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EIGRP Basics

Overview

EIGRP- Enhanced Interior Gateway Routing Protocol

EIGRP is an Advanced Distance Vector routing protocol, as we know RIP uses the 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 kinds of mechanisms to install the route in the Routing Table.

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

EIGRP has a limitation of 255 Routers in a series and by default, a EIGRP router can receive update packets from 100 hops away only. But we can manipulate it according to our requirements. 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

Its uses:-                     

                     ·        Bandwidth

                     ·        Load

                     ·        Delay

                     ·        Reliability

                     ·        MTU

EIGRP uses the DUAL (Diffusing Update Algorithm) algorithm

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

                  ·        This Guarantees that each constructed path is loop-free at every 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 uses AD 90 to install routes in the 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 the routing table with AD 170

             ·        EIGRP uses the 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

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

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

             ·        Whenever the router adds EIGRP Packet to the IP header it uses protocol number 88 to identify the upper layer as EIGRP.

             ·        EIGRP packets can be multicast or unicast

o   Multicast Packets: by default, EIGRP use a 224.0.0.10 multicast address to send all the packets

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

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

             ·        EIGRP uses DUAL Algorithm to find the loop-free path to reach a destination