OPEN SHORTEST PATH FIRST (OSPF)

Presented byBEHZAD ZAHID Operations Engineer-Core

To Understand the OSPF as Dynamic Routing Protocol

Basic Configuration of OSPF on Cisco Routers and Troubleshooting

OBJECTIVE

Prerequisite OSI Model Basics of Routing OSPF

◦ Understanding of OSPF◦ Neighbor Establishment Stages◦ LSP Types and its Propagation

Configuring OSPF on Cisco Routers Trouble Shooting OSPF

OVERVIEW

Understanding of OSI Model◦ Transformation from Data to Bits◦ Significance of Each Layer

Understanding of Routing and Routing table

PREREQUISITES

Routing is a way to get one packet from one destination to the next

A router creates or maintains a table of the available routes.

ROUTING

Path Determination

WHY WE NEED ROUTING

Static routing is simply the process of manually entering routes into a device's routing table.

Static routing is the simplest form of routing, but it is a manual process.

Disadvantage:◦ Not scalable

STATIC ROUTING

Routing device (the router) dynamically learn network destinations and how to get to them and also advertise those destinations to other routers.

Dynamic routing is assisted by routing protocols.

DYNAMIC ROUTING

Exterior Gateway Protocol◦ Border Gateway Protocol

For Large Internetworks Interior Gateway Protocol

◦ Distance Vector RIP

◦ Link State OSPF ISIS

◦ Hybrid EIGRP

TYPES ROUTING PROTOCOL

Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks.

It uses a link state routing algorithm and falls into the group of interior routing protocols(IGP), operating within a single autonomous system (AS).

OSPF

Area◦ A contiguous grouping of routers and router

interfaces.◦ Normal Areas:

Summary LSAs from other areas are injected External LSAs are injected

◦ Stub Areas: Summary LSAs from other areas are injected The default route is injected as a summary route External LSAs are not injected.

OSPF AREA DESIGN AND TERMS

◦ Totally Stubby Areas: No summary LSAs are allowed No external LSAs are allowed The default route is injected as a summary route.

◦ Not-So-Stubby Areas: Type 7 LSAs carry external information within an

NSSA Type 7 LSAs are converted into Type 5 LSAs at the

NSSA ABR No external LSA are allowed, Summary LSAs are

injected

OSPF AREA DESIGN AND TERMS

OSPF AREA DESIGN AND TERMS

Types of Routers in OSPF◦ Internal routers: Routers that have all their

interfaces in the same area and have identical LSDBs.

◦ Backbone routers: Routers having at least one interface connected to area 0.

◦ ABRs: Routers that have interfaces attached to multiple areas.

◦ ASBRs: Routers that have at least one interface attached to an external internetwork (another AS), such as a non‐OSPF network.

OSPF AREA DESIGN AND TERMS

OSPF AREA DESIGN AND TERMS

◦ Designated Router On Multiaccess data links , an ospf

router elected by the routers on that data link to perform special functions (e.g generation of LSAs representing the subnet)

◦ Backup Designated Router A router on Multiaccess data link that

becomes prepared to take over for the DR, should the DR fail.

OSPF AREA DESIGN AND TERMS

OSPF Packet Types:◦ Hello Message

To form neighbor-ship◦ Data Base Descriptor

Has LSA headers (LSID and Seq No)◦ Link State Request

I need complete info of this Network◦ Link State Advertisement

Info of asked Network◦ Link State Update

Set of LSAs◦ Link State Acknowledgement

OSPF AREA DESIGN AND TERMS

OSPF Cost◦ Cost is used to find best router◦ OSPF Metric :

Cost = 100/ BW in MBPS

T1 65E1 48Ethernet 10Fast Ethernet 1Gigabit Ethernet 1

OSPF AREA DESIGN AND TERMS

Hello Interval◦ Interval at which a router sends OSPF Hello

messages on an interface. (10ms for Broadcast) Dear Interval

◦ Timer use to determine neighboring router has failed. (40ms for Broadcast)

Update Destination Address◦ Normally sent to 224.0.0.5◦ All designated Routers 224.0.0.6

OSPF FEATURE SUMMARY

Neighbor Discovery and Forming Neighbor Relationship◦ Sending Hellos Message

Exchanging Topology Database◦ Exchanging DBDs, LSRs, LSUs

Route Computation◦ SPF

OSPF PROCESS

Router ID (must be unique 1.1.1.1 ) Hello and Dead Timers * (10ms and 40ms) Network Mask * Area ID * (e.g Area 1) List of neighbors Router Priority DR/BDR IP address Authentication Password *

OSPF HELLO MESSAGE

Determine Your Router ID◦ Hard coded◦ Highest loopback IP◦ Highest Interface IP

Add interfaces to the Link State Database (dictated by Network Command)

NEIGHBOR FORMING

Exchange hello Messages

NEIGHBOR FORMING

Topology Exchange◦ Master/Slave Relation

NEIGHBOR FORMING

NEIGHBOR STATES

OSPF LSA Types:◦ LSA Type 1 : Router LSA◦ LSA Type 2 : Network LSA (DR Generated)◦ LSA Type 3 : ABR Summary (ABR Generated)◦ LSA Type 4 : ASBR Location (ABR Generated)◦ LSA Type 5 : External LSA (ASBR Generated)◦ LSA Type 7 : NSSA External LSA

LSA TYPES AND FLOODING

Router LSA◦ Each router creates its own type 1 LSA to

represent itself for each area to which it connects.◦ To flood the LSA, the originating router sends it to

its neighbor inside the same area, who in turn to their neighbors.

◦ It lists : Router interface subnet mask and interface ospf cost

(stub network- no DR Elected Interface) IP address of DR (Transit Network- DR elected

interface) Neighbors Router ID (no DR elected interface)

TYPE 1 LSA

◦ OSPF identifies Type 1 LSA using a 32bit LSID.◦ Each router uses its own OSPF router ID value as

the LSID for Type 1 LSA.

TYPE 1 LSA

R5’s type 1 LSA will list these three interfaces as links, plus it will refer to the two working neighbors.

TYPE 1 AND TYPE 3 LSA

LSA FLOODING

LSA FLOODING

SPF requires that the LSDB model the topology with nodes (routers) and connections between nodes (Links).

OSPF can model all the topology inside a single area using TYPE 1 and TYPE 2 LSAs.

MAKING TOPOLOGY

Analyze the LSDB to find all possible routes to reach the subnet

For each possible route, add the ospf interface cost for all outgoing interfaces in that route

Pick the route with the lowest cost

CHOOSING THE BEST OSPF ROUTE

INTRA-AREA COST

INTER-AREA COST

router ospf process-id Configure an OSPF routing process and enter

router configuration mode.

router-id [X.X.X.X] To use a fixed router ID, use the above command in

router configuration mode.

network [ip-address ][wildcard-mask] area [area-id]

To define the interfaces on which OSPF runs and to define the area ID for those interfaces, use the network area command in router configuration mode.

CONFIGURING OSPF

show ip ospf [process-id] To display general information about OSPF

routing processes

show ip ospf neighbor To display OSPF-neighbor information.

TROUBLESHOOTING OSPF

show ip ospf interface [interface-type interface-number]

To display OSPF-related interface information.

show ip ospf [process-id [area-id]] database To display lists of information related to

the OSPF database for a specific router

TROUBLESHOOTING OSPF

Routing OSPF as Dynamic Routing Protocol

◦ Neighbor Forming◦ LSAs Flooding◦ Finding Best Routes

Configuring and Troubleshooting OSPF

SUMMARY

CCNP Route 642-902 Certification Guide CCNA Exploration Companion Guide

◦ Network Fundamentals

REFERENCES