CCNA 3 v3.0 Module 2 Single-Area OSPF

flutteringevergreenNetworking and Communications

Oct 29, 2013 (3 years and 9 months ago)

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CCNA 3 v3.0 Module 2

Single
-
Area OSPF

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Objectives


Link
-
state routing protocol


Single
-
area OSPF concepts


Single
-
area OSPF configuration

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Overview of Link
-
State and Distance Vector
Routing

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Link
-
State Routing Features

Link
-
state routing protocols collect route information
from all other routers in the network or within a
defined area of the network and calculates the best
paths to all destinations in the network.



Respond quickly to network changes



Send triggered updates only when a network
change has occurred



Send periodic updates known as link
-
state
refreshes



Use a hello mechanism to determine the
reachability of neighbors

Features:



Use the hello information and LSAs received from
other routers to build a database about the network



Use the SPF algorithm to calculate the shortest
route to each network



Store the route information in the routing table

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How Routing Information Is Maintained

A
link

is the same as an interface on a router. The
state of the link

is a description of an interface
and the relationship to the neighbor routers. For example, a description of the interface would
include the IP address of the interface, the subnet mask, the type of network that it is connected to,
the routers connected to that network, and so on. The collection of link
-
states form a link
-
state
database which is sometimes called a
topological database
. The link
-
state database is used to
calculate the best paths through the network. Link
-
state routers apply the Dijkstra shortest path
first algorithm against the link
-
state database. This builds the
SPF tree

with the local router as the
root. The best paths are then selected from the SPF tree and placed in the routing table.

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Link
-
State Routing Protocol Algorithms

LSA exchange is triggered by an event
in the network instead of periodic
updates. This speeds up the
convergence process.

A

B

C

D

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Advantages and Disadvantages of Link
-
State Routing

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Comparing Distance Vector and Link
-
State
Routing

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OSPF Overview

OSPF is becoming the preferred IGP
protocol when compared with RIPv1 and
RIPv2 because it is scalable.

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OSPF Terminology

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More OSPF Terminology

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Comparing OSPF Link State with Distance
Vector Routing Protocols

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Shortest Path Algorithm

The best path is the lowest
-
cost path
.

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OSPF Network Types

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OSPF Hello Protocol

Hello packets are addressed to multicast address 224.0.0.5 to all OSPF routers,
sent every 10 seconds by default.

= 1

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Steps in the Operation of OSPF

Discover neighbors

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Steps in the Operation of OSPF

Elect DR and BDR on Multi Access Network

On multi
-
access networks, the Hello protocol elects a designated router (DR) and a
backup designated router (BDR). The Hello carries information about which all neighbors
must agree to form an adjacency and exchange link
-
state information. On multi
-
access
networks the DR and BDR maintain adjacencies with all other OSPF routers on the
network.

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Steps in the Operation of OSPF

Selecting the Best Route

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Basic OSPF Configuration

Process
-
id: a number used to
identify an OSPF process on the
router

Network addresses are configured with a
wildcard mask. Wildcard mask represents
the links or host addresses that can be
present in this segment

Area 0 is called backbone
area

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Basic OSPF Configuration

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Configuring OSPF Loopback Address and
Router Priority

When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF
router ID. If there is no active interface, the OSPF process will not start. If the active interface goes
down, the OSPF process has no router ID and therefore ceases to function until the interface
comes up again.

To ensure OSPF stability there should be an active interface for the OSPF process at all times. A
loopback interface, which is a logical interface, can be configured for this purpose. When a
loopback interface is configured, OSPF uses this address as the router ID, regardless of the value.
On a router that has more than one loopback interface, OSPF takes the highest loopback IP
address as its router ID.

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Setting OSPF Priority

The priorities can be set to any value from 0 to 255. A value of 0
prevents that router from being elected. A router with the highest OSPF
priority will win the election for DR
. The default OSPF priority is 1.

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Modifying OSPF Cost Metric

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Configuring OSPF Authentication


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Configuring OSPF Timers

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OSPF
-

Propagating a Default Route

Router(config)#ip route 0.0.0.0 0.0.0.0 [
interface
|
next
-
hop address
]

Router(config
-
router)#default
-
information originate

This command will propagate the route to all routers in a normal OSPF area.

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Common OSPF Configuration Issues

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Verifying OSPF Configuration


show ip protocol


show ip route


show ip ospf interface


shop ip ospf


show ip ospf neighbor detail


show ip ospf database

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The
debug

and
clear

Commands for OSPF
Verification