Cisco Networking AcademyChapter 6 ReviewConfiguring EIGRP

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IGRP/EIGRP

Similarities:

• share the same distance-vector routing technology

• use the same composite routing metric

• support all the same distance vectors, and their mathematical weights

• EIGRP also uses the IGRP variance feature to provide unequal-cost load balancing.

Differences:

• There is only one minor difference in the algorithm that calculates the composite metric: The IGRP metric is 20 bits long, whereas the EIGRP metric is 32 bits long

IGRP/EIGRP Redistribution

IGRP doesn't have any concept of internal and external routes

EIGRP routes that are translated and redistributed into an IGRP network are treated as native IGRP routes.

Automatic redistribution between IGRP and EIGRP will occur only if the two protocols are configured with the same autonomous system (AS) number

If they have different AS numbers, they will assume that they are part of different networks

EIGRP

A Cisco proprietary routing protocol

Features:

• Rapid Convergence

• Efficient use of bandwidth

• Support for VLSM and CIDR

• Independence from Routed Protocols

• Multiple network-layer support

EIGRP Technologies

Neighbor discovery and recovery

EIGRP routers maintain routing tables by periodic communication to:

• Dynamically learn of new routers that join their network

• Identify routers that become either unreachable or inoperable

• Rediscover routers that had previously been unreachable

Neighbor discovery/recovery process consists of periodically transmitting a small hello packet to neighbors

• establishes the relationship between immediate neighbors (known as an adjacency)

– used to exchange routing metrics supported and other information

EIGRP Technologies

Reliable Transport Protocol (RTP)

used to transport all EIGRP message types through a network

• reliable packets require explicit acknowledgement

– Update

– Query

– Reply

• unreliable packets do not require explicit acknowledgement

– Hello

– ACK

(Cont.)

EIGRP Technologies

Reliable Transport Protocol (RTP)

can support both multicasting and unicasting

• Multicast packets are delivered to multiple, specific destinations simultaneously using a group address

• Unicast packets are explicitly addressed to a single destination

• can support both multicast and unicast transmissions simultaneously for different peers.

EIGRP Technologies

Diffusing Update Algorithm (DUAL)

Finite-state machine

• tracks all routes advertised by neighbors and uses the composite metric of each route to compare them

• selects loop-free path using a successor and remember any feasible successors

– If successor lost - use feasible successor

– If no feasible successor - query neighbors and recompute new successor

EIGRP Technologies

Protocol-specific modulesEIGRP can be easily retrofitted to support any new routed protocols that

may be developed by just adding another protocol-specific module

Each protocol-specific module is responsible for all functions related to its specific routed protocol

EXAMPLE

• The IP-EIGRP module is responsible for:

– 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

EIGRP Neighbor Table

Each EIGRP router maintains a neighbor table that lists adjacent routers

there is a neighbor table for each protocol that EIGRP supports

used to support reliable, sequenced delivery of packets

• One field in each row of the table is populated with the last sequence number of the packet received from that neighbor

• EIGRP uses this field for two purposes:

– Sequence numbers are used to acknowledge specific packets that were delivered reliably

– This field can also alert EIGRP to packets being received out of sequence.

EIGRP Neighbor Table

Key elements:Neighbor address - the network-layer address of the neighbor router

Queue count - the number of packets waiting in queue to be sent

• if this value is constantly higher than zero, then there may be a congestion problem at the router

• a zero means that there are no EIGRP packets in the queue

Smooth Round Trip Timer (SRTT) - the average time it takes to send and receive packets from a neighbor

• used to determine the retransmit interval (RTO)

Hold Time - the interval to wait without receiving anything from a neighbor before considering the link unavailable

EIGRP Routing Table

EIGRP chooses the best (successor) routes to a destination from the topology table and places these routes in the routing table

contains the lowest-metric routes that DUAL calculated for all known destinations

can contain up to six routes to each destination

A separate routing table is maintained for each routed protocol that EIGRP is configured to support.

EIGRP Topology Table

The information needed to calculate a set of distances, and vectors, to all known and reachable destinations

Each EIGRP router maintains a topology table for each configured network protocol

Entries in a topology table can be either in an active or passive state

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

• An active route is one currently being recomputed.

A backup route (feasible successor) is retained

• can have multiple feasible successors for a destination

EIGRP Topology Table

Field Description

BandwidthThe bandwidth of the slowest interface in the path to a destination is used tocalculate the route composite metric

Total delay contains the sum total of delay values in that route

Reliability The reliability of the path

Load The load level of the path

MTUthe size of the smallest maximum transmission unit (MTU) supported by therouter interfaces in the path

Reported distancethe distance reported by an adjacent neighbor to a specific destination(does not include the distance between this router and the adjacent neighbor)

Feasible distance the lowest calculated metric to each destination

Route sourceThe source of the route is the identification number of the router thatoriginally advertised that route

EIGRP Route Tagging

EIGRP classifies routes as either internal or externalInternal routes are those that originated within an EIGRP network

External routes were either learned from a different routing protocol or are static routes that have been injected into EIGRP through redistribution

• External routes include the following information and are identified in the topology table

– The identification number (router ID) of the EIGRP router that redistributed that route into the EIGRP network

– The number of the Autonomous System where that route's destination resides

– The protocol used in that external network

– The cost or metric received from that external protocol

– A tag that can be administratively set and used in route filtering

EIGRP Packet Types

Hello Packetsused to discover (or rediscover) and track other EIGRP routers in the

network

hello packets are transmitted at fixed intervals known as hello intervals

• default interval on relatively low-bandwidth (T1 or less) multipoint circuits (such as multipoint Frame Relay, ATM, and X.25 circuits)

– 60 seconds

• default interval on higher-bandwidth interfaces including point-to-point serial links, multipoint circuits with bandwidth greater than T1, and LANs

– 5 seconds

EIGRP Packet Types

Acknowledgment Packets

used to acknowledge receipt of any EIGRP packet that requires reliable delivery

a hello packet without any data

sent as a unicast packet to a single, specific IP address

EIGRP Packet Types

Update Packets

used to convey routing information to known destinations

• Updates sent to a new neighbor provide a complete dump of topological data

– update packets will be directly addressed to that neighbor

• Updates would be sent whenever a change in either topology or link metric occurred

– update packets would use an IP multicast address to forward to multiple neighbors simultaneously

updates contain only the information that has changed, when it has changed

EIGRP Packet Types

Query Packets

used whenever a router needs specific information from one or all of its neighbors

only sent when a destination becomes active

can be both unicast and multicast

Reply Packets

used to respond to a query

are unicast directly back to the originator of the query

Configuring EIGRP - IP

To enable EIGRP and define the autonomous system

router(config)#router eigrp autonomous-system-number • autonomous-system-number is the number that identifies the autonomous

system

• used to indicate all routers that belong within the internetwork (value must match all routers within the internetwork)

To indicate which networks are part of the EIGRP autonomous system on this router

router(config-router)#network network-number • network-number is the network number that determines which interfaces of the

router are participating in EIGRP and which networks are advertised by the router

The network command configures only connected networks

Configuring EIGRP - IP

When using serial links, it is important to use the bandwidth on the interface to change the bandwidth used for calculating routing metrics

router(config-if)#bandwidth kilobit

• If you do not change the bandwidth for these interfaces, EIGRP assumes that the bandwidth on the link is of T1 speed

• If the link is slower, the router may not be able to converge, routing updates might become lost, or suboptimal path selection may result

Configuring EIGRP - IPX

To enable IPX routing

router(config)#ipx routing

To define EIGRP as the IPX routing protocolrouter(config-ipx-router)#ipx router {eigrp autonomous-system-number |

rip} • If IPX EIGRP is selected, an autonomous system number must be specified

(must be the same for all IPX EIGRP routers in the network)

Indicate which networks are part of the EIGRP autonomous system

router(config-router)#network network-number

Show Commands - IP

Command Description

show ip eigrp neighbors Displays neighbors discovered by EIGRP

show ip eigrp topology Displays the EIGRP topology

show ip route eigrp process-id

Displays the current EIGRP entries in therouting table

show ip protocol process-id

Displays the parameters and current state ofthe active routing protocol

show ip eigrp trafficDisplays the number of EIGRP packetssent/received.and statistics

Show Commands - IPX

Command Description

show ipx route Displays the contents of the IPX routing table

show ipx eigrp neighborsDisplays the IPX neighbors discovered byEIGRP

show ipx eigrp topology Displays the EIGRP topology table

Cisco Networking Academy

End Review

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