Enhanced Interior Gateway Routing Protocol

EIGRP is a Cisco-developed routing protocol that uses the same metric defined by IGRP multiplied by 256. The routing metric in EIGRP is based on bandwidth, delay, load, and reliability. The CCIE Security written exam does not test your understanding of EIGRP too greatly, so this section includes only the relevant topics for the exam.

EIGRP is a Cisco proprietary routing protocol that can be used to route a number of Layer 3 protocols, including IP, IPX, and AppleTalk. This section is concerned only with routing IP.

To ensure EIGRP is as efficient as possible, the following features were built into EIGRP:

■ Rapid convergence—EIGRP uses the Diffusing Update Algorithm (DUAL) to achieve rapid convergence. A Cisco IOS router that runs EIGRP will ensure that any redundant paths are stored and used in case of a network failure.

■ Reduced bandwidth usage—By default, EIGRP uses up to 50 percent of available bandwidth, and this option can be changed with the Cisco IOS command ip bandwidth-percent eigrp as-number percent. By default, EIGRP uses up to 50 percent of the bandwidth defined by the interface bandwidth command. The interface command ip bandwidth-percent eigrp as-number percent can be used to change this value (a good method to use for the CCIE lab).

EIGRP is considered a hybrid routing protocol, meaning that EIGRP uses characteristics of both distance vector and link-state routing protocols to maintain routing tables.

A distance vector protocol counts the number of devices data must flow through to reach a destination—this is called the hop count.

A link-state protocol such as OSPF, discussed later in this book, permits routers to exchange information with one another about the reachability of other networks.

EIGRP Terminology

You need to understand several EIGRP-related terms for the CCIE Security written exam. Table 1-10 defines some of the common terminology used in EIGRP.

58 Chapter 1: General Networking Topics Table 1-10 EIGRP Terms

Term

Meaning

Neighbor

A router in the same autonomous system running EIGRP.

Neighbor table

EIGRP maintains a table with all adjacent routers. To view the EIGRP neighbors, use the Cisco IOS command show ip eigrp neighbors.

Topology table

EIGRP maintains a topology table for all remote destinations discovered by neighboring routers. To view the topology table, the Cisco IOS command is show ip eigrp topology.

Hello

A packet used to monitor and maintain EIGRP neighbor relationships; it is multicast.

Query

A query packet that is sent to neighboring routers when a network path is lost; can be multicast or unicast.

Reply

A reply packet to a query packet; it is unicast.

ACK

Acknowledgment of an update packet, typically a Hello packet with no data; it is unicast.

Holdtime

How long a router waits for a Hello packet before tearing down a neighbor adjacency.

Smooth Round Trip Time (SRTT)

Time taken to send a packet reliably to an acknowledgment. SRTT is the average delta between the time a packet is sent and the arrival of the neighbor's acknowledgment.

Retransmission Timeout (RTO)

The time a router waits for the arrival of the neighbor's acknowledgment.

Feasible distance

Lowest metric to remote network.

Feasibility condition (FC)

A condition under which the sum of a neighbor's cost to a destination and the cost to this neighbor is less than the current successor's cost. If the EIGRP neighbor's reported distance (the metric after the slash) is less than the feasible distance, the feasibility condition is met and that path is a feasible successor.

Feasible successor

A neighboring router with a path whose reported distance is less than the feasible successor.

Successor

A neighboring router that meets the feasibility condition and also contains the best path.

Stuck in Active (SIA)

An EIGRP router waiting for all acknowledgments from neighboring routers for all the queries sent.

Active

When a router is querying neighboring routers about a network path.

Passive

Normal route operation to a remote destination. This means there are no outstanding queries to reply to. This is normal network operation.

EIGRP Configuration Example

This examples describes how to configure a two-router EIGRP network with two Frame Relay links between the two routers, to demonstrate the redundancy mechanism with the EIGRP DUAL algorithm.

Figure 1-15 displays a two-router topology using the same addressing as the RIP example in Figure 1-14.

Figure 1-15 EIGRP Configuration Example

Autonomous System 100 (AS100)

Bandwidth 256

131.108.3.0/30 Frame Relay

172.108.1.1/24

Bandwidth 256

131.108.3.0/30 Frame Relay

172.108.1.1/24

172.108.2.1/24

Bandwidth 128

131.108.10.0/30 Frame Relay

172.108.2.1/24

Bandwidth 128

R1's Loopbacks Loopback0 131.108.4.1/24 Loopback1 131.108.5.1/24 Loopback2 131.108.6.1/24

131.108.10.0/30 Frame Relay

R2's Loopbacks Loopback0 131.108.7.1/24 Loopback1 131.108.8.1/24 Loopback2 131.108.9.1/24

Routers R1 and R2 reside in AS 100, and to enable EIGRP on both routers, you need to start by configuring EIGRP. Example 1-15 displays the EIGRP configuration required on R1 and R2.

Example 1-15 Enabling EIGRP in AS 100

router eigrp 100 network 131.108.0.0

The network command in Example 1-15 enables EIGRP to send and receive updates for interfaces configured with the Class B address, 131.108.0.0. EIGRP will automatically summarize Class A, B, or C addresses.

Example 1-16 displays the IP routing table on R1.

Example 1-16 show ip route on R1 R1#show ip route

Codes: C - connected, D - EIGRP, EX - EIGRP external,

131.108.0.0/16 is variably subnetted, 10 subnets, 2 masks C 131.108.10.0/30 is directly connected, Serial0/1

Example 1-16 show ip route on R1 (Continued)

D

131.1

38.9

2 0/

[90/10639872] via 131

108.3.2, 00

04

27,

Serial0/0

D

131.1

38.8

0/24

[90/10639872] via 131

108.3.2, 00

04

27,

Serial0/0

D

131.1

08.7

0/24

[90/10639872] via 131

108.3.2, 00

04

27,

Serial0/0

C

131.1

38.6

0/24

is directly connected

Loopback2

C

131.1

08.5

0/24

is directly connected

Loopback1

C

131.1

38.4

0/24

is directly connected

Loopback0

C

131.1

08.3

0/30

is directly connected

Serial0/0

D

131.1

58.2

0/24

[90/10537472] via 131

108.3.2, 00:

04

28,

Serial0/0

C

131.1

08.1

0/24

is directly connected

Ethernet0/0

Example 1-16 displays four remote EIGRP entries (designated by D in the routing table) via the serial interface Serial0/0. EIGRP has discovered these networks as the preferred path because the WAN bandwidth is 256 kbps, as opposed to 128 kbps via Serial 0/1. To view the alternate paths, use the show ip eigrp topology Cisco IOS command to display backup paths.

Example 1-17 displays the output of the show ip eigrp topology command on R1. Example 1-17 show ip eigrp topology on R1

R1#show ip eigrp topology

IP-EIGRP

Topology Table for AS(100)/ID(131.108.6.1)

Codes

P

- Passive,

A - Active, U - Update, Q - Query, R

Reply,

r

- reply Status, s - sia Status

P 131

10Í

3.10.0/30,

1 successors, FD is 2169856

via Connected, Serial0/1

via 131.10

8.3.2 (11023872/1761792), Serial0/0

P 131

10Í

3.9.0/24, 1

successors, FD is 2297856

via 131.10

8.3.2 (10639872/128256), Serial0/0

via 131.10

8.10.2 (20640000/128256), Serial0/1

P 131

10Í

3.8.0/24, 1

successors, FD is 2297856

via 131.10

8.3.2 (10639872/128256), Serial0/0

via 131.10

8.10.2 (20640000/128256), Serial0/1

P 131

10Í

3.7.0/24, 1

successors, FD is 2297856

via 131.10

8.3.2 (10639872/128256), Serial0/0

via 131.10

8.10.2 (20640000/128256), Serial0/1

P 131

10Í

3.6.0/24, 1

successors, FD is 128256

via Connected, Loopback2

P 131

10Í

3.5.0/24, 1

successors, FD is 128256

via Connected, Loopback1

P 131

10Í

3.4.0/24, 1

successors, FD is 128256

via Connected, Loopback0

P 131

10Í

3.3.0/30, 1

successors, FD is 2169856

via Connected, Serial0/0

via 131.10

8.10.2 (21024000/1761792), Serial0/1

P 131

10Í

3.2.0/24, 1

successors, FD is 2195456

via 131.10

8.3.2 (10537472/281600), Serial0/0

Example 1-17 show ip eigrp topology on R1 (Continued)

via 131.108.10.2 (20537600/281600), Serial0/1 P 131.108.1.0/24, 1 successors, FD is 281600 via Connected, Ethernet0/0

Example 1-17 shows that the remote network 131.108.2.0 is reachable via two paths, and because the feasible distance is lower through Serial 0/0, that path is injected into the routing table. If, for some reason, the link with Serial 0/0 on R1 fails, the alternate path will be chosen and inserted into the routing table, decreasing convergence times.

When EIGRP loses a path to a remote network, it sends requests to neighboring routers for alternative ways to reach the failed network. The neighboring router that returns the most favorable routes is called the feasible successor; in Figure 1-15, that router is R2.

NOTE The Cisco CD Documentation defines the state (active, passive, and more) of a given network with the following:

■ P (Passive)—Indicates that no EIGRP computations are being performed for this destination.

■ A (Active)—Indicates that EIGRP computations are being performed for this destination.

■ U (Update)—Indicates that an update packet was sent to this destination.

■ Q (Query)—Indicates that a query packet was sent to this destination.

■ R (Reply)—Indicates that a reply packet was sent to this destination.

■ r (Reply status)—A flag that is set after the software has sent a query and is waiting for a reply.

Cisco.com was the source for this material, http://www.cisco.com/univercd/cc/td/doc/product/ software/ios122/122cgcr/fiprrp_r/1rfeigrp.htm#wp1018743.

EIGRP also supports an authentication mechanism. To enable authentication of EIGRP packets, use the ip authentication key-chain eigrp command in interface configuration mode. Chapter 8, "CCIE Security Self-Study Lab," contains an example of this command and its proper use.

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