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Chapter 11: Hybrid: Enhanced Interior Gateway Routing Protocol (EIGRP)As internetworks grew in scale and diversity in the early 1990s, new routing protocols were needed. Cisco developed Enhanced Interior Gateway Routing Protocol (IGRP) primarily to address many of the limitations of IGRP and RIP. As WANs were growing, so was the need for a routing protocol that would use efficient address space on WAN links, as well as the LAN networks. OSPF was available, but the CPU-intensive tasks that it had to perform often overloaded the small processors of many edge or remote routers of that time. The configuration was also more complex than that of RIP or IGRP. A routing protocol was needed that could support VLSM and that could scale with large internetworks, yet that was less CPU-intensive than OSPF. In 1994, Cisco answered the call by releasing Enhanced IGRP in Cisco IOS Software Release 9.21. Today, EIGRP is used as the routing protocol on many large government and commercial internetworks. It has proven to be very stable, flexible, and fast. In addition to these characteristics, the ease of EIGRP configuration makes it one of the most popular routing protocols among network engineers.
EIGRP can be referred to as a hybrid protocol. It combines most of the characteristics of traditional distance vector protocols with some characteristics of link-state protocols. Specifically, EIGRP is "enhanced" by using four routing technologies:
- Neighbor discovery/recovery
- Reliable Transport Protocol (RTP)
- DUAL finite-state machine
- Protocol-dependent modules
Technical Overview of EIGRPEIGRP offers many advantages over other routing protocols, including the following:
- Support for VLSM-EIGRP is a classless routing protocol and carries the subnet mask of the route in its update.
- Rapid convergence-By using the concept of feasible successors, defined by DUAL, EIGRP is capable of preselecting the next best path to a destination. This allows for very fast convergence upon a link failure.
- Low CPU utilization-Under normal operation, only hellos and partial updates are sent across a link. Routing updates are not flooded and are processed only periodically.
- Incremental updates-EIGRP does not send a full routing update; it sends only information about the changed route.
- Scalable-Through the use of VLSM and a complex composite metric, EIGRP networks can be vast in size.
- Easy configuration-EIGRP supports hierarchical network design, but it does not require the strict configuration guidelines, such as the ones needed for OSPF.
- Automatic route summarization-EIGRP will perform automatic summarization on major bit boundaries.
- MD5 route authentication-As of Cisco IOS Software Release 11.3, EIGRP can be configured to perform MD5 password authentication on route updates.
EIGRP is a classless routing protocol. It directly interfaces to IP as protocol 88. EIGRP uses the multicast address of 22.214.171.124 for hellos and routing updates instead of an all-hosts broadcast like RIP uses. EIGRP also employs a system of hello and hold timers to maintain neighbors. Aside from the initial routing update, partial routing updates are sent only when network topology changes occur. The updates are also bounded, which means that updates are sent only to pertinent routers. Like IGRP, EIGRP uses a composite metric to calculate the best path to a destination. The sections that follow take a closer look at how EIGRP makes use of metrics, neighbors, reliable transport, and DUAL in its operation.
Early releases of EIGRP had stability issues over low-speed serial links and problems maintaining many neighbors. Cisco significantly enhanced EIGRP with Cisco IOS Software Releases 10.3(11), 11.0(8), and 11.1(3)- early releases of EIGRP are sometimes referred to as EIGRP version 1. Cisco currently ships routers with IOS 12.0 and above.
EIGRP MetricsEIGRP uses metrics in the same way as IGRP. Each route in the route table has an associated metric. EIGRP uses a composite metric much like IGRP, except that it is modified by a multiplier of 256. Recall from Chapter 10, "Distance Vector Protocols: Interior Gateway Routing Protocol (EIGRP)," that bandwidth, delay, load, reliability, and MTU are the submetrics. Like IGRP, EIGRP chooses a route based primarily on bandwidth and delay, or the composite metric with the lowest numerical value. When EIGRP calculates this metric for a route, it calls it the feasible distance to the route. EIGRP calculates a feasible distance to all routes in the network. The following list is a detailed description of the five EIGRP submetrics:
- Bandwidth-Bandwidth is expressed in units of kilobits. It must be statically config-ured to accurately represent the interfaces that EIGRP is running on. For example, the default bandwidth of a 56-kbps interface and a T1 interface is 1544 kbps. To accurately adjust the bandwidth, use the bandwidth kbps interface subcommand. Table 11-1 highlights some common bandwidth values.
- Delay-Delay is expressed in microseconds. It, too, must be statically configured to accurately represent the interface that EIGRP is running on. The delay on an interface can be adjusted with the delay time_in_microseconds interface subcommand. Common delay values are represented in Table 11-1.
- Reliability-Reliability is a dynamic number in the range of 1 to 255, where 255 is a 100 percent reliable link and 1 is an unreliable link.
- Load-Load is the number in the range of 1 to 255 that shows the output load of an interface. This value is dynamic and can be viewed using the show interfaces command. A value of 1 indicates a minimally loaded link, whereas 255 indicates a 100 percent loaded link.
- MTU-The maximum transmission unit (MTU) is the recorded smallest MTU value in the path, usually 1500.
NOTE Whenever you are influencing routing decisions in IGRP or EIGRP, use the metric of delay over bandwidth. Changing bandwidth can affect other routing protocols, such as OSPF. Changing delay affects only IGRP and EIGRP.
EIGRP uses a composite metric (CM) that is derived from the five submetrics. When EIGRP computes the composite metric, it uses a formula that involves five constants or "k" values. The constant values have default value such as the following:
k1 = k3 = 1 and k2 = k4 = k5 = 0
By setting k2, k4, and k5 to 0, it essentially nullifies the submetrics of load, reliability, and MTU. This is precisely why you should first use delay and then bandwidth when trying to influence which routes EIGRP prefers. The formula EIGRP uses to calculate the composite metric is as follows:
CM = 256 x ([k1 x BWmim + (k2 x BWmim ) / (256-LOAD) + k3 x DELAYsum ] x X)
where the following is true:
With the k values set at the default value you haveBWmim = 107 / bandwidth_of_slowest_link DELAYsum = Ÿ° (delays_along_the_path) X = k5 / (reliability + k4) if and only if k1<>1, if k1 = 1 then X = 1
k1 = k3 = 1 k2 = k4 = k5 = 0 CM = 256 x (BWmim + DELAYsum )
The router calculation of the composite metric will always differ slightly from the result when it is performed by longhand. This is because of the way the router handles floating-point mathematics; there will be slight rounding discrepancies.
Using the default values of constants, k1 = k3 = 1 and k2 = k4 = k5 =0, the formula quickly breaks down to this:
(256 x [BWmim and DELAYsum])
Substituting the constants, you have the following:
CM = 256 x ([1 x BWmim + (0 x BWmim) / (256-LOAD) + 1 x DELAYsum] x 1) CM = 256 x ([BWmim + (0) / (256-LOAD) + DELAYsum] x 1) CM = 256 x (BWmim + DELAYsum)
For reference, the metric is computed the same way for IGRP, except the result of bandwidth and delay is not multiplied by 256, and the DELAY sum variable is divided by 10.
CM = (k1 x BWmin + [k2 x BWmin] / [256-LOAD] + [k3 x DELAYsum] x X)
where the following is true:
BWmin = 107/ bandwidth_of_slowest_link DELAYsum = S(delays_along_the_path) /10 X = k5 / (reliability + k4) if and only if k1<>1, if k1=1 then X=1 k1=k3=1 k2=k4=k5=0
With k values set at the default value, you have:
CM = BWmin + DELAYsum
To demonstrate composite metric calculation, refer to Figure 11-1. In this example, EIGRP calculates a composite metric on the alpha router to 172.16.1.0/24, which resides on the charlie router.
Assuming that the bandwidth statements been set by an astute engineer, the lowest band-width on the path between alpha and charlie routers would be 56. Therefore, you have
BWmim = 10 7 / 56 = 178571
The delay is the summation of the delays on the outbound interfaces only. The summation ends with the delay on the interface in which the final subnet resides. From alpha to bravo, the delay is 20000; from bravo to charlie, it is 1000; this includes the final interface on charlie, which has a delay of 1000. Therefore, you have
DELAYsum = 20000 + 1000 + 1000 = 22000
The composite metric now yields the following:
CM = 256 x (178571) + 256 x (22000) = 46277485
The submetrics and the composite metric can be confirmed by performing the show ip route 172.16.1.0 command on the alpha router, as in Example 11-1. Remember, because of rounding errors, the metric does not match exactly....