routing protocol ripv1, ripv2 eigrp o ospf · routing protocol ripv1, ripv2, eigrp ospf 2 outline o...

Routing Protocol

RIPv1, RIPv2, EIGRP OSPF

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Outline

o RIPv1, RIPv2

o EIGRP

o OSPF

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RIPv1

RIP Characteristics

• A classful, Distance Vector (DV) routing protocol

• Metric = hop count

• Routes with a hop count > 15 are unreachable

• Updates are broadcast every 30 seconds

4

RIPv1�RIP Message Format

�RIP header - divided into 3 fields

- Command field

- Version field

- Must be zero

�Route Entry - composed of 3 fields

- Address family identifier

- IP address

- Metric

5

RIPv1

RIP Operation

RIP uses 2 message types:

�Request message

• This is sent out on startup by each RIP enabled

interface

• Requests all RIP enabled neighbors to send routing

table

�Response message

• Message sent to requesting router containing routing

table6

RIP in action

RIP request

dst. 255.255.255.255

RIP reply

dst. request router every 30 sec.

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RIPv1

o IP addresses initially

divided into classes

- Class A

- Class B

- Class C

o RIP is a classful

routing protocol

- Does not send subnet

masks in routing updates

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Automatic SummarizationBoundary Routers

� RIP automatically summarizes classful networks

� Boundary routers automatic summarize RIP subnets from one major network to another.

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RIPv2Difference between RIPv1 & RIPv2

RIPv1• A classful distance vector routing protocol

• Does not support discontiguous subnets

• Does not support VLSM

• Does not send subnet mask in routing update

• Routing updates are broadcast

RIPv2• A classless distance vector routing protocol that is an

enhancement of RIPv1’s features.

• Next hop address is included in updates

• Routing updates are multicast

• The use of authentication is an option

10

Similarities between RIPv1 & RIPv2

o Use of timers to prevent routing loops

o Use of split horizon or split horizon with poison reverse

o Use of triggered updates

o Maximum hop count of 15

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RIPv1 Limitations

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RIPv1 Limitations

o RIPv1 – a classfull routing protocol

-Subnet mask are not sent in updates

-Summarizes networks at major network boundaries

-if network is discontiguous and RIPv1 configured,

convergence will not be reached

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RIPv1 Limitations

o RIPv1 does not support VLSM

Reason: RIPv1 does not

send subnet mask in

routing updates

o RIPv1 does summarize routes

to the Classfull boundary

Or uses the Subnet mask

of the outgoing interface

to determine which

subnets to advertise

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RIPv1 Limitations

o No CIDR Support

o In the diagram R2 will

not include the static

route in its update

Reason: Classful routing

protocols do not support

CIDR routes that are

summarized with a

smaller mask than the

classful subnet mask

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Configuring RIPv2o Comparing RIPv1 & RIPv2 Message Formats

- RIPv2 Message format is similar to RIPv1 but has 2 extensions

� 1st extension is the subnet mask field

� 2nd extension is the addition of next hop address

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Configuring RIPv2

oEnabling and Verifying RIPv2

oConfiguring RIP on a Cisco router

By default it is running RIPv1

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Configuring RIPv2

o Configuring RIPv2 on a

Cisco router

-Requires using the

version 2 command

-RIPv2 ignores RIPv1

updates

o To verify RIPv2 is

configured use the

show ip protocols

command

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Configuring RIPv2

o Auto-Summary &

RIPv2

o RIPv2 will

automatically

summarize routes at

major network

boundaries and can

also summarize routes

with a subnet mask

that is smaller than the

classfull subnet mask

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oDisabling Auto-

Summary in

RIPv2

oTo disable

automatic

summarization

issue the no auto-

summary

command

Configuring RIPv2

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Configuring RIPv2

o Verifying RIPv2 Updates

o When using RIPv2 with automatic summarization

turned off

Each subnet and mask has its own specific

entry, along with the exit interface and next-hop

address to reach that subnet.

o To verify information being sent by RIPv2 use the

debug ip rip command

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VLSM & CIDR

o RIPv2 and VLSM

o Networks using a

VLSM IP addressing

scheme

Use classless routing

protocols (i.e. RIPv2)

to disseminate network

addresses and their

subnet masks

22

VLSM & CIDR

oCIDR uses Supernetting

Supernetting is a bunch of contiguous

classful networks that is addressed as a single

network.

23

VLSM & CIDR

o To verify that

supernets are

being sent and

received use the

following

commands

-Show ip route

-Debug ip rip

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Summary RIP v1 & v2

Routing

Protocol

Distance

Vector

Classless

Routing

Protocol

Uses

Hold-

Down

Timers

Use of

Split Horizon or

Split Horizon w/

Poison Reverse

Max

Hop

count

= 15

Auto

Summary

Support

CIDR

Supports

VLSM

Uses

Authen-

tication

RIPv1 Yes No Yes Yes Yes Yes No No No

RIPv2 Yes Yes Yes Yes Yes Yes Yes Yes Yes

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EIGRP

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EIGRP

Roots of EIGRP: IGRP (Cisco)�Developed in 1985 to overcome

RIPv1’s limited hop count

�Distance vector routing protocol

�Metrics used by IGRP

�Bandwidth (used by default)

�Delay (used by default)

�Reliability

�LoadDiscontinued support starting with IOS 12.2(13)T & 12.2(R1s4)S

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EIGRP

EIGRP Message Format

o EIGRP Header� Data link frame header - contains source and

destination MAC address

� IP packet header - contains source & destination IP address

� EIGRP packet header - contains AS number

� Type/Length/Field - data portion of EIGRP message

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EIGRPo EIGRP packet header contains

� Opcode field

� Autonomous System number

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EIGRP EIGRP Parameters contains

� Weights

� Hold time

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EIGRP o TLV: IP internal contains

� Metric field

� Subnet mask field

� Destination field

� in Autonomous System

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EIGRP o TLV: IP external contains

� Fields used when external

� routes are imported into

� EIGRP routing process� Redistribute form other

Autonomous System

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EIGRP

Protocol Dependent

Modules (PDM)

o EIGRP uses PDM to route

several different protocols

i.e. IP, IPX & AppleTalk

o PDMs are responsible for

the specific routing task

for each network layer

protocol

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EIGRPReliable Transport Protocol (RTP)o Purpose of RTP

� Used by EIGRP to transmit and receive EIGRP packets

o Characteristics of RTP� Involves both reliable & unreliable

deliveryof EIGRP packet� Reliable delivery requires

acknowledgment from destination� Unreliable delivery does not require

an acknowledgement from destination

� Packets can be sent � Unicast� Multicast

• Using address 224.0.0.1034

EIGRP Packet Types

o EIGRP uses five different packet types, some in pairs.

� Hello Packets

� Update Packets

� Acknowledgement Packets

� Query and Reply Packets

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EIGRP Packet Typeso Hello packets

� Used to discover & form adjacencies with neighbors

� Hello packets are multicasts and use unreliable delivery.

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EIGRP Packet Types

o Update packets

� Used to propagate routing

information

� Unlike RIP, EIGRP does not

send periodic updates. Update

packets are sent only

when necessary.

o ACK packets

� Used to acknowledge

receipt of update,

query & reply packets

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EIGRP Packet Types

o Query & Reply packets

�Used by DUAL for

searching for networks

�Query packets

- Can use

•Unicast

•Multicast

�Reply packet

- Use only

•unicast

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EIGRP – Hello Protocol o Purpose of Hello Protocol

� To discover & establish adjacencies with neighbor routers

o Characteristics of hello protocol� Time interval for sending hello packet

�Most networks it is every 5 seconds

�Multipoint non broadcast multi-access networks• Unicast every 60 seconds

- Holdtime� This is the maximum time router

should wait before declaring a neighbor down

� Default holdtime

• 3 times hello interval

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EIGRP – Bounded Updateso EIGRP only sends update when there is a change in route status

o Routing update is called “Partial update”� A partial update includes only the route information that has

changed – The whole routing table is NOT sent

o and also considered as “Bounded update”� When a route changes, only those devices that are impacted will

be notified of the change

o EIGRP’s use of partial bounded updates minimizes use of bandwidth

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EIGRP – DUAL

Diffusing Update Algorithm (DUAL)

� Purpose

• EIGRP’s primary method for preventing routing loops

� Advantage of using DUAL

• Provides for fast convergence time by keeping a list of

loop-free backup routes

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EIGRP

o Administrative Distance (AD)

� Defined as the trustworthiness of the source route

o EIGRP default administrative distances

� Summary routes = 5

� Internal routes = 90

� Imported routes = 170

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EIGRP

Authentication

o EIGRP can

� Encrypt routing information

� Authenticate routing information

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EIGRP

o Topology Example

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EIGRP

o EIGRP will

automatically

summarize routes at

classful boundaries

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Basic EIGRP Configuration o Autonomous System (AS) & Process IDs

� Autonomous System

� This is a collection of networks under the control of

a single authority (reference RFC 1930)

� AS Numbers are assigned by IANA

� Entities needing AS numbers

� ISP

� Internet Backbone prodiers

� Institutions connecting to

other institutions using

AS numbers

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Basic EIGRP Configuration

o EIGRP autonomous

system number actually

functions as

a process ID

o Process ID represents

an instance of the

routing protocol

running on a router

o Example Router(config)#router eigrp autonomous-

system

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The router eigrp command

o All routers in the EIGRP routing domain must use the same process ID number (autonomous-system number)

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Basic EIGRP ConfigurationThe Network Command

o Functions of the network command� Enables interfaces to transmit & receive EIGRP updates

� Includes network or subnet in EIGRP updates

o Example: Router(config-router)#network network-address

o When EIGRP is configured on R2, DUAL sends a notification message to the console stating that a neighbor relationship with another EIGRP router has been established.

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Basic EIGRP ConfigurationThe network Command with a Wildcard Mask

- This option is used when you want to configure EIGRP to

advertise specific subnets

- Example

� Router(config-router)#network network-address [wildcard-mask]

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Basic EIGRP ConfigurationVerifying EIGRP

o EIGRP routers must establish adjacencies with their neighbors

before any updates can be sent or received

o Command used to view neighbor table and verify that EIGRP

has established adjacencies with neighbors is

show ip eigrp neighbors

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If a neighbor is not listed after adjacencies have been

established with a router's neighbors

o Are both routers configured with the same EIGRP

process ID?

o Is the directly connected network included in the

EIGRP network statements?

o Is the passive-interface command configured to

prevent EIGRP Hello packets on the interface?

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o The show ip protocols

command is also used

to verify that EIGRP is

enabled

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Examining the Routing Table

o The show ip routecommand is also used to verify EIGRP

o EIGRP routes are denoted in a routing table by the letter “D”

o By default , EIGRP automatically summarizes routes at major network boundary

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Basic EIGRP Configurationo Introducing the Null0 Summary Route

� Null0 is not a physical interface

� In the routing table summary routes are sourced from Null0�Reason: routes are used for advertisement purposes

� EIGRP will automatically include a null0 summary route as child route when 2 conditions are met�At least one subnet is learned via EIGRP

�Automatic summarization is enabled

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o R3’s routing table

shows that the

172.16.0.0/16

network is

automatically

summarized by R1

& R3

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EIGRP Metric CalculationEIGRP Composite Metric & the K Values

o EIGRP uses the following values in its composite metric� -Bandwidth, delay, reliability, and load

o The composite metric used by EIGRP

� formula used has values K1 ��K5• K1 & K3 = 1• others K values = 0

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EIGRP Metric Calculationo Use the sh ip protocols command to verify the K values

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EIGRP Metric Calculation

EIGRP Metrics

o Use the show interfacescommand to view metrics

o EIGRP Metrics

� Bandwidth – EIGRP uses a static bandwidth to calculate metric

� Most serial interfaces use a default bandwidth value of 1.544Mbos (T1)

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EIGRP Metrics

o Delay is the defined as the measure of time it takes for a packet to traverse a route

� -it is a static value based on link type to which interface is connected

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EIGRP Metric Calculationo Reliability (not a default EIGRP metric)

� -A measure of the likelihood that a link will fail

� -Measure dynamically & expressed as a fraction of 255

� the higher the fraction the better the reliability

o Load (not a default EIGRP metric)

� A number that reflects how much traffic is using a link

� Number is determined dynamically and is expressed as a fraction of 255�The lower the fraction the less the load on the link

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Using the Bandwidth Command

o Modifying the interface bandwidth

- Use the bandwidth command

- Example

Router(config-if)#bandwidth kilobits

o Verifying bandwidth

Use the show interface command

Note : bandwidth command does not change the link’s physical bandwidth

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EIGRP Metric Calculationo The EIGRP metric can be determined by examining the

bandwidth delay

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o EIGRP uses the lowest bandwidth (BW) in its metric

calculation

Calculated BW = reference BW / lowest BW(kbps) * 256

o Delay – EIGRP uses the cumulative sum of all outgoing

interfaces

� Calculated Delay = the sum of outgoing interface delays

* 256

o EIGRP Metric = calculated BW + calculated delay

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Link Bandwidth

(kbps)

Delay (µµµµs)

R2 – R1 1,544 20,000

R2 – ISP 128 20,000

R2 – R3 1,024 20,000

R1 – ISP 10,000 1,000

R3 – ISP 100,000 100

ISP – Net A 100,000 100

(107/ 1,544) x 256 1,657,856 (107/ 64) x 256 40,000,000

(107/ 1,024) x 256 2,499,840 (107/ 10,000) x 256 256,000

(107/ 128) x 256 20,000,000 (107/ 100,000) x 256 25,600

Example : R2 to Network A

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DUAL Briefo EIGRP uses the Diffusing Update Algorithm (DUAL).

o EIGRP does not send periodic updates and route entries do not age out.

o EIGRP uses a lightweight Hello protocol to monitor connection status with its neighbors.

o Only changes in the routing information, such as a new link or a link becoming unavailable cause a routing update to occur.

o EIGRP's DUAL maintains a topology table separate from the routing table, which includes both the best path to a destination network and any backup paths that DUAL has determined to be loop-free.

o Loop-free means that the neighbor does not have a route to the destination network that passes through this router.

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DUAL Concepts

The Diffusing Update Algorithm (DUAL) is

used to prevent looping

69

DUAL Conceptso Successor

a neighboring router that is used for packet forwarding and is the least-cost route to the destination network

o Feasible distance

the lowest calculated metric to reach the destination network

FD is the metric listed in the routing table entry

70

DUAL ConceptsFeasible Successors, Feasibility Condition & Reported Distance

�Feasible Successor

-This is a loop free

backup route to

same destination as

successor route

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DUAL Concepts

o Reported distance(RD)

The metric that a router reports to a neighbor about its own cost to that network

Feasible Successors, Feasibility Condition & Reported Distance

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DUAL Concepts

o Feasibility Condition (FC)

Met when a neighbor’s

RD is less than the

local router’s FD to the

same destination

network

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DUAL Conceptso Topology Table:

Successor & Feasible Successor

o EIGRP Topology table� Viewed using the show ip eigrp topologycommand

� Contents of table include:

• all successor routes

• all feasible successor routes

�

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DUAL Concepts

o EIGRP

Topology Table

dissected

75

DUAL Concepts

Topology Table: No Feasible Successor

o A feasible successor may not be present because

the feasibility condition may not be met

� -In other words, the reported distance of the

neighbor is greater than or equal to the

current feasible distance

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77

DUAL Concepts

o Finite Sate Machine (FSM)

� An abstract machine that defines a set of

possible states something can go through,

what event causes those states and what

events result form those states

� FSMs are used to describe how a device,

computer program, or routing algorithm will

react to a set of input events

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DUAL Concepts

o DUAL FSM

� Selects a best

loop-free path to

a destination

� Selects

alternate routes

by using

information in

EIGRP tables

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DUAL Concepts

Finite State Machines (FSM)

o To examine output from EIGRP’s finite state machine us the debug eigrp fsm command

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More EIGRP Configurations

The Null0 Summary Route

o By default, EIGRP uses the Null0 interface to discard

any packets that match the parent route but do not

match any of the child routes

o EIGRP automatically includes a null0 summary route as

a child route whenever both of the following conditions

exist

� One or subnets exists that was learned via EIGRP

� Automatic summarization is enabled

81


The Null0 Summary Route

82


Disabling Automatic Summarization

o The auto-summary command permits EIGRP to automatically summarize at major network boundaries

o The no auto-summary command is used to disable automatic summarization

� This causes all EIGRP neighbors to send updates that will not be automatically summarized

this will cause changes to appear in both

-routing tables

-topology tables

83


Manual Summarization

o Manual summarization can include supernets

� Reason: EIGRP is a classless routing protocol &

include subnet mask in update

o Command used to configure manual

summarization

� Router(config-if)#ip summary-address eigrp as-number

network-address subnet-mask

84


o Configuring a summary route in EIGRP

85


EIGRP Default Routes

o “quad zero” static default route

� -Can be used with any currently supported routing protocol

� -Is usually configured on a router that is connected a network outside the EIGRP domain

o EIGRP & the “Quad zero” static default route

� Requires the use of the redistribute staticcommand to disseminate default route in EIGRP updates

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Fine-Tuning EIGRP

o EIGRP bandwidth utilization

-By default, EIGRP uses only up to 50% of interface bandwidth

for EIGRP information

-The command to change the percentage of bandwidth used by

EIGRP is

Router(config-if)#ip bandwidth-percent eigrp as-number percent

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More EIGRP Configurationso Configuring Hello Intervals and Hold Times

-Hello intervals and hold times are configurable on a per-interface basis

-The command to configure hello interval is

Router(config-if)#ip hello-interval eigrp as-number seconds

o Changing the hello interval also requires changing the hold time to a value greater than or equal to the hello interval

-The command to configure hold time value is

Router(config-if)#ip hold-time eigrp as-number seconds

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Summary

o Background & History

� EIGRP is a derivative of IGRP�EIGRP is a Cisco proprietary distance vector routing

protocol released in 1994

o EIGRP terms and characteristics

� EIGPR uses RTP to transmit & receive EIGRP packets

� EIGRP has 5 packet type:�Hello packets

�Update packets

�Acknowledgement packets

�Query packets

�Reply packets

� Supports VLSM & CIDR

89

Summary

o EIGRP terms and characteristics

� EIGRP uses a hello protocol

�Purpose of hello protocol is to discover & establish

adjacencies

� EIGRP routing updates

�Aperiodic

�Partial and bounded

�Fast convergence

90

Summary

o EIGRP commands

� The following commands are used for EIGRP configuration

RtrA(config)#router eigrp [autonomous-system #]

RtrA(config-router)#network network-number

� The following commands can be used to verify EIGRP

Show ip protocols

Show ip eigrp neighbors

Show ip route

91

Summary

o EIGRP metrics include

�Bandwidth (default)

�Delay (default)

�Reliability

�Load

92

Summary

o DUAL

� Purpose of DUAL�To prevent routing loops

� Successor�Primary route to a destination

� Feasible successor�Backup route to a destination

� Feasible distance�Lowest calculated metric to a destination

� Reported distance�The distance towards a destination as advertised by an

upstream neighbor

93

Summaryo Choosing the best route

� After router has received all updates from directly connected neighbors, it can calculate its DUAL

�1st metric is calculated for each route

�2nd route with lowest metric is designated successor & is placed in routing table

�3rd feasible successor is found

• Criteria for feasible successor: it must have lower reported distance to the destination than the installed route’s feasible distance

• Feasible routes are maintained in topology table94

Summary

o Automatic summarization

�On by default

�Summarizes routes on classful boundary

�Summarization can be disabled using

the following command

RtrA(config-if)#no auto-summary

95

Link-State Routing

o Link state routing protocols

• Also known as shortest path first algorithms

• These protocols built around Dijkstra’s SPF

96

Link-State Routing

Dikjstra’s algorithm also known as the shortest path first

(SPF) algorithm

97

Link-State Routing

o The shortest path to a destination is not necessarily

the path with the least number of hops

98

Link-State Routing Protocols

2 link state routing protocols used for routing IP

� Open Shortest Path First (OSPF)

� Intermediate System-Intermediate System (IS-IS)

99

OSPF

100

OSPFBackground of OSPF

o Began in 1987

o 1989 OSPFv1 released in RFC 1131

This version was experimental & never deployed

o 1991 OSPFv2 released in RFC 1247

o 1998 OSPFv2 updated in RFC 2328 (current version)

o 1999 OSPFv3 published in RFC 2740 (for IPv6)

101

OSPF packetOSPF Message Encapsulationo OSPF packet type

There exist 5 typeso OSPF packet header

Contains - Router ID and area ID and Type code for OSPF packet type

o IP packet header

Contains - Source IP address, Destination IP address, & Protocol field set to 89

� Data link frame header

Contains - Source MAC address and Destination MAC address

102

OSPF Packet Types

1. Hello - Hello packets are used to establish and maintain adjacency with other OSPF routers.

2. DBD - The Database Description (DBD) packet contains an abbreviated list of the sending router's link-state database and is used by receiving routers to check against the local link-state database.

3. LSR - Receiving routers can then request more information about any entry in the DBD by sending a Link-State Request (LSR).

4. LSU - Link-State Update (LSU) packets are used to reply to LSRs as well as to announce new information. LSUs contain seven different types of Link-State Advertisements (LSAs).

5. LSAck - When an LSU is received, the router sends a Link-State Acknowledgement (LSAck) to confirm receipt of the LSU.

103

OSPF

Hello Protocol : OSPF Hello Packet

� Purpose of Hello Packet

• Discover OSPF neighbors & establish adjacencies

• Before two routers can form an OSPF neighbor adjacency, they must agree on three values: Hello interval, Dead interval, and network type.

• Advertise guidelines on which routers must agree to become neighbors

• Used by multi-access networks to elect a designated router and a backup designated router

104

OSPF

105

OSPFo Hello Packets continued

Contents of a Hello Packet, router ID of transmitting routero OSPF Hello Intervals

� Usually multicast (224.0.0.5 = all OSPF routers)

� Sent every 10 seconds on multiaccess and point-to-point segments and 30 seconds for NBMA (non broadcast multiple-access) segments

o OSPF Dead Intervals

� This is the time that must transpire

before the neighbor is considered

down

� Default time is 4 times

the hello interval

106

OSPF

o Hello protocol packets contain information that is

used in electing

� Designated Router (DR)

- DR is responsible for updating all other OSPF routers

� Backup Designated Router (BDR)

- This router takes over DR’s responsibilities if DR fails

107

OSPF Link State Updateo Purpose of a Link State Update (LSU)

Used to deliver link state advertisementso Purpose of a Link State Advertisement (LSA)

Contains information about neighbors & path costs

108

OSPF Algorithm

o OSPF routers build & maintain

link-state database containing

LSA received from other

routers

� Information found in database

is utilized upon execution of

Dijkstra SPF algorithm

� SPF algorithm used to create

SPF tree

� SPF tree used to populate

routing table

109

OSPF Administrative Distance

o Default Administrative Distance for OSPF is 110

110

OSPF Authentication� Purpose is to encrypt & authenticate routing information

� This is an interface specific configuration

�Routers will only accept routing information from other routers that have been configured with the same password or authentication information

111

Basic OSPF Configuration

o Topology Example

• Discontiguous IP addressing scheme

• Since OSPF is a classless routing protocol the subnet mask is configured in

112

Basic OSPF ConfigurationThe router ospf command

o To enable OSPF on a router use the following command

R1(config)#router ospf process-id

Process id

� A locally significant number between 1 and 65535

this means it does not have to match other OSPF routers in order to establish adjacencies with those neighbors.

113

Basic OSPF Configurationo OSPF network command - Requires entering:

network address

wildcard mask - the inverse of the subnet mask

area-id - area-id refers to the OSPF area.

OSPF area is a group of routers that share link state information Example:

Router(config-router)#network network-address wildcard-ask area area-id

-The area area-id refers to the OSPF area.

-An OSPF area is a group of routers that share link-state information.

-All OSPF routers in the same area must have the same link-state information in their link-state databases.

114


115


o Router ID

�This is an IP address used to identify a router

�3 criteria for deriving the router ID�Use IP address configured with OSPF router-id command

-Takes precedence over loopback and physical interface addresses

�If router-id command not used then router chooses highest IP address of any loopback interfaces

�If no loopback interfaces are configured then the highest IP address on any active interface is used

116

Basic OSPF ConfigurationOSPF Router IDo Commands used to verify current router ID

�Show ip protocols�Show ip ospf�Show ip ospf interface

117


OSPF Router ID

o Router ID & Loopback addresses- Highest loopback address will be used as router ID if router-id command isn’t used

- Advantage of using loopback address

the loopback interface cannot fail �� OSPF stabilityo The OSPF router-id command

� Introduced in IOS 12.0

� Command syntax�Router(config)#router ospf process-id

�Router(config-router)#router-id ip-address

o Modifying the Router ID�Use the command Router#clear ip ospf process 118

Basic OSPF ConfigurationVerifying OSPFo Use the show ip ospf command to verify & trouble shoot

OSPF networksCommand will display the following:� Neighbor adjacency

-No adjacency indicated by -�Neighboring router’s Router ID is not displayed�A state of full is not displayed

-Consequence of no adjacency-�No link state information exchanged�Inaccurate SPF trees & routing tables

119


Command Description

show ip protocols Displays OSPF process ID, router ID, networks

router is advertising & administrative distance

show ip ospf Displays OSPF process ID, router ID, OSPF area

information & the last time SPF algorithm

calculated

show ip ospf interface Displays hello interval and dead interval

Verifying OSPF - Additional Commands

120


Examining the routing table

o Use the show ip route command to display the routing table

-An “O” at the beginning of a route indicates that the router source is OSPF

-Note OSPF does not automatically summarize at major network boundaries

121

OSPF Metric

o OSPF uses cost as the metric for determining the best route

-The best route will have the lowest cost

-Cost is based on bandwidth of an outgoing interface

“The lower the cost, the more likely the interface is to be

used to forward data traffic."

� Cost is calculated using the formula

108 / bandwidth

-Reference bandwidth

� defaults to 100 Mbps

� can be modified using

auto-cost reference-bandwidth command122

OSPF Metric

123

OSPF Metric

o COST of an OSPF route

Is the accumulated value from one router to the next

124

OSPF Metrico Usually the actual speed of a link is different than the default

bandwidth

�This makes it imperative that the bandwidth value reflects link’s actual speed�Reason: so routing table has best path information

o The show interface command will display interface’s bandwidth

-Most serial link default to 1.544Mbps

125


Modifying the Cost of a link

o Both sides of a serial link should be configured with the same bandwidth

�Commands used to modify bandwidth value�Bandwidth command

Example: Router(config-if)#bandwidth bandwidth-kbps

�ip ospf cost command – allows you to directly specify interface cost

Example:R1(config)#interface serial 0/0/0

R1(config-if)#ip ospf cost 1562

126


127


Modifying the Cost of the link

o Difference between bandwidth command & the ip

ospf cost command

�ip ospf cost command

�Sets cost to a specific value

�bandwidth command

�Link cost is calculated

128


129

OSPF and Multiaccess NetworksChallenges in Multi-access Networks

o OSPF defines five network types:

�Point-to-point

�Broadcast Multiaccess

�non-broadcast multi-access

(NBMA)

�Point-to-multipoint

�Virtual links

130

OSPF and Multi-access Networks

131

OSPF in Multi-access Networkso 2 challenges presented by multi-access networks

�Multiple adjacencies

�Extensive LSA flooding

132

OSPF in Multi-access Networkso Extensive flooding of LSAs

For every LSA sent out there must be an acknowledgement of receipt sent back to transmitting router.

consequence: lots of bandwidth consumed and chaotic traffic

133

OSPF in Multi-access Networkso Solution to LSA flooding issue is the use of

�Designated router (DR)

�Backup designated router (BDR)

o DR & BDR selection

�Routers are elected to send & receive LSA

o Sending & Receiving LSA

�DR others only form full adjacencies with the DR and BDR in the network.

�DR others send LSAs via multicast 224.0.0.6 to DR & BDR

�DR forward LSA via multicast address 224.0.0.5 to all other routers

134

OSPF in Multi-access Networks

DR/BDR Election Process

o DR/BDR elections DO NOT occur in point to point networks

135

OSPF in Multi-access Networkso DR/BDR elections will take place on multi-access networks

as shown below

136

OSPF in Multi-access NetworksCriteria for getting elected DR/BDR

1. DR: Router with the highest OSPF interface priority.

2. BDR: Router with the second highest OSPF interface priority.

3. If OSPF interface priorities are equal, the

highest router ID is used to break the tie.

137

OSPF in Multi-access Networks

o Timing of DR/BDR Election

� Occurs as soon as 1st router has its interface

enabled on multi-access network

• When a DR is elected it remains as the DR until one

of the following occurs

-The DR fails.

-The OSPF process on the DR fails.

-The multi-access interface on the DR fails.

138

OSPF in Multiaccess Networks

o Manipulating the election process

-If you want to influence the election of DR & BDR then

do one of the following

�Boot up the DR first, followed by the BDR, and then boot all

other routers,

OR

�Shut down the interface on all routers, followed by a no

shutdown on the DR, then the BDR, and then all other routers.

139

OSPF in Multiaccess Networks

OSPF Interface Priority

o Manipulating the DR/BDR election process continued

�Use the ip ospf priority interface command.

�Example:Router(config-if)#ip ospf priority {0 - 255}

�Priority number range 0 to 255•0 means the router cannot become the DR or BDR

•1 is the default priority value

140

More OSPF Configuration

Redistributing an OSPF Default Route

o Topology includes a link to ISP

�Router connected to ISP

�Called an autonomous system border router

�Used to propagate a default route

•Example of static default route

R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1

•Requires the use of the default-information originatecommand

•Example of default-information originate command

R1(config-router)#default-information originate

141


Fine-Tuning OSPF

o Since link speeds are getting faster it may be necessary to change reference bandwidth values

�Do this using the auto-cost reference-bandwidthcommand

�Example:

� R1(config-router)#auto-cost reference-bandwidth 10000

142


Fine-Tuning OSPF

o Modifying OSPF timers

�Reason to modify timers

�Faster detection of network failures

�Manually modifying Hello & Dead intervals

�Router(config-if)#ip ospf hello-interval seconds

�Router(config-if)#ip ospf dead-interval seconds

�Point to be made

�Hello & Dead intervals must be the same

between neighbors

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Summary

o RFC 2328 describes OSPF link state concepts and operations

o OSPF Characteristics

�A commonly deployed link state routing protocol

�Employs DRs & BDRs on multi-access networks

�DRs & BDRs are elected

�DR & BDRs are used to transmit and receive LSAs

�Uses 5 packet types:1: HELLO

2: DATABASE DESCRIPTION

3: LINK STATE REQUEST

4: LINK STATE UPDATE

5: LINK STATE ACKNOWLEDGEMENT144

Summary

o OSPF Characteristics

�Metric = cost�Lowest cost = best path

o Configuration

�Enable OSPF on a router using the following command�R1(config)#router ospf process-id

�use the network command to define which interfaces will participate in a given OSPF process�Router(config-router)#network network-address

wildcard-mask area area-id

145

Summary

oVerifying OSPF configuration

�Use the following commands

�show ip protocol

�show ip route

�show ip ospf interface

�show ip ospf neighbor

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