CCIE R&S Written Overview: EIGRP

EIGRP Overview

• Enhanced Interior Gateway Routing Protocol

– Successor to Interior Gateway Routing Protocol (IGRP)

• Cisco proprietary “hybrid” protocol

– Both Distance Vector and Link State Behavior

– Really “Advanced Distance Vector”

EIGRP Features

• “Classless” protocol

– Supports VLSM and summarization

• Multiple routed protocol support

– IPv4, IPX, & Appletalk

• Uses its own transport protocol

– IP protocol 88 (EIGRP)

• Reliable Transport Protocol (RTP)

– Uses multicast to and unicast

• Forms active neighbor adjacencies

– Guarantees packet delivery and supports partial updates

• Guarantees loop-free topology

– Diffusing Update Algorithm (DUAL)

• Fast convergence

– Fastest of all IGP in certain designs

• Granular Metric

– Hybrid metric derived from multiple factors

• Unequal Cost Load Balancing

– Only IGP that supports true load distribution

• Summarization

– Like RIPv2 supports auto-summary and manual summaries

• Control Plane Security

– Supports MD5 based authentication

Forming EIGRP Adjacencies

• Neighbors are discovered with HELLO packets

– Sent to from primary IP address

• Neighbors must agree on…

– IPv4 subnet

– Autonomous System Number

– Authentication

– Metric Weightings (K values)

• Neighbors do not need to agree on timers

– Opposite of OSPF timer logic

Sending EIGRP Updates

• Once neighbors are found, EIGRP UPDATE messages used to exchange routes

– Sent as multicast to or as unicast

• Update messages describe attributes of a route

– Prefix + Length

– Next-Hop

– Bandwidth

– Delay

– Load

– Reliability


– Hop Count

– External Attributes

Calculating the EIGRP Topology

• All routes learned from all neighbors make up the EIGRP “topology table”

• Once topology is learned, DUAL runs to choose loop-free best path to each destination

– Best path has the lowest “composite metric”

• Composite metric calculated from…

– Administrative Weighting (K values)

– Bandwidth

– Delay

– Load

– Reliability

• Path with lowest composite metric is considered best and installed in IP routing table

• Only best route is advertised to other EIGRP neighbors

• One or more backup routes can also be precalculated per destination

EIGRP Loop Prevention

• EIGRP guarantees loop-free topology through usage of…

– Split Horizon

• Don’t advertise routes out the link they came in on

– DUAL Feasibility Condition

• If your metric is lower than mine, you are loop-free

EIGRP Reconvergence

• Active EIGRP neighbor adjacency reduces convergence time

– Adjacent neighbors’ hello packets contain “hold time”

– If no hello is received within hold time, neighbor declared unreachable

• When neighbor is lost…

– Paths via that neighbor are removed from topology and routing table

– If backup routes exist, they become new best paths and are inserted in routing table

• In this case EIGRP can have sub-second convergence

– If no backup routes exist, DUAL must run again

• When best path is lost and no backup routes exist, route goes into

“active” state and “active timer” starts

– Stable routes not in active state are considered “passive”

• EIGRP “QUERY” message is reliably sent to remaining neighbors asking if there is an alternate route

– QUERY is propagated to all neighbors within EIGRP “QUERY domain” or “flooding domain”

• Summarization and EIGRP Stub feature limits the QUERY domain

– Neighbors respond with EIGRP “REPLY” packet indicating if alternate route is available

• If alternate route exists, DUAL recalculates new best path

• If no alternate route, prefix removed from topology table

• If active timer expires and no REPLY received, route is declared “Stuck-In-Active” (SIA) and removed from topology table