EIGRP Concepts in a Nutshell

EIGRP is an advanced distance-vector routing protocol that relies on features commonly associated with link-state protocols. Administrative Distance

Internal 90; Summary 5; External 170

Features of EIGRP• Classless Routing Protocol (VLSM, CIDR)• Faster convergence times • Multiprotocol support: • Rapid Convergence and Better handling of

routing loops – (DUAL) • Efficient Use of Bandwidth

– Partial, bounded updates: Incremental updates only to the routers that need them.

– Minimal bandwidth consumption: no more that 50% of link’s bandwidth EIGRP packets.

• PDM (Protocol Dependent Module)• Unequal-cost load balancing

Metrics: Bandwidth, Delay, Reliability, Load

Default Metricbandwidth = (10,000,000/bandwidth kbps) * 256delay = (delay/10) * 256 32 bit metric for bandwidth and delay

Routing Table Metric: Slowest of bandwidth plus the sum of the delays of all outgoing interfaces from “this router” to the destination network.

External routes (redistributed) are tagged as external routes (EX)

Hellos • Used by the neighbor discovery and recovery

process. • Multicast• Unreliable delivery• T1 and faster : Hello interval 5 seconds, hold

time 15 seconds • slower than T1: Hello interval 60 seconds, hold

time 180 seconds • If a neighbor is not heard from for the duration of

the hold time (three times hello interval), EIGRP considers that neighbor down, and DUAL must step in to reevaluate the routing table.

• EIGRP routers do not need to have the same hello intervals and hold down intervals

Automatic redistribution between IGRP and EIGRP as long as “AS” numbers are the same.

Operation of EIGRP RTP, PDM, Neighbor Discovery, DUAL

RTP Reliable RTP is used for EIGRP queries,

updates and replies (224.0.0.10) Unreliable RTP is used for EIGRP Hello’s and

Ack’s

Neighbor Adjacency• Dynamically learn of new routes that join their

network • Identify routers that become either unreachable

or inoperable • Rediscover routers that had previously been

unreachable

Acknowledgements (ACK)• Hello packets with no data• Unicast• Unreliable delivery

DUALThis engine contains all the logic used to calculate and compare routes in an EIGRP network DUAL selects alternate routes quickly by using

the information in the EIGRP tables. If a link goes down, DUAL looks for a feasible

successor in its neighbor and topology tables.

Updates• Transmitted only when necessary• Unicast when sent to a specific router• Multicast when sent to multiple routers• Reliable delivery

PDMDifferent PDMs can be added to EIGRP as new routed protocols are enhanced or developed

Queries and Replies• Used by DUAL FSM• Queries: Multicast or Unicast• Replies: Unicast• Reliable delivery

Successor – Current Route A passive route is one that is stable and

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Feasible Successor - A backup route available for use. An active route is a route in the process of

being recomputed by DUAL.

SIA (Stuck in Active) – When query does not get a reply.

If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL moves the route from passive state to active state.

Feasible distance (FD) is the minimum distance (metric) along a path to a destination network.

Reported distance (RD) is the distance (metric) towards a destination as advertised by an upstream neighbor. Reported distance is the distance reported in the queries, the replies and the updates.

A neighbor meets the feasible condition(FC) if the reported distance by the neighbor is less than to the current feasible distance (FD) of this router. "If a neighbors metric is less than mine, then I know the neighbor doesn't have a loop going through me."

A feasible successor is a neighbor whose reported distance (RD) is less than or equal to the current feasible distance (FD). Feasible successor is one who meets the feasible condition (FC).

Topology Table Listing of all successors and feasible successors

Neighbor Table Lists adjacent routers.

CommandsRouter(config)#router eigrp autonomous-system-number

• This value must match all routers within the internetwork.

Router(config-router)#network network-number [wildcard-mask]• The network command configures only connected networks.

Router(config-router)#eigrp log-neighbor-changes • This command enables the logging of neighbor adjacency changes to monitor the stability of the

routing system and to help detect problems.

Router(config-if)#bandwidth kilobits• When configuring serial links using EIGRP it is important to configure the bandwidth setting on the

interface. If the bandwidth setting is not changed for these interfaces EIGRP assumes the default bandwidth on the link instead of the true bandwidth.

Router(config-if)#ip bandwidth-percent eigrp AS percentage• By default, EIGRP is set to use only up to 50% of the bandwidth of an interface to exchange routing

information.

Router(config-router)#no auto-summary • To disable automatic summarization.• EIGRP automatically summarizes routes at the classful boundary

Router(config-if)#ip summary-address eigrp autonomous-system-number ip-address mask administrative-distance

• Summary route is sourced from Null0, and not an actual interface

A Null0 route is created in the routing table whenever EIGRP includes subnetted routes and with the default no auto-summary command. It is a type of discard route.Bandwidth and Frame Relay: The links that have the same configured CIR are presented as a single subinterface with a bandwidth reflecting the aggregate CIR of all the circuits.

EIGRP Commands in a Nutshell

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Enabling EIGRP RoutingRouter(config)# router eigrp AS number (Must be the same on all routers)

Router(config-router)# network network-address [wildcard mask]

EIGRP Interface commandsRouter(config-if)# ip summary-address eigrp as-number network-address mask

RTC(config-if)#ip summary-address eigrp 2446 2.1.0.0 255.255.0.0

Router(config-if)# no auto-summary auto-summary on by default. EIGRP summarizes automatically between

classful boundaries. Must be used for VLSM.

Router(config-if)#bandwidth kilobits• Configures the bandwidth used by routing metrics on the outgoing

interface.

Router(config-if)#ip bandwidth-percent eigrp as-number bandwidth-percentage• By default, EIGRP is set to use only up to 50% of the bandwidth of an

interface to exchange routing information.

Router(config-router)#eigrp log-neighbor-changes • This command enables the logging of neighbor adjacency changes to monitor

the stability of the routing system and to help detect problems.

RTA(config-router)# variance number The variance command instructs the router to include routes with a metric

less than or equal to n times the minimum metric route for that destination, where n is the number specified by the variance command.

MiscellaneousRouter(config-router)# default-metric 56 100 255 10 1500 {k values)

Show commandsRouter# show ip eigrp neighbors {displays neighbors}Router# show ip eigrp topologyRouter# show ip eigrp topology all links

displays topology, active/passive (well) state, successorsRouter# debug eigrp fsmRouter# debug eigrp packet

Router# show ip route eigrp {EIGRP routes in routing table}Router# show ip protocols

AS number, filtering, redistribution, neighbors, distanceRouter# show ip eigrp traffic EIGRP packets sent and received

RedistributionExample 1: EIGRP and IGRP redistributes automatically as long as the same process-id is used.

Router(config)# router eigrp 44 and Router(config)# router igrp 44

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