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
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
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
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