EIGRP concepts and terminology

EIGRP concepts and terminology


3.1.2 This page will discuss the three tables that EIGRP uses to store network information.
EIGRP routers keep route and topology information readily available in RAM so they can react quickly to changes. Like OSPF, EIGRP saves this information in several tables and databases.
EIGRP saves routes that are learned, in specific ways. Routes are given a particular status and can be tagged to provide additional useful information.
The following three tables are maintained by EIGRP:

• Neighbor table
• Topology table
• Routing table

The neighbor table is the most important table in EIGRP. Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports.
When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This information is stored in the neighbor data structure. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the amount of time a router treats a neighbor as reachable and operational. If a hello packet is not received within the hold time, then the hold time expires. When the hold time expires, the Diffusing Update Algorithm (DUAL), which is the EIGRP distance vector algorithm, is informed of the topology change and must recalculate the new topology.
The topology table is made up of all the EIGRP routing tables in the autonomous system. DUAL takes the information supplied in the neighbor table and the topology table and calculates the lowest cost routes to each destination. EIGRP tracks this information so that EIGRP routers can identify and switch to alternate routes quickly. The information that the router learns from the DUAL is used to determine the successor route, which is the term used to identify the primary or best route. This information is also entered into the topology table.
Every EIGRP router maintains a topology table for each configured network protocol. All learned routes to a destination are maintained in the topology table.
The topology table includes the following fields:

• Feasible distance (FD) - This is the lowest calculated metric to each destination. For example, the feasible distance to 32.0.0.0 is 2195456.
• Route source - The identification number of the router that originally advertised that route. This field is populated only for routes learned externally from the EIGRP network. Route tagging can be particularly useful with policy-based routing. For example, the route source to 32.0.0.0 is 200.10.10.10 through 200.10.10.10.
• Reported distance (RD) - The distance reported by an adjacent neighbor to a specific destination. For example, the reported distance to 32.0.0.0 is /281600 as indicated by (2195456/281600).
• Interface information - The interface through which the destination can be reached.
• Route status - The status of a route. Routes are identified as being either passive, which means that the route is stable and ready for use, or active, which means that the route is in the the process of being recomputed by DUAL.

The EIGRP routing table holds the best routes to a destination. This information is retrieved from the topology table. EIGRP routers maintain a routing table for each network protocol.
A successor is a route selected as the primary route to reach a destination. DUAL identifies this route from the information contained in the neighbor and topology tables and places it in the routing table. There can be up to four successor routes for any particular destination. These can be of equal or unequal cost and are identified as the best loop-free paths to a given destination. A copy of the successor routes is also placed in the topology table.
A feasible successor (FS) is a backup route. These routes are identified at the same time as the successors, but these routes are only kept in the topology table. Multiple feasible successors for a destination can be retained in the topology table although it is not mandatory.
A router views the feasible successors as neighbors downstream, or closer to the destination than it is. Feasible successor cost is computed by the advertised cost of the neighbor router to the destination. If a successor route goes down, the router will look for an identified feasible successor. This route will be promoted to successor status. A feasible successor must have a lower advertised cost than the current successor cost to the destination. If a feasible successor is not identified from the current information, the router places an Active status on a route and sends out query packets to all neighbors in order to recompute the current topology. The router can identify any new successor or feasible successor routes from the new data that is received from the reply packets that answer the query requests. The router will then place a Passive status on the route.
The topology table can record additional information about each route. EIGRP classifies routes as either internal or external. EIGRP adds a route tag to each route to identify this classification. Internal routes originate from within the EIGRP AS.
External routes originate outside the EIGRP AS. Routes learned or redistributed from other routing protocols, such as RIP, OSPF, and IGRP, are external. Static routes that originate outside the EIGRP AS are external. The tag can be configured to a number between 0-255 to customize the tag.  
The next page will list some advantages of EIGRP

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