Chapter 9: EIGRP

Cisco Press

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Note that the stub option still requires the stub router to form neighbor relationships, even in receive-only mode. The stub router simply performs less work and reduces the query scope.

Example 9-6 also shows the EIGRP hello interval and hold time being set. These parameters can be set per interface using the interface subcommands ip hello-interval eigrp asn seconds and ip hold-time eigrp asn seconds, respectively. The default EIGRP hello interval defaults to 5 seconds on most interfaces, with NBMA interfaces whose bandwidth is T1 or slower using a hello interval of 60 seconds. The hold time defaults to 15 and 180 seconds, respectively—three times the default hello interval. However, if you change the hello interval, the hold time default does not automatically change to three times the new hello interval; instead, it remains at 15 or 180 seconds.

EIGRP Load Balancing

EIGRP allows for up to six equal-metric routes to be installed into the IP routing table at the same time. However, because of the complex EIGRP metric calculation, metrics may often be close to each other, but not exactly equal. To allow for metrics that are somewhat close in value to be considered equal, and added to the IP routing table, you can use the variance multiplier command. The multiplier defines a value that is multiplied by the lowest metric (in other words, the FD, which is the metric of the successor route). If any other routes have a better metric than that product of variance * FD, those other routes are considered equal, and added to the routing table.


Note - EIGRP allows only FS routes to be considered for addition as a result of using the variance command. Otherwise, routing loops could occur.


Once the multiple routes for the same destination are in the routing table, EIGRP allows several options for balancing traffic across the routes. Table 9-5 summarizes the commands that impact how load balancing is done with EIGRP, plus the other commands related to installing multiple EIGRP routes into the same subnet. Note that these commands are all subcommands under router eigrp.

Table 9-5: EIGRP Route Load-Balancing Commands

Router EIGRP SubcommandMeaning
varianceAny FS route whose metric is less than the variance value multiplied by the FD is added to the routing table (within the restrictions of the maximum-paths command).
maximum-paths {1..6}The maximum number of routes to the same destination allowed in the routing table. Defaults to 4.
traffic-share balancedThe router balances across the routes, giving more packets to lower-metric routes.
traffic-share minAlthough multiple routes are installed, sends traffic using only the lowest-metric route.
traffic-share balanced across-interfacesIf more routes exist than are allowed with the maximum-paths setting, the router chooses routes with different outgoing interfaces, for better balancing.
No traffic-share command configuredBalances evenly across routes, ignoring EIGRP metrics.

EIGRP Configuration Options That Are Similar to RIP

Although EIGRP and RIPv2 differ quite a bit in their underlying operation, several of their features are configured almost identically. This section details these features. You can refer to Chapter 8, "RIP Version 2," for more information on the configuration syntax for these features.

Authentication—EIGRP configures authentication almost exactly like RIP. EIGRP authentication commands use a keyword of eigrp asn instead of rip, using the ASN configured by the router eigrp command. For example, the interface subcommand ip eigrp 1 authentication key-chain carkeys enables EIGRP MD5 authentication on the interface, pointing to a key chain called "carkeys." Also, EIGRP does not support simple-text authentication, instead defaulting to MD5. Therefore, it does not use an equivalent of the ip rip authentication mode {text | md5} command.

Route filtering—Configured with the distribute-list command, EIGRP route filtering is configured identically to RIP route filtering—with one important difference in the underlying operation. With RIP, incoming filters prevent the information from getting into the IP routing table; with EIGRP, an incoming filter prevents topology information from getting into the topology table.

Offset lists—EIGRP uses the same syntax for the offset-list command as RIP, but with an interesting underlying bit of logic. EIGRP does not advertise an integer metric, but rather advertises the components of the metric, including constraining bandwidth and cumulative delay. An EIGRP offset list increments only the delay value in the EIGRP metric. For instance, an inbound offset list adding an offset of 1 results in a net increase in the metric of 256, because it increases delay by 1, and EIGRP multiplies the delay setting by 256.

Autosummarization—EIGRP, like RIP, defaults to use auto-summarization; like RIPv2, autosummarization can be disabled with the no auto-summary command under router eigrp.

Split horizon—EIGRP bounds its updates using split horizon logic, like RIP. Split horizon can be disabled per interface by using the no ip split-horizon eigrp asn interface subcommand. Note that, like RIP, most interfaces default to split horizon, with the notable exception of a physical serial interface configured for Frame Relay.

Clearing IP routing tables—The clear ip route * command clears the IP routing table. However, because EIGRP keeps all possible routes in its topology table, a clear ip route * does not cause EIGRP to send any messages or learn any new topology information; the router simply refills the IP routing table with the best routes from the existing topology table. The clear ip eigrp neighbor command clears all neighbor relationships, which clears the entire topology table on the router. The neighbors then come back up, send new updates, and repopulate the topology and routing tables. The clear command also allows for clearing all neighbors that are reachable out an interface, or based on the neighbor's IP address. (The generic syntax is clear ip eigrp neighbors [ip-address | interface-type interface-number].)

Foundation Summary

This section lists additional details and facts to round out coverage of the topics in this chapter. Unlike most of the Cisco Press Exam Certification Guides, this book does not repeat information presented in the "Foundation Topics" section of the chapter. Please take the time to read and study the details in this section of the chapter, as well as review the items in the "Foundation Topics" section noted with a Key Point icon.

Table 9-6 lists some of the most popular Cisco IOS commands related to the topics in this chapter. Also refer to Table 9-4 for a few additional commands related to load balancing.

Table 9-6: Command Reference for Chapter 9

CommandCommand Mode and Description
router eigrp as-numberGlobal config; puts user in EIGRP configuration mode for that AS
network ip-address [wildcard-mask]EIGRP config mode; defines matching parameters, compared to interface IP addresses, to pick interfaces on which to enable EIGRP
distribute-list [access-list-number | name] {in | out} [interface-type | interface-number]EIGRP config mode; defines ACL or prefix list to use for filtering EIGRP updates
ip split-horizon eigrp asnInterface subcommand; enables or disables split horizon
passive-interface [default] {interface-type interface-number}EIGRP config mode; causes EIGRP to stop sending Hellos on the specified interface, and thereby to also stop receiving and/or sending updates
ip hello-interval eigrp asn secondsInterface subcommand; sets the interval for periodic Hellos sent by this interface
ip hold-time eigrp asn secondsInterface subcommand; sets the countdown timer to be used by a router's neighbor when monitoring for incoming EIGRP messages from this interface
auto-summaryEIGRP config mode; enables automatic summarization at classful network boundaries
metric weights tos k1 k2 k3 k4 k5EIGRP config mode; defines the per-ToS K values to be used in EIGRP metric calculations
ip bandwidth-percent eigrp asn percentInterface subcommand; defines the maximum percentage of interface bandwidth to be used for EIGRP messages
timers active-time [time-limit | disabled]EIGRP config mode; sets the time limit for how long a route is in active state before becoming stuck-in-active
show ip route eigrp asnUser mode; displays all EIGRP routes in the IP routing table
show ip eigrp topology [as-number | [[ip-address] mask]] [active | all-links | pending | summary | zero-successors]User mode; lists different parts of the EIGRP topology table, depending on the options used
show ip eigrp interfaces [interface-type interface-number] [as-number]User mode; lists EIGRP protocol timers and statistics per interface
show ip eigrp traffic [as-number]User mode; displays EIGRP traffic statistics
show ip protocolsUser mode; lists EIGRP timer settings, current protocol status, automatic summarization actions, and update sources
show ip eigrp asn neighborsUser mode; lists EIGRP neighbors
clear ip eigrp neighbors [ip-address | interface-type interface-number]Enable mode; disables current neighbor relationships, removing topology table entries associated with each neighbor
clear ip route {network [mask] | *}Enable mode; clears the routing table entries, which are then refilled based on the current topology table
show ip interface [type number]User mode; lists many interface settings, including split horizon
eigrp log-neighbor-changesEIGRP subcommand; displays log messages when neighbor status changes; enabled by default

Table 9-7 summarizes the types of EIGRP packets and their purposes.

Table 9-7: EIGRP Message Summary

Key PointEIGRP PacketPurpose
 HelloIdentifies neighbors, exchanges parameters, and is sent periodically as a keepalive function
 UpdateInforms neighbors about routing information
 AckAcknowledges Update, Query, and Response packets
 QueryAsks neighboring routers to verify their route to a particular subnet
 ReplySent by neighbors to reply to a Query
 GoodbyeUsed by a router to notify its neighbors when the router is gracefully shutting down

Memory Builders

The CCIE Routing and Switching written exam, like all Cisco CCIE written exams, covers a fairly broad set of topics. This section provides some basic tools to help you exercise your memory about some of the broader topics covered in this chapter.

Fill in Key Tables from Memory

First, take the time to print Appendix F, "Key Tables for CCIE Study," which contains empty sets of some of the key summary tables from the "Foundation Topics" section of this chapter. Then, simply fill in the tables from memory, checking your answers when you review the "Foundation Topics" section tables that have a Key Point icon beside them. The PDFs can be found on the CD in the back of the book, or at http://www.ciscopress.com/title/1587201410.

Definitions

Next, take a few moments to write down the definitions for the following terms:

hello interval, full update, partial update, Route Tag field, Next Hop field, MD5, DUAL, Hold timer, K value, neighbor, adjacency, RTP, SRTT, RTO, Update, Ack, query, Reply, Hello, Goodbye, RD, FD, feasibility condition, successor route, feasible successor, input event, local computation, active, passive, going active, stuck-in-active, query scope, EIGRP stub router, limiting query scope, variance

Refer to the CD-based glossary to check your answers.

Further Reading

Jeff Doyle's Routing TCP/IP, Volume I, Second Edition, (Cisco Press) has several excellent examples of configuration, as well as several examples of the DUAL algorithm and the Active Query process.

EIGRP Network Design Solutions, by Ivan Pepelnjak, contains wonderfully complete coverage of EIGRP, at least as it existed at the latest publication date. It also has great, detailed examples of the Query process.

Copyright © 2007 Pearson Education. All rights reserved.

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