Configuring IP Enhanced 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.

• Fast convergence---The DUAL algorithm allows routing information to converge as quickly as any currently available routing protocol.

• Partial updates---Enhanced IGRP sends incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table. This feature minimizes the bandwidth required for Enhanced IGRP packets.

• Less CPU usage than IGRP---This occurs because full update packets need not be processed each time they are received.

• Neighbor discovery mechanism---This is a simple hello mechanism used to learn about neighboring routers. It is protocol-independent.

• Variable-length subnet masks (VLSMs).

• Arbitrary route summarization.

• Scaling---Enhanced IGRP scales to large networks.

Enhanced IGRP has the following four basic components:

• Neighbor discovery/recovery

• Reliable transport protocol

• DUAL finite state machine

• Protocol-dependent modules

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 sent 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, which 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.

Command	Purpose
Router(config-router)#offset-list [access-list-number | name] {in | out} offset [type number]	Apply an offset to routing metrics.

To disable automatic summarization, use the following command in router configuration mode:

Command	Purpose
Router(config-router)#no auto-summary	Disable automatic summarization.

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.

Before you can enable Enhanced IGRP route authentication, you must enable IP Enhanced IGRP.

	Command	Purpose
Step1	Router(config-if)#ip authentication mode eigrp autonomous-system md5	Enable MD5 authentication in IP Enhanced IGRP packets.
Step2	Router(config-if)#ip authentication key-chain eigrp autonomous-system key-chain	Enable authentication of IP Enhanced IGRP packets.
Step3	Router(config-if)#exit Router(config)#	Exit to global configuration mode.
Step4	Router(config)#key chain name-of-chain	Identify a key chain. (Match the name configured in Step 1.)
Step5	Router(config-keychain)#key number	In key chain configuration mode, identify the key number.
Step6	Router(config-keychain)#key-st ring text	In key chain key configuration mode, identify the key string.
Step7	Router(config-keychain)#accept -lifetime start-time {infinite | end-time | duration seconds}	Optionally specify the time period during which the key can be received.
Step8	Router(config-keychain)#send-l ifetime start-time {infinite | end-time | duration seconds}	Optionally specify the time period during which the key can be sent.

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 Network Time Protocol (NTP) and calendar commands in the "Performing Basic System Management" chapter of the CiscoIOS Configuration Fundamentals Configuration Guide.

To change the interval between hello packets, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#ip hello-interval eigrp autonomous-system-number seconds	Configure the hello interval for an IP Enhanced IGRP routing process.

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, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#ip hold-time eigrp autonomous-system-number seconds	Configure the hold time for an IP Enhanced IGRP routing process.

NoteDo not adjust the hold time without advising your technical support personnel.

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, including networks in which you have IP Enhanced IGRP configured, you may want to disable split horizon.

To disable split horizon, use the following command in interface configuration mode:

Command	Purpose
Router(config-if)#no ip split-horizon eigrp autonomous-system-number	Disable split horizon.

Router A

ip authentication mode eigrp 1 md5
ip authentication key-chain eigrp 1 holly
key chain holly
key 1
key-string 0987654321
accept-lifetime infinite
send-lifetime 04:00:00 Dec 4 1996 04:48:00 Dec 4 1996
exit
key 2
key-string 1234567890
accept-lifetime infinite
send-lifetime 04:45:00 Dec 4 1996 infinite

Router B

ip authentication mode eigrp 1 md5
ip authentication key-chain eigrp 1 mikel
key chain mikel
key 1
key-string 0987654321
accept-lifetime infinite
send-lifetime 04:00:00 Dec 4 1996 infinite
exit
key 2
key-string 1234567890
accept-lifetime infinite
send-lifetime 04:45:00 Dec 4 1996 infinite
 

Router 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.