EIGRP Theory

EIGRP- Enhanced Interior Gateway Routing Protocol
This is only meant to be a brief overview.  I will be posting about the DUAL algorithm, the "feasibility condition" and terminology (anyone who has worked with EIGRP knows how confusing the chosen terms can be), and configuration/verification in their own separate sets of posts.

EIGRP-The "hybrid" routing protocol.
EIGRP is known as a hybrid routing protocol.  Its metric can make use of bandwidth, delay, reliability, and load.  By default, only bandwidth and delay are used.  EIGRP does not send periodic updates, and routing information does not expire.  Instead, EIGRP is aware of its neighbor's connection status through the use of hello messages.  If these messages are not received within a specific interval time, and the hold timer expires, EIGRP declares the route to be down and can immediately change to a backup route (known as a feasible successor route) if it exists, or it will use the Diffusing Update Algorithm (DUAL) to send a series of queries to neighboring routers in search of a valid path to the destination subnet.

EIGRP can operate in both a classful and a classless addressing scheme.  It does enable auto-summarization by default, so this is something to watch out for if working in a discontinguous network, or one that utilizes VLSM.  Unlike OSPF, any router can make use of summary routes in order to reduce the size of the routing table.

EIGRP Neighbors
The concept of neighbors is similar to OSPF, but simpler as there are not different states of the neighbor relationship-routers either form a neighbor relationship, or do not form a relationship.

Routers may form a neighbor relationship if:
-They are operating in the same subnet
-The routers are both configured with the same Autonomous System (AS) number
-The K values on both routers match
-Both routers pass authentication, if configured

If these four parameters are successfully met, the two routers become neighbors and begin by initiating a full topology transfer using the Reliable Transport Protocol (RTP).  After this topology exchange, the routers continue to send periodic hellos to the multi-cast address of 224.0.0.10.  In the event of a topology change, the router will notify its neighbor with an EIGRP update message using either the multicast address of 224.0.0.10 on multi-access networks, or the router's unicast IP address in a point to point type of relationship.  Two terms are used to describe EIGRP's update messages-bounded and partial.  A partial update will contain new information such as when a route changes, or a link goes down.  These are only sent in the event of a topology change.  The term bounded means that the updates are only sent to routers that are affected by this new change in the network topology.

Timers
The default hello interval depends on the link type being used.  Usually, a router configured to operate on a multi-point nonbroadcast multiaccess (NMBA) network such as Frame Relay, will have a default hello interval of 60 seconds.  The default hold time is 3x the hello interval.  On most other networks, the hello interval defaults to 5 seconds.  Unlike OSPF, these timers do not have to match in order to form a valid neighbor relationship, however if the timers are configured incorrectly, and the hold timer expires before receiving another hello message from its neighbor, EIGRP will drop the adjacency.

DUAL FSM
DUAL FSM (finite state machine) is the algorithm used by EIGRP to calculate the best path to a destination subnet.  It works to provide loop-free paths, and backup loop-free paths, which can be put to immediate use.  If a link goes down, DUAL makes a logical decision depending on whether it has any valid backup routes in its topology table-directly install a backup route (feasible successor route-this will be explained in further detail later), or if a feasible successor route does not exist, it will place the destination subnet in a "active" state and send queries to its neighbors.  If the neighbors have a valid route to the destination subnet, one of the types of messages they will send back is a reply message with information about the route.