1 © 2003, cisco systems, inc. all rights reserved. ccna 3 v3.0 module 3 eigrp cisco networking...

1© 2003, Cisco Systems, Inc. All rights reserved.

CCNA 3 v3.0 Module 3 EIGRP

Cisco Networking Academy


Objectives

• EIGRP concepts

• EIGRP configuration

• Troubleshooting Routing protocols


EIGRP Design Features

• Rapid Convergence

• Efficient use of bandwidth

• Support for VLSM and CIDR

• Multiple network-layer support

• Independence from routed protocols


Comparing EIGRP with IGRP

• Comparisons between EIGRP and IGRP fall into the following major categories:

–Compatibility mode – IGRP and EIGRP are compatible with each other.

–Metric calculation – EIGRP scales the metric of IGRP by a factor of 256. That is because EIGRP uses a metric that is 32 bits long, and IGRP uses a 24-bit metric.

–Hop count – IGRP has a maximum hop count of 255 and EIGRP has a maximum hop count limit of 224.

–Automatic protocol redistribution – EIGRP automatically redistributes IGRP routes from the same AS

–Route tagging – EIGRP will tag routes learned from IGRP or any outside source as external because they did not originate from EIGRP routers. IGRP cannot differentiate between internal and external routes.


EIGRP and IGRP Metric Calculation


Using EIGRP with IGRP

When configuring EIGRP it is only necessary to configure the classful network number of the subnet the interface belongs to.


EIGRP Concepts

• Every EIGRP router maintains a topology table for each configured network protocol (IP, IPX).

• All learned routes to a destination are maintained in the topology table.

• The best routes from the topology table are installed into the routing table (similar to OSPF).


EIGRP Technologies

• Neighbor discovery and recovery (Hello Packets)

• Reliable Transport Protocol (Layer 4)

• DUAL finite-state machine algorithm (Routing Decisions)

• Protocol-dependent modules (PDMs)

• By forming adjacencies, EIGRP routers:

1. Dynamically learn of new routes that join their network

2. Identify routers that become either unreachable or inoperable

3. Rediscover routers that had previously been unreachable


Building Neighbor Tables


Discover Routes


EIGRP Technologies

Four key technologies set EIGRP apart from IGRP

– Neighbor discovery and recovery

– Reliable Transport Protocol (RTP)

– PDMs

– DUAL finite-state machine (FSM)


Neighbor Discovery/Recovery

• EIGRP routers establish adjacencies with neighbor routers by using small hello packets.

• An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors (and their routes) remain viable.


Reliable Transport Protocol

• RTP (Reliable Transport Protocol)

– transport-layer protocol

• EIGRP is protocol-independent; that is, it doesn’t rely on TCP/IP to exchange routing information the way RIP, IGRP, and OSPF do.

• To stay independent of IP, EIGRP uses its own, proprietary transport-layer protocol to guarantee delivery of routing information: RTP. (Don’t confuse with Real-Time Protocol)


Protocol Dependent Modules

• PDM (Protocol-dependent module)

• EIGRP is modular

• Different PDMs can be added to EIGRP as new routed protocols are enhanced or developed:

– IPv4, IPv6, IPX, and AppleTalk


PDMs

Each PDM is responsible for all functions related to its specific routed protocol.

The IP-EIGRP module is responsible for the following:

– Sending and receiving EIGRP packets that bear IP data

– Notifying DUAL of new IP routing information that is received

– Maintaining the results of DUAL’s routing decisions in the IP routing table

– Redistributing routing information that was learned by other IP-capable routing protocols


Diffusing Update Algorithm (DUAL)

• The DUAL finite state machine (FSM) is the routing decision process for EIGRP.

• DUAL tracks all routes advertised by all neighbors.

• DUAL uses the distance information to select the best routes to install into the routing table.

• This distance, or metric is the feasible distance.

• DUAL is the EIGRP distance vector algorithm part of the hybrid routing protocol.


DUAL FSM (Finite State Machine)

• DUAL selects alternate routes quickly by using the information in the EIGRP topology tables.

• If a link goes down, DUAL looks for a feasible successor in its neighbor and topology tables.


EIGRP Successors and Feasible Successors

• A successor is a neighboring router used for packet forwarding that has a least cost path to a destination, based on the feasible distance, that is guaranteed not to be part of a routing loop.

• Feasible successors are viewed by a router as neighbors that are downstream with respect to the destination.


DUAL FSM

• Feasible successors are routes that represent the next lowest-cost paths to a destination without introducing routing loops.

• Feasible successor routes can be used in case the existing route fails; packets to the destination network are immediately forwarded using the feasible successor, which at that point, is promoted to the status of successor.

• If a feasible successor is not found, the route is flagged as Active, or unusable at present.

• Once a feasible successor is found the route is placed back into the Passive state.


Calculating a Feasible Successor

• Reported distance is the total metric along a path to a destination network as advertised by an upstream neighbor.

• A feasible successor is a path whose reported distance is less than the feasible distance.


EIGRP Successors and Feasible Successors


Feasible Successor Route Selection Rules


Successors and Feasible Successors

• A passive route is one that is stable and available for use.

• An active route is a route in the process of being recomputed by DUAL.

• Recomputation happens if a route becomes unavailable and DUAL can’t find any feasible successors.


Route Tagging with EIGRP

• All external routes are included in the topology table and are tagged with the following information: – Identification number, known as router ID, of the EIGRP

router that redistributed the route into the EIGRP network

– AS number of the destination

– Protocol used in that external network

– Cost or metric received from that external protocol

– Configurable administrator tag


Data Structure

The five EIGRP packet types are as follows:

1. Hello (used to discover, verify, and rediscover neighbor routers)

2. Acknowledgment

3. Update

4. Query

5. Reply


EIGRP Packet Types


Default Hello Intervals and Hold Times for EIGRP

The default hold time is three times the hello interval


EIGRP Hello Packets

• 5-second hello:

– broadcast media, such as Ethernet, Token Ring, and FDDI

– point-to-point serial links, such as PPP or HDLC leased circuits, Frame Relay point-to-point subinterfaces, and ATM

– point-to-point subinterfaces

– high bandwidth (greater than T1) multipoint circuits, such as ISDN PRI and Frame Relay

• 60-second hello:

– multipoint circuits T1 bandwidth or slower, such as Frame Relay multipoint interfaces, ATM multipoint interfaces, ATM

– switched virtual circuits, and ISDN BRIs


EIGRP Hello packets

• If a neighbor is not heard from for the duration of the hold time, EIGRP considers that neighbor down, and DUAL must step in to reevaluate the routing table.

– By default, the hold time is three times the hello interval, but an administrator can configure both timers as desired.


EIGRP Hello packets

• Unlike OSPF routers, EIGRP routers do not need to have the same hello intervals and hold down intervals.


Acknowledgement Packets

• Acknowledgement packets, which are “dataless” hello packets, are used to ensure reliable communication.

– Unlike multicast hellos, acknowledgement packets are unicast.

– Acknowledgements can be made by piggybacking on other kinds of EIGRP packets, such as reply packets.


Update Packet

Update packets are used when a router discovers a new neighbor.

– An EIGRP router sends unicast update packets to that new neighbor so that it can add to its topology table.

– More than one update packet may be needed to convey all of the topology information to the newly discovered neighbor.


Update Packet

• Update packets are also used when a router detects a topology change. In this case, the EIGRP router sends a multicast update packet to all neighbors alerting them to the change.

• All update packets are sent reliably.


Query and Reply Packets

• EIGRP routers use query packets whenever it needs specific information from one, or all, of its neighbors.

– A reply packet is used to respond to a query.


Query and Reply Packets

• If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL places the route in the active state.

– the router multicasts a query to all neighbors, searching for a successor to the destination network.

– Neighbors must send replies that either provide information on successors, or indicate that no successor information is available.

• Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.


EIGRP Configuration


EIGRP Route Summarization

• EIGRP automatically summarizes routes at the classful boundary.

• The two routers below will auto-summarize to each other because the 2.1.1.0/24 and 2.2.2.0/24 networks are separated by a 10.1.1.0/30 network.

• This separation of subnets from one major network number by a different major network number is known as discontiguous subnets.


EIGRP Automatically Summarizes Based on Class


Turning off Auto-Summarization

• To turn off auto-summarization:

Router(config)# router eigrp 1

Router(config-router)# no auto-summary


Manual Summarization with EIGRP

With EIGRP, a summary address can be manually configured by configuring a prefix network.

Manual summarization of EIGRP routes is done at interface configuration mode.

Router(config-if)# ip summary-address eigrp 1 2.1.0.0 255.255.0.0


Using the ip bandwidth-percent command


Verifying EIGRP


EIGRP debug Commands


Troubleshooting Routing Protocols

1. Analyze the network failure, make a clear problem statement.

2. Gather the facts needed to help isolate possible causes.

3. Consider possible problems based on the facts that have been gathered.

4. Create an action plan based on the remaining potential problems.

5. Implement the action plan, performing each step carefully while testing to see whether the symptom disappears.

6. Analyze the results to determine whether the problem has been resolved. If it has, the process is complete.

7. If the problem has not been resolved, create an action plan based on the next most likely problem in the list. Return to Step 4, change one variable at a time, and repeat the process until the problem is solved.

8. Once the actual cause of the problem is identified, try to solve it.


Troubleshooting RIP Configuration

• Layer 1 or Layer 2 connectivity issues exist.

• VLSM subnetting is configured. VLSM subnetting cannot be used with RIPv1

• Mismatched RIPv1 and RIPv2 routing configurations exist.

• Network statements are missing or incorrectly assigned.

• The outgoing interface is down.

• The advertised network interface is down.


Troubleshooting IGRP Configuration


Troubleshooting EIGRP Configuration


Troubleshooting OSPF Configuration

1 © 2003, cisco systems, inc. all rights reserved. ccna 3 v3.0 module 3 eigrp cisco networking...

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