EIGRP technologies

EIGRP technologies
3.1.4

This page will discuss some of the new technologies that EIGRP includes. Each new technology represents an improvement in EIGRP operation efficiency, speed of convergence, or functionality relative to IGRP and other routing protocols. These technologies fall into one of the following four categories:

• Neighbor discovery and recovery
• Reliable Transport Protocol
• DUAL finite-state machine algorithm
• Protocol-dependent modules

Simple distance vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors, much the same way that OSPF routers do.
EIGRP routers establish adjacencies as described in Figure . EIGRP routers use small hello packets to accomplish this. Hellos are sent by default every five seconds. An EIGRP router assumes that as long as it receives hello packets from known neighbors, those neighbors and their routes remain viable or passive. The following are possible when EIGRP routers form adjacencies:

• Dynamically learn of new routes that join the network
• Identify routers that become either unreachable or inoperable
• Rediscover routers that had previously been unreachable

Reliable Transport Protocol (RTP) is a transport layer protocol that guarantees ordered delivery of EIGRP packets to all neighbors. On an IP network, hosts use TCP to sequence packets and ensure their timely delivery. However, EIGRP is protocol-independent. This means it does not rely on TCP/IP to exchange routing information the way that RIP, IGRP, and OSPF do. To stay independent of IP, EIGRP uses RTP as its own proprietary transport layer protocol to guarantee delivery of routing information.
EIGRP can call on RTP to provide reliable or unreliable service as the situation warrants. For example, hello packets do not require the overhead of reliable delivery because they are frequent and should be kept small. The reliable delivery of other routing information can actually speed convergence because then EIGRP routers do not wait for a timer to expire before they retransmit.
With RTP, EIGRP can multicast and unicast to different peers simultaneously. This allows for maximum efficiency.
The centerpiece of EIGRP is the DUAL, which is the EIGRP route-calculation engine. The full name of this technology is DUAL finite-state machine (FSM). An FSM is an algorithm machine, not a mechanical device with parts that move. FSMs define a set of possible states that something can go through, the events that cause those states, and the events that result from those states. Designers use FSMs to describe how a device, computer program, or routing algorithm will react to a set of input events. The DUAL FSM contains all the logic used to calculate and compare routes in an EIGRP network.
DUAL tracks all the routes advertised by neighbors. Composite metrics of each route are used to compare them.   DUAL also guarantees that each path is loop free. DUAL inserts lowest cost paths into the routing table. These primary routes are known as successor routes. A copy of the successor routes is also placed in the topology table.
EIGRP keeps important route and topology information readily available in a neighbor table and a topology table. These tables supply DUAL with comprehensive route information in case of network disruption. DUAL uses the information in these tables to select alternate routes quickly. If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table.
One of the best features of EIGRP is its modular design. Modular, or layered designs, prove to be the most scalable and adaptable. Support for routed protocols, such as IP, IPX, and AppleTalk, is included in EIGRP through PDMs. In theory, EIGRP can add PDMs to easily adapt to new or revised routed protocols such as IPv6.
Each PDM is responsible for all functions related to its specific routed protocol. The IP-EIGRP module is responsible for the following functions:

• Send and receive EIGRP packets that bear IP data
• Notify DUAL of new IP routing information that is received
• Maintain the results of DUAL routing decisions in the IP routing table
• Redistribute routing information that was learned by other IP-capable routing protocols

The next page will discuss the EIGRP packet types

EIGRP design features

EIGRP design features
3.1.4

This page will describe some key design features of EIGRP.
EIGRP operates quite differently from IGRP. EIGRP is an advance distance vector routing protocol, but also acts as a link-state protocol in the way that it updates neighbors and maintains routing information. The following are advantages of EIGRP over simple distance vector protocols:

• Rapid convergence
• Efficient use of bandwidth
• Support for VLSM and CIDR.
• Multiple network layer support
• Independence from routed protocols.

Independence from routed protocols means that protocol-dependent modules (PDMs) protect EIGRP from lengthy revision. As routed protocols evolve, they may need new protocol modules, but changes to EIGRP will not be necessary.
EIGRP routers converge quickly because they rely on DUAL. DUAL guarantees loop-free operation throughout a route computation which allows all routers involved in a topology change to synchronize at the same time.
EIGRP sends partial, bounded updates and makes efficient use of bandwidth. EIGRP uses minimal bandwidth when the network is stable. EIGRP routers do not send the complete tables, but instead, send partial, incremental updates. This is similar to OSPF operation, except that EIGRP routers send these partial updates only to the routers that need the information, not to all routers in an area. For this reason, they are called bounded updates. Instead of timed routing updates, EIGRP routers use small hello packets to keep in touch with each other. Though exchanged regularly, hello packets do not use up a significant amount of bandwidth.
EIGRP supports IP, IPX, and AppleTalk through PDMs. EIGRP can redistribute IPX-RIP and IPX SAP information to improve overall performance. In effect, EIGRP can take over for these two protocols. EIGRP routers receive routing and service updates, and update other routers only when changes in the SAP or routing tables occur. In EIGRP networks, routing updates occur in partial updates.
EIGRP can also take over for the AppleTalk RTMP. As a distance vector routing protocol, RTMP relies on periodic and complete exchanges of routing information. To reduce overhead, EIGRP uses event-driven updates to redistributes AppleTalk routing information. EIGRP also uses a configurable composite metric to determine the best route to an AppleTalk network. RTMP uses hop count, which can result in suboptimal routing. AppleTalk clients expect RTMP information from local routers, so EIGRP for AppleTalk should be run only on a clientless network, such as a WAN link.
The next page will discuss some EIGRP technologies.

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