In this article we will look at Enhanced Interior Gateway Routing Protocol (EIGRP) which is enhanced version of IGRP.
Introduction
The Interior Gateway Routing Protocol (IGRP) is a routing protocol developed in the mid-1980s by Cisco Systems. Cisco’s principal goal in creating IGRP was to provide a robust protocol for routing within an autonomous system (AS). In the mid-1980s, the most popular intra-AS routing protocol was the Routing Information Protocol (RIP). Although RIP was quite useful for routing within small-to-moderate sized, relatively homogeneous internetworks, its limits were being pushed by network growth.
In particular, RIP’s small hop-count limit (16) restricted the size of internetworks, and its single metric (hop count) did not allow for much routing flexibility in complex environments. Cisco’s initial IGRP implementation worked in Internet Protocol (IP) networks. Cisco developed Enhanced IGRP in the early 1990s to improve the operating efficiency of IGRP. Cisco introduced an enhanced version of IGRP that combines the advantages of link state protocols with the advantages of distance vector protocols.
EIGRP Features
Enhanced IGRP includes the following features:
Fast convergence: Enhanced IGRP uses DUAL (Diffusing Update Algorithm) to achieve convergence quickly. A router running Enhanced IGRP stores all of its neighbors’ routing tables so that it can quickly adapt to alternate routes. If no appropriate route exists, Enhanced IGRP queries its neighbors to discover an alternate route. These queries propagate until an alternate route is found.
Variable length subnet masks: Enhanced IGRP includes full support for variable length subnet masks. Subnet routes are automatically summarized on a network number boundary. In addition, Enhanced IGRP can be configured to summarize on any bit boundary at any interface.
Partial, bounded updates: Enhanced IGRP does not make periodic updates. Instead, it sends partial updates only when the metric for a route changes. Propagation of partial updates is automatically bounded so that only those routers that need the information are updated. As a result of these two capabilities, Enhanced IGRP consumes significantly less bandwidth than IGRP.
Multiple network-layer support: Enhanced IGRP includes support for AppleTalk, IP, and Novell NetWare. The AppleTalk implementation redistributes routes learned from the Routing Table Maintenance Protocol (RTMP). The IP implementation redistributes routes learned from OSPF, Routing Information Protocol (RIP), IS-IS, Exterior Gateway Protocol (EGP), or Border Gateway Protocol (BGP). The Novell implementation redistributes routes learned from Novell RIP or Service Advertisement Protocol (SAP).
EIGRP Technologies
Enhanced IGRP features four new technologies:
Neighbor discovery/recovery: Used by routers to dynamically learn about other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. This process is achieved with low overhead by periodically sending small hello packets. As long as a router receives hello packets from a neighboring router, it assumes that the neighbor is functioning, and they can exchange routing information.
Reliable Transport Protocol (RTP): Responsible for guaranteed, ordered delivery of Enhanced IGRP packets to all neighbors. It supports intermixed transmission of multicast or unicast packets. For efficiency, only certain Enhanced IGRP packets are transmitted reliably. RTP has a provision for sending multicast packets quickly when unacknowledged packets are pending, which helps ensure that convergence time remains low in the presence of varying speed links.
DUAL finite state machine: Embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses distance information to select efficient, loop-free paths and selects routes for insertion in a routing table based on feasible successors. A feasible successor is a neighboring router used for packet forwarding that is a least-cost path to a destination that is guaranteed not to be part of a routing loop.
Protocol-dependent modules: Responsible for network-layer protocol-specific requirements. For example, the IP-Enhanced IGRP module is responsible for sending and receiving Enhanced IGRP packets that are encapsulated in IP. IP-Enhanced IGRP is also responsible for parsing Enhanced IGRP packets and informing DUAL of the new information that has been received. IP-Enhanced IGRP asks DUAL to make routing decisions, the results of which are stored in the IP routing table. IP-Enhanced IGRP is responsible for redistributing routes learned by other IP routing protocols.
EIGRP Packets
Following are different EIGRP packet types:
Hello: It determines the neighbors’ router and also serves as a keep-alive mechanism between the routers. If Router X is connected with Router Y and the Router X is not receiving the hello packets from Router Y then it assumes that Router Y is not reachable and the network is down.
Update: Updates are to send the information about the route to its neighbors. When a new router is found, the update packets sent to the neighbor to build up the topology table.
Query: Queries are used specifically for requesting route info. They act as multi-part until they send received queries as the response. It will send the queries only when the destination state is active.
Reply: Reply packets respond to a query which indicates the originator router that it does not need to go into Active state as reliable successors for the destination network. Replies are sent when destinations go into Active state. For the reply packet, an acknowledgment is sent.
ACK: Acknowledgment packet will be sent to Enhanced Interior Gateway Routing Protocol Query, Update and Reply packets. It is shared with unicast address and, also acknowledgment not sent to Hello packets.
EIGRP Tables
To perform different functions, EIGRP creates three tables which are:
- Neighbor Table
- Topology Table
- Routing Table
Neighbor Table: The neighbor table contains information about routers and neighborship relation-ships with whom routes have been established. Command to list router information: ‘show ip eigrp neighbors’. The Neighbor Table has Fields like H: Handle, Address, Interface, Hold Time, Uptime, Smooth Round Trip Time, Retransmission Timeout, Queue Count, Sequence Number.
Topology Table: The topology table holds information about all the paths to networks understood by EIGRP routers. Command to list router information- ‘show ip eigrp topology’. Topology table holds the following fields Passive, Feasible Distance, Advertised distance, Feasible distance.
Routing Table: The routing table stores the routes which are currently active in sending packets to the network. It stores the optimal route for the destination from the sender. Command to List Router Information: ‘show ipv6 route’.
Suryateja Pericherla, at present is a Research Scholar (full-time Ph.D.) in the Dept. of Computer Science & Systems Engineering at Andhra University, Visakhapatnam. Previously worked as an Associate Professor in the Dept. of CSE at Vishnu Institute of Technology, India.
He has 11+ years of teaching experience and is an individual researcher whose research interests are Cloud Computing, Internet of Things, Computer Security, Network Security and Blockchain.
He is a member of professional societies like IEEE, ACM, CSI and ISCA. He published several research papers which are indexed by SCIE, WoS, Scopus, Springer and others.
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