Explain And Optimize EIGRP Metrics

The enhanced interior gateway routing protocol is the advanced distance vector routing protocol which is used widely on the networks to assist automate routing configuration and decisions. It uses the diffusing update algorithm to improve the efficiency of the protocol and also to prevent the calculation error while attempting to find the best route to the remote network. The EIGRP determines the value of the path with the help of 4 different metrics such as bandwidth, delay, reliability and load. You will get knowledge about the EIGRP and optimization of EIGRP metrics.


The EIGRP is the Cisco proprietary protocol, which has a wide range of features such as partial updates, fast convergence, VLSM support, use of unicast and multicast communication, multiple network layer support. Seamless connectivity over all the data link layer topologies and protocols.


The EIGRP uses a variety of metrics also called as metric components or component metrics, to describe the selected technical properties of the route. These metric components are Load, delay, bandwidth, reliability, hop count and MTU. With that, the first 4 are combined together using the well known formula to create the single number which is called as the composite metric or else where there are no risk of any confusion, simply called as metric, cost or distance. The single composite metric is used by the EIGRP to select the best route toward the destination. Actually, the concept of combining many technical measures of the route into the single number came in the IGRP which strived to be very smarter than the routing information protocol and also covers more properties of the route in the single metric value. When the EIGRP was developed, to serve as the drop in replacement for the IGRP, it retook their system of the metric calculation.


The main component metrics as described in the coming section are called as the classic metrics. It is the standard set of the metrics supported by the entire current EIGRP implementation. The EIGRP shipped with the recent IOS version can also supports which are called as wide metrics that expand the allowable range of the existing classic metrics.



Bandwidth metric:

The bandwidth is the static metric which is assigned to every router interface by using the bandwidth interface level commands. This metric component, meaning is so obvious. It explains the speed of transmission of the interface. The command of bandwidth expresses an interface bandwidth in terms of the kilobits per second. Suppose, no bandwidth commands are configured explicitly, then IOS will assign an implicit bandwidth value to every interface based on its operational characteristic and hardware type. With the selected types of interface, along with the Ethernet interface, an implicit bandwidth value will reflect a true speed which is negotiated by an interface with the link partner. Like that other types of interface, the value has no real relation to an interface capacity. While calculating a composite metric to the destination, the EIGRP takes a minimal bandwidth along with the route into an account. It can be done by simply comparing the bandwidth as advertised by the neighboring router to a bandwidth of an interface toward the advertising neighbor as well as taking a lower value of both. With the classic metrics, the EIGRP is able to describe the bandwidth in the range of 10 Gbps to 1kbps.



Delay metric:

The delay is the static metric which is assigned to every interface using a delay interface command. The delay metric component will estimate the serialization of the delay incurred by an interface. The static value is not related to any of the true characteristic of the interface, then the delay metric component of the interface may be seen more same as an average delay incurred by an interface for the typical traffic. The delay command expresses the delay in inconvenient units, may be in terms of tens of microsecond. Then configuring the delay of the 123 on the interface defines it's delay to be 1230 microseconds. The difference between these units used in the output of show interface as well as in the delay command is subtle, but also significant. The output of show interface always displays the value ten times greater than the configured value. Suppose the delay is not at all configured explicitly, the IOS will assign the implicit delay value to all interfaces based on the type of interface hardware.


While calculating a composite metric to the destination, the EIGRP takes a total delay into consideration. It is done by simply taking a delay as advertised by the neighboring router and also summing it with an interface delay toward an advertising neighbor. With the classic metrics, the EIGRP is able to describe a delay in the range between 10 to 167,772,140 microseconds. The delay of 16,777,215 tens of the microseconds is mostly used to denote the infinite distance and it is the key to the capability of advertising the unreachable network. The route poisoning, split horizon with the poisoned reverse, withdrawing the route, all the mentioned techniques in the EIGRP use the maximum delay as the indication of the unreachable route.



Reliability metric:

The reliability is the dynamic estimated metric of the interface which evaluate the reliability or a ratio in between the successfully received count and all received frame counts. The ratio is expressed as the fraction of 255. The reliability of 255 expressing the 100 percent reliability, the reliability of 230 is expressing the 90 percent reliability and the reliability of 26 expressing the 10 percent reliability. A reliability metric is updated dynamically by the IOS. While calculating a composite metric to the destination, the EIGRP takes a minimal reliability into consideration. It can be done by just comparing the reliability as advertised by the neighboring router to a reliability of an interface toward an advertising neighbor and also takes the lower value of both. In that, there is an essential fact regarding an EIGRP reliability metric handling. When the EIGRP advertising the information about a path reliability as well as optionally factors that into a composite metric, the EIGRP does not send any updates when any changes in the interface reliability. In short, the change in the value interface's reliability will not trigger the sending EIGRP updates. A reliability metric of the route is simply the snapshot of its current reliability when it is last advertised.


To really understand it, recall the EIGRP metric is retaken from the IGRP that was the timer driven protocol. On the other hand, the EIGRP is the event driven protocol. Suppose the reliability metric change was the trigger event for an EIGRP, then it could potentially influence the oscillation of the routing table into a network, bringing a traffic off and on the unreliable link and also aggravating a swing in a reliability metric more, creating the feedback loop. The reliability metric components is retaken into the EIGRP primarily to ease for the smooth transition from an IGRP. In EIGRP, a reliability metric component is currently a relic carried across its predecessor, with no specific usability.



Load metric:

The load is the dynamically estimated metric of the interface which measures the volume of traffic flowing via the interface in relation to the maximum capability. As same as reliability, a load is expressed as the fraction of 255, with a load of 1 representing an interface which is empty and a load of 255 express the fully utilized interface. To report for the large differences in a momentary load caused by the bursty traffic, the IOS computes the exponentially weighted average by the momentary load which smooths out the short lived load swing. Since the interface may be utilized differently in the egress and ingress data flow direction, the IOS maintains 2 independent load metrics counter, the Txload for the outgoing traffic as well as the Rxload for the incoming traffic.


While calculating a composite metric to the destination, the EIGRP takes a maximal Txload into consideration. It is done by simply comparing the load like advertised by the neighboring router to a Txload of an interface towards an advertising neighbor and also taking a higher value of both. The EIGRP handling of a load metric component is same as that of the reliability. When optionally factored and advertised into a composite metric, the Txload value changes on an interface will not trigger the EIGRP updates. A load metric of the route is a snapshot of the current load while it is last advertised. A load, along with a reliability is the relic from an IGRP with no specific usability in the EIGRP.



MTU metric:

MTU stands for for the maximum transmission unit. There is a widespread confusion about the MTU metric component in the EIGRP and the use of MTU. As same that of the reliability and bandwidth, the EIGRP advertises a minimum MTU along a route to a destination. The MTU is unused completely even though carried in the EIGRP message in a best path selection process. The use of MTU in the EIGRP path selection algorithm will never implemented.



Hop count metric:

This metric component is just the counter of the routers on a path toward a destination. It is simply a fallback security measure. The EIGRP routers can also be configured to advertise the every route in its hop count over the predefined threshold as unreachable, hence breaking the potential routing loops. This limit is 100 as default. It can be configured as in the range of 255 to 1. A hop count is not factored into a composite metric calculation and it does not create an impact on the path selection.


Calculating composite metric: as EIGRP treats every metric component in a different way, the EIGRP routers will exchange these metrics as a separate value. To get at the single metric value, every router has to be independently Compute the final composite metric by its own. Those composite metric is mostly used locally on the router and it will not advertise as the single number in the EIGRP messages. The one and only exception to that rule is when the route is redistributed from one of the EIGRP process to the other. However, in this case, the redistributed route's composite metric is retaken particularly for the diagnostic purposes and also carried separately from a seed component metrics indicated in a redistribute command which activates a redistribution and also defines the starting metrics for the redistributed routes. By using the delay, load, reliability and load, each of the EIGRP router computes the composite metric value by using the given formula.



In the above formula, the constants k1 through k5 is commonly called as K value. It is the weight constants in the range of 0-255 which can be tweaked to influence an impact of the individual metric on the overall metric. It is a crucial in which all the EIGRP routers in the autonomous system will compute the metric in a same way. Hence, the K value on all the routers has to match. By default, the k1 and k3 are set to one and l the other k values are set to zero, causing the EIGRP to take only delay and bandwidth into consideration. If the k5 is 0, then all term in the right side parentheses {k5/(k4+Rmin)}, is not used in a metric computation. The BWscaled term is the one which tells how many times that the minimal bandwidth along a path is smaller than the 107-Kbps = a0- Gbps link, which is multiplied by 256. The LoMax stands for the maximum load and Rmin stands for the minimum reliability.


In this section, there are lots of hidden details about the EIGRP are included. It is important to learn these EIGRP metrics to perform the day to day task for the network professionals. This section will get you to understand the variety of metrics such as bandwidth, delay, load, reliability, hop count, and maximum transmission unit. Each metric component is used in the variety of areas where it needs and the usage depends upon the requirements only. Because, in that metrics some will take minimum and some may take maximum metrics.


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