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Network Protocols Configuration Guide, Part 1
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IP Overview
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Configuring IP Addressing
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Configuring IP Services
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Configuring RSVP
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Configuring On-Demand Routing
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Configuring RIP
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Configuring IGRP
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Configuring OSPF
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Configuring IP Enhanced IGRP
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Configuring Integrated IS-IS
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Configuring BGP
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Configuring IP Routing Protocol-Independent Features
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Configuring IP Multicast Routing
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Table Of Contents
Cisco's IP Enhanced IGRP Implementation
Enhanced IGRP Configuration Task List
Transition from IGRP to Enhanced IGRP
Log Enhanced IGRP Neighbor Adjacency Changes
Configure the Percentage of Link Bandwidth Used
Adjust the IP Enhanced IGRP Metric Weights
Apply Offsets to Routing Metrics
Configure Summary Aggregate Addresses
Configure Enhanced IGRP Route Authentication
Configure Enhanced IGRP's Protocol-Independent Parameters
Adjust the Interval between Hello Packets and the Hold Time
Monitor and Maintain Enhanced IGRP
IP Enhanced IGRP Configuration Examples
Configuring IP Enhanced IGRP
This chapter describes how to configure IP Enhanced Interior Gateway Routing Protocol (Enhanced IGRP). For a complete description of the IP Enhanced IGRP commands listed in this chapter, refer to the "IP Enhanced IGRP Commands" chapter of the Network Protocols Command Reference, Part 1. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
Refer to the Network Protocols Configuration Guide, Part 2 for information on AppleTalk Enhanced IGRP or IPX Enhanced IGRP.
For protocol-independent features that work with IP Enhanced IGRP, see the chapter "Configuring IP Routing Protocol-Independent Features."
Enhanced IGRP is an enhanced version of the IGRP developed by Cisco Systems, Inc. Enhanced IGRP uses the same distance vector algorithm and distance information as IGRP. However, the convergence properties and the operating efficiency of Enhanced IGRP have improved significantly over IGRP.
The convergence technology is based on research conducted at SRI International and employs an algorithm referred to as the Diffusing Update Algorithm (DUAL). This algorithm guarantees loop-free operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations. The convergence time with DUAL rivals that of any other existing routing protocol.
Cisco's IP Enhanced IGRP Implementation
IP Enhanced IGRP provides the following features:
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Automatic redistribution—IP IGRP routes can be automatically redistributed into Enhanced IGRP, and IP Enhanced IGRP routes can be automatically redistributed into IGRP. If desired, you can turn off redistribution. You can also completely turn off IP Enhanced IGRP and IP IGRP on the router or on individual interfaces.
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Enhanced IGRP offers the following features:
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Enhanced IGRP has the following four basic components:
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Neighbor discovery/recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. Neighbor discovery/recovery is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, the Cisco IOS software can determine that a neighbor is alive and functioning. Once this status is determined, the neighboring routers can exchange routing information.
The reliable transport protocol is responsible for guaranteed, ordered delivery of Enhanced IGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some Enhanced IGRP packets must be transmitted reliably and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities (such as Ethernet) it is not necessary to send hellos reliably to all neighbors individually. Therefore, Enhanced IGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, and this is indicated in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.
The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors but there are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL will test for feasible successors. If there are feasible successors, it will use any it finds in order to avoid unnecessary recomputation.
The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the IP Enhanced IGRP module, which is responsible for sending and receiving Enhanced IGRP packets that are encapsulated in IP. It is also responsible for parsing Enhanced IGRP packets and informing DUAL of the new information received. IP Enhanced IGRP asks DUAL to make routing decisions, but the results are stored in the IP routing table. Also, IP Enhanced IGRP is responsible for redistributing routes learned by other IP routing protocols.
Enhanced IGRP Configuration Task List
To configure IP Enhanced IGRP, complete the tasks in the following sections. At a minimum, you must enable IP Enhanced IGRP. The remaining tasks are optional.
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See the section "IP Enhanced IGRP Configuration Examples" at the end of this chapter for configuration examples.
Enable IP Enhanced IGRP
To create an IP Enhanced IGRP routing process, perform the following tasks, beginning in global configuration mode:
IP Enhanced IGRP sends updates to the interfaces in the specified networks. If you do not specify an interface's network, it will not be advertised in any IP Enhanced IGRP update.
Transition from IGRP to Enhanced IGRP
If you have routers on your network that are configured for IGRP, and you want to make a transition to routing Enhanced IGRP, you must designate transition routers that have both IGRP and Enhanced IGRP configured. In these cases, perform the tasks as noted in the previous section, "Enable IP Enhanced IGRP," and also read the chapter, "Configuring IGRP" in this document. You must use the same autonomous system number in order for routes to be redistributed automatically.
Log Enhanced IGRP Neighbor Adjacency Changes
You can enable the logging of neighbor adjacency changes to monitor the stability of the routing system and to help you detect problems. By default, adjacency changes are not logged. To enable such logging, perform the following task in global configuration mode:
Configure the Percentage of Link Bandwidth Used
By default, Enhanced IGRP packets consume a maximum of 50 percent of the link bandwidth, as configured with the bandwidth interface configuration command. You might want to change that value if a different level of link utilization is required or if the configured bandwidth does not match the actual link bandwidth (it may have been configured to influence route metric calculations).
To configure the percentage of bandwidth that may be used by Enhanced IGRP on an interface, perform the following task in interface configuration mode:
Configure the percentage of bandwidth that may be used by Enhanced IGRP on an interface.
ip bandwidth-percent eigrp percent
Adjust the IP Enhanced IGRP Metric Weights
You can adjust the default behavior of IP Enhanced IGRP routing and metric computations. For example, this adjustment allows you to tune system behavior to allow for satellite transmission. Although IP Enhanced IGRP metric defaults have been carefully selected to provide excellent operation in most networks, you can adjust the IP Enhanced IGRP metric. Adjusting IP Enhanced IGRP metric weights can dramatically affect network performance, so be careful if you adjust them.
To adjust the IP Enhanced IGRP metric weights, perform the following task in router configuration mode:
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By default, the IP Enhanced IGRP composite metric is a 32-bit quantity that is a sum of the segment delays and the lowest segment bandwidth (scaled and inverted) for a given route. For a network of homogeneous media, this metric reduces to a hop count. For a network of mixed media (FDDI, Ethernet, and serial lines running from 9600 bps to T1 rates), the route with the lowest metric reflects the most desirable path to a destination.
Apply Offsets to Routing Metrics
An offset list is the mechanism for increasing incoming and outgoing metrics to routes learned via Enhanced IGRP. This is done to provide a local mechanism for increasing the value of routing metrics. Optionally, you can limit the offset list with either an access list or an interface. To increase the value of routing metrics, perform the following task in router configuration mode:
Apply an offset to routing metrics.
offset-list [access-list-number | name] {in | out} offset [type number]
Disable Route Summarization
You can configure IP Enhanced IGRP to perform automatic summarization of subnet routes into network-level routes. For example, you can configure subnet 131.108.1.0 to be advertised as 131.108.0.0 over interfaces that have subnets of 192.31.7.0 configured. Automatic summarization is performed when there are two or more network router configuration commands configured for the IP Enhanced IGRP process. By default, this feature is enabled.
To disable automatic summarization, perform the following task in router configuration mode:
Route summarization works in conjunction with the ip summary-address eigrp interface configuration command, in which additional summarization can be performed. If automatic summarization is in effect, there usually is no need to configure network level summaries using the ip summary-address eigrp command.
Configure Summary Aggregate Addresses
You can configure a summary aggregate address for a specified interface. If there are any more specific routes in the routing table, IP Enhanced IGRP will advertise the summary address out the interface with a metric equal to the minimum of all more specific routes.
To configure a summary aggregate address, perform the following task in interface configuration mode:
Configure a summary aggregate address.
ip summary-address eigrp autonomous-system-number address mask
See the "Route Summarization Example" at the end of this chapter for an example of summarizing aggregate addresses.
Configure Enhanced IGRP Route Authentication
IP Enhanced IGRP route authentication provides MD5 authentication of routing updates from the IP Enhanced IGRP routing protocol. The MD5 keyed digest in each Enhanced IGRP packet prevents the introduction of unauthorized or false routing messages from unapproved sources.
Before you can enable Enhanced IGRP route authentication, you must enable IP Enhanced IGRP.
To enable authentication of IP Enhanced IGRP packets, perform the following tasks beginning in interface configuration mode:
Each key has its own key identifier (specified with the key number command), which is stored locally. The combination of the key identifier and the interface associated with the message uniquely identifies the authentication algorithm and MD5 authentication key in use.
You can configure multiple keys with lifetimes. Only one authentication packet is sent, regardless of how many valid keys exist. The software examines the key numbers in order from lowest to highest, and uses the first valid key it encounters. Note that the router needs to know the time. Refer to the NTP and calendar commands in the "Performing Basic System Management" chapter of the Configuration Fundamentals Configuration Guide.
For an example of route authentication, see the section "Route Authentication Example" at the end of this chapter.
Configure Enhanced IGRP's Protocol-Independent Parameters
Enhanced IGRP works with AppleTalk, IP, and IPX. The bulk of this chapter describes IP Enhanced IGRP. However, this section describes Enhanced IGRP features that work for AppleTalk, IP, and IPX. To configure such protocol-independent parameters, perform one or more of the tasks in the following sections:
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For more protocol-independent features that work with IP Enhanced IGRP, see the chapter "Configuring IP Routing Protocol-Independent Features."
Adjust the Interval between Hello Packets and the Hold Time
You can adjust the interval between hello packets and the hold time.
Routing devices periodically send hello packets to each other to dynamically learn of other routers on their directly attached networks. This information is used to discover who their neighbors are, and to learn when their neighbors become unreachable or inoperative.
By default, hello packets are sent every 5 seconds. The exception is on low-speed, nonbroadcast, multiaccess (NBMA) media, where the default hello interval is 60 seconds. Low speed is considered to be a rate of T1 or slower, as specified with the bandwidth interface configuration command. The default hello interval remains 5 seconds for high-speed NBMA networks. Note that for the purposes of Enhanced IGRP, Frame Relay and SMDS networks may or may not be considered to be NBMA. These networks are considered NBMA if the interface has not been configured to use physical multicasting; otherwise they are not considered NBMA.
You can configure the hold time on a specified interface for a particular IP Enhanced IGRP routing process designated by the autonomous system number. The hold time is advertised in hello packets and indicates to neighbors the length of time they should consider the sender valid. The default hold time is three times the hello interval, or 15 seconds. For slow-speed NBMA networks, the default hold time is 180 seconds.
To change the interval between hello packets, perform the following task in interface configuration mode:
Configure the hello interval for an IP Enhanced IGRP routing process.
ip hello-interval eigrp autonomous-system-number seconds
On very congested and large networks, the default hold time might not be sufficient time for all routers to receive hello packets from their neighbors. In this case, you may want to increase the hold time.
To change the hold time, perform the following task in interface configuration mode:
Configure the hold time for an IP Enhanced IGRP routing process.
ip hold-time eigrp autonomous-system-number seconds
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Disable Split Horizon
Split horizon controls the sending of IP Enhanced IGRP update and query packets. When split horizon is enabled on an interface, these packets are not sent for destinations for which this interface is the next hop. This reduces the possibility of routing loops.
By default, split horizon is enabled on all interfaces.
Split horizon blocks route information from being advertised by a router out of any interface from which that information originated. This behavior usually optimizes communications among multiple routing devices, particularly when links are broken. However, with nonbroadcast networks (such as Frame Relay and SMDS), situations can arise for which this behavior is less than ideal. For these situations, you may want to disable split horizon.
To disable split horizon, perform the following task in interface configuration mode:
Monitor and Maintain Enhanced IGRP
To delete neighbors from the neighbor table, perform the following task in EXEC mode:
Delete neighbors from the neighbor table.
clear ip eigrp neighbors [ip-address | interface]
To display various routing statistics, perform the following tasks in EXEC mode:
IP Enhanced IGRP Configuration Examples
This section contains the following examples:
Route Summarization Example
The following example configures route summarization on the interface and also configures the auto-summary feature. This configuration causes IP Enhanced IGRP to summarize network 10.0.0.0 out Ethernet interface 0 only. In addition, this example disables auto summarization.
interface Ethernet 0ip summary-address eigrp 1 10.0.0.0 255.0.0.0!router eigrp 1network 172.16.0.0no auto-summaryRoute Authentication Example
The following example enables MD5 authentication on IP Enhanced IGRP packets in autonomous system 1. shows the scenario.
Figure 23 Enhanced IGRP Route Authentication Scenario
Router A
ip authentication mode eigrp 1 md5ip authentication key-chain eigrp 1 hollykey chain hollykey 1key-string 0987654321accept-lifetime infinitesend-lifetime 04:00:00 Dec 4 1996 04:48:00 Dec 4 1996exitkey 2key-string 1234567890accept-lifetime infinitesend-lifetime 04:45:00 Dec 4 1996 infiniteRouter B
ip authentication mode eigrp 1 md5ip authentication key-chain eigrp 1 mikelkey chain mikelkey 1key-string 0987654321accept-lifetime infinitesend-lifetime 04:00:00 Dec 4 1996 infiniteexitkey 2key-string 1234567890accept-lifetime infinitesend-lifetime 04:45:00 Dec 4 1996 infiniteRouter A will accept and attempt to verify the MD5 digest of any Enhanced IGRP packet with a key equal to 1. It will also accept a packet with a key equal to 2. All other MD5 packets will be dropped. Router A will send all Enhanced IGRP packets with key 2.
Router B will accept key 1 or key 2, and will send key 1. In this scenario, MD5 will authenticate.
