EIGRP - Enhanced Interior Gateway Routing Protocol

Master EIGRP for a successful Cisco certification journey

EIGRP Introduction

EIGRP is a Advanced distance Vector routing protocol, as we know RIP uses concept of Hop count to find the best path to a specific route and OSPF uses Link properties to find the best path for a specific route, So Here also EIGRP uses both kind of mechanism to install the route in Routing table.

First router check the packet hop count whether it can receive it or not and then if hop count is verified then it goes to metric values to install the route with calculated metric value.

EIGRP has a limitation of 255 Routers in a series and by default a EIGRP router can receive update packet from 100 hops away only. But we can manipulate it according to our requirement. The value we can change from 1 to 255.

So later we will see how EIGRP check its hop count and how we can manipulate it using commands*

As we have discussed after checking Hop count it goes to metric calculation, so EIGRP Uses Link properties to do the metric calculation

     It use:-

                    • Bandwidth

                    • Load

                    • Delay

                    • Reliability

                    • MTU

EIGRP uses DUAL (Diffusing Update Algorithm) algorithm

                    • Constructs least-cost path to all reachable destination in a network consisting of nodes and edges

                    • This Guarantees that each constructed path is loop free at very instant including periods of topology changes and reconvergence.

                    • Dual computes the new best loop free path by using the feasibility condition.

                    • A feasibility condition is used to verify path the loop free or not.

Properties of EIGRP

• Whenever a EIGRP router gets a route from internal AS Routers it use AD 90 to install routes in Routing table

• Whenever a EIGRP router gets a route from external AS or other domain routes using redistribution then routers in EIGRP install routes in routing table with AD 170

• EIGRP uses concept of Autonomous number AS to run EIGRP, AS number size is 16 bit and we can have a range from 1 to 65535 to configure on a router.

• EIGRP use timers

                    Hello: 5 Sec, in every 5 sec router sends a hello to all active eigrp interfaces

                    Hold: 15 Sec, routers consider a neighbor relationship down if did not get hello packet till next 15sec of first Hello received.

• Whenever router add EIGRP Packet to IP header it use protocol number 88 to identify upper layer is EIGRP.

• EIGRP packets can be multicast or unicast

                    Multicast Packets: by default EIGRP use multicast address to send all the packets

                    Unicast Packets: when we use unicast neighborship, the router sends all the packet unicast to its neighbor

• EIGRP routers share partial update when they detect a new change in the update, only when they form EIGRP neighborship they send full update.

• EIGRP uses DUAL Algorithm to find loop free path to reach a destination

As Number Concept

                    • EIGRP uses concept of Autonomous number to form the neighborship

                    • All the routers in the same domain should have same AS number to form the neighborship with other router

                    • AS number size is 16 bit

                    • Range can be from 1 to 65535

Hello Packet Content

                    • AS number Must match

                    • K values Must match

                    • Hold time Can be different

                    • Seq number

                    • Ack number

                    • Flags

      To form EIGRP neighborship AS number and K Values should be same on both routers

Router State:

  EIGRP routers can have 2 states

      Pending: if did not get reply of Null update

      Full: when all the parameters are accurate and neighborship is up

Route State:


      A route is considered to be in the passive state when at least one neighbor provides the loop free path and least metric to use.

      A route in Passive state is usable and present in Routing table


      A route that is not considered to be in the active state when the route do not pass the feasibility condition & do not provide least metric

      A route in Active state is unusable & router coordinates with neighbor to find a loop free path.


               Check out our Complete EIGRP Video on Youtube