EIGRP Routing Protocols

EIGRP use five types of packets to communicate:

  > Hello    : used to identify neighbors. They are sent as periodic multicasts

  > Update : used to advertise routes, only sent as multicasts when something is changed

  > Ack      : acknowledges receipt of an update. In fact, Ack is Hello packet without data. It is always unicast and uses   UDP.

  > Query  : used to find alternate paths when all paths to a destination have failed

  > Reply  : is sent in response to query packets to instruct the originator not to recompute the route because feasible successors exist. Reply packets are always unicast to the originator of the query

Advertised distance(AD):	the cost from the neighbor to the destination.
Feasible distance (FD):	The sum of the AD plus the cost between the local router and the next-hop router
Successor:	The primary route used to reach a destination. The successor route is kept in the routing table. Notice that successor is the best route to that destination.
Feasible successor:	The backup route. To be a feasible successor, the route must have an AD less than the FD of the current successor route

EIGRP Neighbor,Topogoly and Routing Table

EIGRP routers will start sending hello packets to other routers just like OSPF does, if you send hello packets and you receive them you will become neighbors. EIGRP neighbors will exchange routing information which will be saved in the topology table. The best path from the topology table will be copied in the routing table.

Neighbor table: The neighbor relationships are tracked in this table which are the basis for EIGRP routing and convergence activity. The address and the interface of a neighbor is discovered and recorded in a new entry of the neighbor table, whenever a new neighbor is discovered. These tables are used for reliable and sequenced delivery of packets.

Topology table: Routers use topology table which route traffic in a network. All routing tables inside the autonomous system are available in this table, where the router is positioned. Each router uses routing protocol and maintains a topology table for each configured network protocol. The routes leading to a destination are found in the topology table.

Route Table: The routes of particular destinations are stored in the routing tables. The information contains the network topology that is immediately around it. The primary goal of routing protocols and routes is the construction of routing tables. Network id, cost of the packet path and next hop are the details are available in the routing table.



Normal Configuration:

**************
      R1
**************

Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#inter g0/0
Router(config-if)#ip address 1.1.1.1 255.255.255.252
Router(config-if)#exit
Router(config)#router eigrp 100
Router(config-router)#network 1.1.1.0
Router(config-router)#exit
Router(config)#exit
Router#
%SYS-5-CONFIG_I: Configured from console by console

Router#

**************
      R2
**************

Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#inter g0/0
Router(config-if)#ip address 1.1.1.2 255.255.255.252
Router(config-if)#exit
Router(config)#router eigrp 100
Router(config-router)#network 1.1.1.0
Router(config-router)#exit
Router(config)#exit
Router#

  In CCNA Exam Lab troubleshoot the eigrp with just changing AS number of this topology

Thats all. Thanks