EIGRP Concept / Comparing EIGRP and IGRP

EIGRP Concept 
Comparing EIGRP and IGRP
3.1.1

Cisco released EIGRP in 1994 as a scalable and improved version of its proprietary distance vector routing protocol, IGRP. This page will explain how EIGRP and IGRP compare to each other. The distance vector technology and distance information found in IGRP is also used in EIGRP.
EIGRP has improved convergence properties and operates more efficiently over IGRP. This allows a network to have improved architecture as well as retain the current investment in IGRP.
The comparisons between EIGRP and IGRP fall into the following major categories:

• Compatibility mode
• Metric calculation
• Hop count
• Automatic protocol redistribution
• Route tagging

IGRP and EIGRP are compatible with each other. This compatibility provides seamless interoperability with IGRP routers. This is important as users can take advantage of the benefits of both protocols. EIGRP offers multiprotocol support, but IGRP does not.
EIGRP and IGRP use different metric calculations. EIGRP scales the metric of IGRP by a factor of 256. That is because EIGRP uses a metric that is 32 bits long, and IGRP uses a 24-bit metric. EIGRP can multiply or divide by 256 to easily exchange information with IGRP.
IGRP has a maximum hop count of 255. EIGRP has a maximum hop count limit of 224. This is more than adequate to support large, properly designed internetworks.
To enable dissimilar routing protocols such as OSPF and RIP to share information requires advanced configuration. Redistribution, or route sharing, is automatic between IGRP and EIGRP as long as both processes use the same AS number. In Figure , RTB automatically redistributes routes learned from EIGRP to the IGRP AS, and vice versa.
EIGRP tags routes learned from IGRP or any outside source as external because they did not originate from EIGRP routers. IGRP cannot differentiate between internal and external routes.
Notice that in the show ip route command output for the routers in Figure , EIGRP routes are flagged with D, and external routes are denoted by EX. RTA identifies the difference between the 172.16.0.0 network, which was learned through EIGRP, and the 192.168.1.0 network that was redistributed from IGRP. In the RTC table, the IGRP protocol makes no such distinction. RTC, which uses IGRP only, just sees IGRP routes, despite the fact that both 10.1.1.0 and 172.16.0.0 were redistributed from EIGRP.
The Interactive Media Activity will help students recognize the characteristics of IGRP and EIGRP.
The next page will explain EIGRP in greater detail.

Module 3: EIGRP

Module 3: EIGRP


EIGRP is a Cisco-proprietary routing protocol that is based on IGRP.
EIGRP supports CIDR and VLSM which allows network designers to maximize address space. When compared to IGRP which is a classful routing protocol, EIGRP boasts faster convergence times, improved scalability, and superior management of routing loops.
Furthermore, EIGRP can replace Novell RIP and AppleTalk Routing Table Maintenance Protocol (RTMP). EIGRP serves both IPX and AppleTalk networks with powerful efficiency.
EIGRP is often described as a hybrid routing protocol that offers the best of distance vector and link-state algorithms.
EIGRP is an advanced routing protocol that relies on features commonly associated with link-state protocols. Some of the best features of OSPF, such as partial updates and neighbor discovery, are similarly put to use by EIGRP. However, EIGRP is easier to configure than OSPF.
EIGRP is an ideal choice for large, multi-protocol networks built primarily on Cisco routers.
This module covers common EIGRP configuration tasks. The emphasis is on ways in which EIGRP establishes relationships with adjacent routers, calculates primary and backup routes, and responds to failures in known routes to a particular destination.
A network is made up of many devices, protocols, and media that allow data communication to occur. When a network component does not work correctly, it can affect the entire network. In any case, network administrators must quickly identify and troubleshoot problems when they arise. The following are some reasons why network problems occur:

• Commands are entered incorrectly
• Access lists are constructed or placed incorrectly
• Routers, switches, or other network devices are misconfigured
• Physical connections are bad

A network administrator should troubleshoot in a methodical manner with the use a general problem-solving model. It is often useful to check for physical layer problems first and then move up the layers in an organized manner. Although this module focuses on how to troubleshoot Layer 3 protocols, it is important to troubleshoot and eliminate any problems that may exist at the lower layers.
This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.  
Students who complete this module should be able to perform the following tasks:

• Describe the differences between EIGRP and IGRP
• Describe the key concepts, technologies, and data structures of EIGRP
• Understand EIGRP convergence and the basic operation of the Diffusing Update Algorithm (DUAL)
• Perform basic EIGRP configuration
• Configure EIGRP route summarization
• Describe the processes used by EIGRP to build and maintain routing tables
• Verify EIGRP operations
• Describe the eight-step process for general troubleshooting
• Apply a logical process to troubleshoot routing
• Use the show and debug commands to troubleshoot RIP
• Use the show and debug commands to troubleshoot IGRP
• Use the show and debug commands to troubleshoot EIGRP

Use the show and debug commands to troubleshoot OSPF 

Summary

Summary


This page summarizes the topics discussed in this module.
An essential difference between link-state routing protocols and distance vector protocols is how they exchange routing information. Link-state routing protocols respond quickly to network changes, send triggered updates only when a network change has occurred, send periodic updates known as link-state refreshes, and use a hello mechanism to determine the reachability of neighbors.
A router running a link-state protocol uses the hello information and LSAs it receives from other routers to build a database about the network. It also uses the shortest path first (SPF) algorithm to calculate the shortest route to each network.
To overcome the limitations of distance vector routing protocols, link-state routing protocols use link-state advertisements (LSAs), a topological database, the shortest path first (SPF) algorithm, a resulting SPF tree, and a routing table of paths and ports to each network to determine the best paths for packets.
A link is the same as an interface on a router. The state of the link is a description of an interface and the relationship to its neighboring routers. Link-state routers advertise with LSAs the states of their links to all other routers in the area so that each router can build a complete link-state database. They form special relationships with their neighbors and other link-state routers. Link state routers are a good choice for complex, scalable networks. The benefits of link-state routing over distance vector protocols include faster convergence and improved bandwidth utilization. Link-state protocols support classless interdomain routing (CIDR) and variable-length subnet mask (VLSM).
Open Shortest Path First (OSPF) is a link-state routing protocol based on open standards. The Open in OSPF means that it is open to the public and is non-proprietary. OSPF routers elect a Designated Router (DR) and a Backup Designated Router (BDR) that serve as focal points for routing information exchange in order to reduce the number of exchanges of routing information among several neighbors on the same network. OSPF selects routes based on cost, which in the Cisco implementation is related to bandwidth. OSPF selects the fastest loop-free path from the shortest-path first tree as the best path in the network. OSPF guarantees loop-free routing. Distance vector protocols may cause routing loops. When a router starts an OSPF routing process on an interface, it sends a hello packet and continues to send hellos at regular intervals. The rules that govern the exchange of OSPF hello packets are called the Hello protocol. If all parameters in the OSPF Hello packets are agreed upon, the routers become neighbors.
Each router sends link-state advertisements (LSA) in link-state update (LSU) packets. Each router that receives an LSA from its neighbor records the LSA in the link-state database. This process is repeated for all routers in the OSPF network. When the databases are complete, each router uses the SPF algorithm to calculate a loop free logical topology to every known network. The shortest path with the lowest cost is used in building this topology, therefore the best route is selected.
This routing information is maintained. When there is a change in a link-state, routers use a flooding process to notify other routers on the network about the change. The Hello protocol dead interval provides a simple mechanism for determining that an adjacent neighbor is down