Today is Monday, so we will just get started with some brief history, protocol features, and the composite metric.
EIGRP stands for Enhanced Interior Gateway Routing Protocol and was released by Cisco in 1992 as a proprietary protocol. EIGRP is a member of the Distance Vector Protocol family which base the route calculation on how far away the destination is. EIGRP however is considered an Advanced Distance Vector Protocol because it uses severeal link-state features like dynamic neighbor discovery.
EIGRP is an enhancement of the IGRP protocol with the goal to provide rapid convergence and prevent routing loops in the topology. Features of EIGRP include the following:
- Convergence - EIGRP was designed to achieve rapid convergence using the DUAL (Diffusing Update Algorithm). If EIGRP has no route in the local routing table to reach the destination it is instantly looking in the topology table for an alternative route. If the topology table also offers no route to the target EIGRP queries its neighbors to discover an laternative route, these requests are propagated until an alternative route is found or until it is clear that the destination is unreachable. The DUAL also guarantees a 100% Loop Frees topology.
- Design - Unlike OSPF`s concept of multiple areas the goal of EIGRP is to provide a design which is flexible and easy to scale. EIGRP is also capable to operate effectively in LAN and WAN enviroments without requiring different configurations for different layer 2 protocols such as Ethernet or Frame Relay
- Updates - EIGRP does not send out periodic updates instead it uses partial updates which are beeing triggered by the cange of the path or metric of a route. The content of these updates is also limited to the recent changes tp keep the updated as small as possible. Additionally only routers who require the information are beeing updated.
- VLSM Support - With the support of discontinuos subnets and Variable lenght subnet masking (VLSM) EIGRP is consideres a classless routing protocol if the auto summary function is turned off.
- Load Balancing - EIGRP supports equal and unequal loadbalancing and is able to calculate how many packts should be send out different interfaces based on the different paths metric value.
- Multiple network Layer support - IP (4 & 6), IPX and Apple Talk protocols are natively supported.
Every EIGRP router is creating 3 databases, to store information about the network and to determine the best route to a destination:
- Routing Table - Home of the best routes to each destination. Packets are beeing forwarded based on the information in the routing table.
- Neighbor table - EIGRP uses hello packets to discover neighbors. If a neighbor is found EIGRP will try to form an adjacency with his new friend. Established adjacencys are stored in the neighbor table, including the neighbors address and the interface through which it can be reached.
- Topology table - Whenever a new neighbor is discovered by EIGRP information about all known routes are beeing exchanged. This makes the topology table a copy of alle neighbors routing table. This feature is used for instantly finding a backup route if a link goes down.
EIGRP calculates the metric using a complex formula. It is referred to as composite metric. EIGRP uses five different components (so called K-values) to determine the value of the metric. One of them, MTU (Maximu Transmission Unit), is not factored in directly but changes the way the value is calculated.
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| EIGRP metrc formula |
By default EIGRP uses only two parameters in this formula:
- Bandwidth
- Delay
All K-values can be seen using: 'show ip protocols' command if EIGRP has been configured. The K-values listed are as follows:
- K1 - Bandwidth
- K2 - Load
- K3 - Delay
- K4 - Reliability
- K5 - MTU
Tomorrow we will take a look how EIGRP Operates an cover an example how adjacencys are beeing formed and routes are choosen.
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