Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network to help automate routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary protocol, available only on Cisco routers, but Cisco converted it to an open standard in 2013.

" So now EIGRP is a Open Standard Routing Protocol, It means EIGRP can be used in other vendors as well"

Now question arises, Why EIGRP replaced IGRP Protocol ?
One of the major reasons for this was because the design of the Internet Protocol had been changed to support classless IPv4 addresses, which IGRP could not support.

" EIGRP Replaces IGRP protocol because IGRP doesn't support Classless inter domain routing"

Fig 1.1- EIGRP

EIGRP uses the following tables to store information:-
Routing table, Neighbor Table and Topology Table, Lets Start discussing about all these tables.

Routing Table:-That 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.
Neighbor Table: The neighbor table keeps a record of the IP addresses of routers that have a direct physical connection with this router. Routers that are connected to this router indirectly, through another router are not recorded in this table as they are not considered neighbors.
Topology Table: The topology table stores routes that it has learned from neighbor routing tables. Unlike a routing table, the topology table does not store all routes, but only routes that have been determined by EIGRP. The topology table also records the metrics for each of the listed EIGRP routes, the feasible successor and the successors. Routes in the topology table are marked as "passive" or "active". Passive indicates that EIGRP has determined the path for the specific route and has finished processing. Active indicates that EIGRP is still trying to calculate the best path for the specific route. Routes in the topology table are not usable by the router until they are inserted into the routing table. The topology table is never used by the router to forward traffic

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

" So EIGRP protocol with so many features is better to use rather than any RIP, IGRP protocols"

EIGRP Technical Specifications:-

EIGRP is a distance vector routing protocol that uses the diffusing update algorithm (DUAL)
EIGRP routing information exchanged to a router from another router within the same autonomous system has a default with AD 90.
EIGRP routing information that has come from an EIGRP-enabled router outside the autonomous system has a default AD of 170
EIGRP Summary routes has a default AD of 5.
EIGRP does not operate using the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). This means that EIGRP does not use a port number to identify traffic.
Since EIGRP does not use TCP for communication, it implements Cisco's Reliable Transport Protocol (RTP) to ensure that EIGRP router updates are delivered to all neighbors completely.
EIGRP uses port no 88 on RTC to identify traffic.
The reliable transport protocol also contains other mechanisms to maximize efficiency and support Multicasting.

" So EIGRP protocol having AD 90 for internal, AD 170 for external and AD 5 for summary routes"

" EIGRP is not Application layer protocol, as it doesn't using TCP or UDP for identify Traffic, while it uses RTC with port no 88 to identify traffic and RTC is the part of EIGRP"

EIGRP composite and vector metrics:-

Bandwidth: Minimum Bandwidth (in kilobits per second) along the path from router to destination network.

Load: Number in range 1 to 255; 255 being saturated

Delay: Delay, in 10s of microseconds, along the path from router to destination network

Reliability: Number in range 1 to 255; 255 being the most reliable

MTU: Minimum path Maximum Transmission Unit (MTU).

\bigg [ \bigg ( K_1 \cdot {\text{Bandwidth}}_{E} + \frac{K_2 \cdot {\text{Bandwidth}}_{E}}{256-\text{Load}} + K_3 \cdot {\text{Delay}}_{E} \bigg ) \cdot \frac {K_5}{K_4 + \text{Reliability}} \bigg ] \cdot 256


where the various constants ( $K_1$ through $K_5$ ) can be set by the user to produce varying behaviors. An important and unintuitive fact is that if $K_5$ is set to zero, the term $\tfrac {K_5}{K_4 + \text{Reliability}}$ is not used (i.e. taken as 1).
The default is for $K_1$ and $K_3$ to be set to 1, and the rest to zero, effectively reducing the above formula to $({\text{Bandwidth}}_{E} + \text{Delay}_{E}) \cdot 256$ .

This is all about the basics of Part-1 in EIGRP, we will discuss in Part-2 about the IOS configurations, Feasible Successor, Active and passive state, Feasibility condition and a running example with Scenario.

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Enhanced Interior Gateway Routing Protocol (EIGRP)