as 10^7 divided by the bandwidth in Kbps.

Delay: The cumulative delay along the path. Each interface has a configured delay (in tens of microseconds). The sum of all interface delays on the path.

Example: A path through a 100 Mbps link (delay 10µs) and a 1 Gbps link (delay 1µs):

• Bandwidth: 10^7 / 100,000 = 100
• Delay: (10 + 1) / 10 = 1.1 (in tens of µs)
• Metric ≈ 256 × (100 + 1.1) = ~25,882

DUAL — The Diffusing Update Algorithm

The DUAL algorithm is what separates EIGRP from simple distance-vector protocols. It guarantees loop-free paths at all times, even during convergence.

Key Terms

Feasible Distance (FD): The best metric from this router to the destination network (the metric of the Successor route).

Reported Distance (RD) / Advertised Distance (AD): The metric a neighbour reports for reaching the destination.

Successor: The best-metric next hop to a destination. This is the route installed in the routing table.

Feasible Successor (FS): A backup next hop that satisfies the Feasibility Condition: its Reported Distance must be less than the current Feasible Distance. This guarantees the backup path cannot loop through us.

Feasibility Condition: RD < FD. If a neighbour’s reported distance is less than our current best metric, that neighbour cannot be routing through us — so using it as a backup is loop-free.

Instant Failover with a Feasible Successor

When a Feasible Successor exists:

Primary: RouterA → RouterB → Destination (FD = 1000)
Backup:  RouterA → RouterC → Destination (RD = 800 < FD 1000 ✓ — Feasible Successor)

If RouterB goes down, RouterA instantly activates the RouterC path — no queries, no recalculation, no topology change propagation. Convergence time: milliseconds.

Active State — When No Feasible Successor Exists

When the Successor fails and there is no Feasible Successor, the route enters Active state:

1. The router sends Query packets to all neighbours asking “Do you have a path to this network?”
2. Neighbours respond with Reply packets (or send Queries further if they also lose their route)
3. When all Replies are received, the router recalculates and selects a new Successor

This query/reply mechanism is bounded — it propagates only as far as routers that use the failed path, not across the entire network like OSPF’s LSA flooding.

EIGRP Packet Types

EIGRP uses five packet types, delivered reliably via its own RTP (Reliable Transport Protocol):

EIGRP Neighbour Relationship

Neighbour requirements: Same AS number, same K-values (metric weights), compatible authentication, same subnet. Mismatched K-values prevent adjacency even if all other parameters match.

Hello interval: 5 seconds on LAN, 60 seconds on serial WAN. Hold time: 3× Hello interval (15s / 180s). If no Hello is received within the hold time, the neighbour is declared dead and routes through it are removed.

EIGRP Configuration (Cisco IOS)

! Classic EIGRP (IOS 12.x style)
router eigrp 100                        ! AS 100
 network 192.168.1.0 0.0.0.255          ! Advertise this network
 network 10.0.0.0 0.255.255.255
 no auto-summary                        ! Required — disable classful summarisation
 passive-interface GigabitEthernet0/2   ! Don't send Hellos out this interface

! Named EIGRP (IOS 15.x / IOS-XE — preferred)
router eigrp ENTERPRISE
 !
 address-family ipv4 unicast autonomous-system 100
  !
  af-interface GigabitEthernet0/2
   passive-interface
  !
  topology base
   redistribute ospf 1 metric 1000000 100 255 1 1500
  !
  network 192.168.1.0 0.0.0.255
  network 10.0.0.0 0.255.255.255
  eigrp router-id 1.1.1.1

EIGRP vs OSPF — When to Use Each

Rule of thumb: Use EIGRP in homogeneous Cisco environments where simplicity and fast convergence are priorities. Use OSPF in multi-vendor environments or when strict standards compliance is required.

Unequal-cost load balancing is a unique EIGRP feature: the variance multiplier allows traffic to be distributed across paths with different metrics, proportional to their metric difference. OSPF and most other protocols only do equal-cost load balancing.

References

• RFC 7868 — Cisco’s Enhanced Interior Gateway Routing Protocol (EIGRP)