EIGRP - Enhanced Interior Gateway Routing Protocol

Dynamic routing Protocol performs the same function as static routing Protocol does. In dynamic routing Protocol, if the destination is unreachable then an another entry, in the routing table, to the same destination can be used. One of the routing Protocol is EIGRP.

EIGRP:
Enhanced Interior Gateway Routing Protocol (EIGRP) is a dynamic routing Protocol which is used to find the best path between any two layer 3 device to deliver the packet. EIGRP works on network layer Protocol of osi model and uses the protocol number 88.It uses metric to find out best path between two layer 3 device (router or layer 3 switch) operating EIGRP. 

EIGRP uses the Diffusing-Update Algorithm (DUAL) to determine the most efficient (least cost) route to a destination. A DUAL finite state machine contains decision information used by the algorithm to determine the least-cost route (which considers distance and whether a destination path is loop-free).

Routers running EIGRP must become neighbors before exchanging routing information. To dynamically discover neighbors, EIGRP routers use the multicast address of 224.0.0.10. Each EIGRP router stores routing and topology information in three tables:

• Neighbor table – stores information about EIGRP neighbors
• Topology table – stores routing information learned from neighboring routers
• Routing table –  stores the best routes

Neighbor of EIGRP:EIGRP must establish neighbor relationships with other EIGRP neighboring routers before exchanging routing information. To establish a neighbor relationships, routers send hello packets every couple of seconds. Hello packets are sent to the multicast address of 224.0.0.10.

The following fields in a hello packet must be the identical in order for routers to become neighbors:

• ASN (autonomous system number)
• subnet number
• K values (components of metric)

Routers send hello packets every couple of seconds to ensure that the neighbor relationship is still active. By default, routers considers the neighbor to be down after a hold-down timer has expired. Hold-down timer is, by default, three times the hello interval. On LAN network the hold-down timer  is 15 seconds.

Feasible and reported distance

Two terms that you will often encounter when working with EIGRP are feasible and reported distance. Let’s clarify these terms:

• Feasible distance (FD) – the metric of the best route to reach a network. That route will be listed in the routing table.
• Reported distance (RD) – the metric advertised by a neighboring router for a specific route. It other words, it is the metric of the route used by the neighboring router to reach the network.

Successor and feasible successor

Another two terms that appear often in the EIGRP world are successor and feasible successor. A successor is the route with the best metric to reach a destination. That route is stored in the routing table. A feasible successor is a backup path to reach that same destination that can be used immediately if the successor route fails. These backup routes are stored in the topology table.

Topology of EIGRP:

EIGRP toplogy table contains all learned routes to a destination. The table holds all routes received from a neighbor, successors and feasible successors for every route, and interfaces on which updates were received. The table also holds all locally connected subnets included in an EIGRP process.

Best routes (the successors) from the topology table are stored in the routing table. Feasible successors are only stored in the topology table and can be used immediately if the primary route fails.

Routing of EIGRP:

EIGRP of Routing contains rules by which traffic is forwarded in a network. If the router does not contain a valid path to the destination, the traffic is discarded and the path is not there in the routing table.EIGRP supports the following features:-

• Support for Classless Inter-Domain Routing (CIDR) and variable length subnet masking. Routes are not summarized at the classful network boundary unless auto summary is enabled.
•  Support for load balancing on parallel links between sites.
• The ability to use different authentication passwords at different times.
•  MD5 authentication between two routers.
• Sends topology changes, rather than sending the entire routing table when a route is changed.
• Periodically checks if a route is available and propagates routing changes to neighboring routers if any changes have occurred.
• Runs separate routing processes for Internet Protocol (IP), IPv6, IPX and AppleTalk through the use of protocol-dependent modules (PDMs).
• Backwards compatibility with the IGRP routing protocols

EIGRP Cost Calculation

As Enhanced Interior Gateway Routing Protocol (EIGRP) is a hybrid vector routing protocol as it contains features of both distance vector routing protocol and link-state routing protocol. It is a network layer protocol which uses protocol number 88.

Cost Calculation:
Composite matrix is used to calculate the cost and also used for neighbourship discovery purpose. It has values:
K1(bandwidth)- 1
K2(load)- 0
K3(delay) -1
K4(reliability)- 0
K5(MTU)-0
As only bandwidth and delay is used to calculate the cost. The formula used for cost

calculation is:

EIGRP Metric = 256*((K1*Bandwidth) + (K2*Bandwidth)/(256-Load) + K3*Delay)*(K5/(Reliability + K4)))

As values of K1 and K3 are set to 1, and K2, K4 and K5 are set to 0. Therefore the formula becomes:
Metric = 256*( Bandwidth +Sum of all Delay)
Where the bandwidth = (10^7/least bandwidth) and
Delay = (sum of all delays /10)