Objectives

After completing this chapter you will be able to:

Describe hierarchical routing in OSPFDescribe the 3 protocols in OSPF, the Hello, Exchange

and Flooding protocolsDescribe the OSPF databaseOutline the advantages and disadvantages of OSPF

Functional Requirements of OSPF

A more descriptive routing metric was introducedOSPF can discover multiple best paths to a given

destinationOSPF supports a 2 level routing hierarchyOSPF supports Variable Length Subnet Masks (VLSM)OSFP packets have a space reserved for authenticationOSPF is an example of a link state algorithm that adjusts

to network changes quicker than RIP and is more robust

Hierarchical Routing

Area 1

Area 2

Area 3

Hierarchical OSPF Internetwork

Area 1

Router 4

Router 1

Router 5

H2Router 8

Router 7

Router 11

Router 13

H1

Area 2

Area 3

Router 12

Area 0(Backbone)

Router 2Router 3

Router 6

Router 9

Router 10

Hierarchical Routing in OSPF

Area 0.0.0.0

Area 0.0.0.1

Area 0.0.0.2

Area 0.0.0.3

X

Y Z

W

Hierarchical OSPF Network Example

LAN 4

Router 1

Router 2

Router 3

Router 6

Router 4Router 5

LAN 3

LAN 5

LAN 2Router 2Router 3

Area 3.0.0.0

Area 0.0.0.0

Area 4.0.0.0External Network

ABR

ABR

ASBR

Multiple copies of algorithm

Inter area routing

Intra area routing

Intra area routing

OSPF Area Types

Transit AreasA transit area includes any area capable of propagating or originating Type-5 AS external LSAs

Stub AreasStub areas do not import external route information( External LSAs Type 5). Instead, network traffic to destinations not local to the area or AS is directed to the closest area border router advertising a default route

Not-so-stubby areas (NSSA)The NSSA (not-so-stubby-area) defines a new OSPF area similar to the stub area in that External LSAs (Type-5) are not propagated into the area nor may they originate in a stub area (via an ASBR). The area may contain an AS border router that may inject NSSA LSAs (Type-7) into the area.

OSPF Packet Header Format

Version(1) Type Message Length

Router IP address

Area ID

Checksum

Authentication (octets 0-3)

0 8 16 24 31

Authentication Type

Authentication (octets 4-7)

Type Meaning1 Hello2 Database description3 Link status request4 Link status update5 Link status

acknowledgementIP protocol 89

IP Packet

24 byte header

Sending and Receiving an OSPF Packet

Version(1) Type Message Length

Router IP address

Area ID

Checksum

Authentication (octets 0-3)

Authentication Type

Authentication (octets 4-7)

Version = 2

Type = 1 (hello)

Length = entire length

Router IP address = 192.168.10.1

Area ID = 3.0.0.0

Checksum = 16 bit checksum of entire packet

Authentication = 64 bits

Check on IP level

Check IP protocol number (89)

OSPF version must be 2

IP address must be on same network as receiving interface

Area ID must be = 3.0.0.0

Checksum must be successful

Authentication must be successful

The Protocols within OSPF

Hello protocol-To check that the links are operational.

-To elect the Designated Router (DR) and the Backup Designated Router (BDR).

Exchange protocol-Performs initial OSPF database synchronisation between two adjacent routers.

Flooding protocol-The flooding protocol is used to maintain the two databases in adjacent routers in synchronisation

Hello Protocol

BDR DR

HELLO

Priority = 1 Priority = 2

Priority = 7

Priority = 4 Priority = 3

Hello Protocol - OSPF Hello Message Format

Network Mask

Backup Designated RouterDesignated router

OSPF Header with Type = 1

Neighbour(1) IP Address

……..

Dead Timer Hello Int Gway Prio

Neighbour(2) IP Address

Neighbour(n) IP Address

Database Synchronisation

10.1.1.5 10.1.1.7

OSPF Hello

OSPF Hello: I heard 10.1.1.5

Database Description: Seq=x

Database Description: Seq=x, 5 LSA headers

Database Description: Seq=x+1,1 LSA header

Database Description: Seq=x+1

Link State Request Packet

Link State Update Packet

Link State Update Packet

Exchange Protocol - OSPF Database Description Message

Must be Zero

Link IDLink Type

OSPF Header with Type = 2

Advertising Router

Link Sequence Number

……..Link Checksum

Database Sequence Number

Link Age

I M S

The fields starting at

Link Type to Link Age are repeated for each link

Exchange Protocol & Flooding Protocol - OSPF Link Status Request Message Format

OSPF Header with Type = 3

Link Type

Link ID

Advertising Router

………

Exchange Protocol & Flooding Protocol - OSPF Link Status Update Message Format

OSPF Header with Type = 4

Number of Link Status Advertisements

Link Status Advertisement (1)

………………

Link Status Advertisement (n)

Exchange Protocol & Flooding Protocol - Header Format used for all Link State

Advertisements

Link Age Link Type

Link ID

Advertising Router

Link Sequence Number

Link Checksum Length

The OSPF Database - Router Links State Record

--0---EB

Link ID

Link Data

Type #TOS TOS 0 metric

Type #TOS TOS x metric

---0---- Number of links

The OSPF Database - The Network Links State Record

Attached router

---------

Network mask

Attached router

The OSPF Database - The Summary Links State Record

Network Mask

TOS=0 0 TOS 0 metric

TOS=x 0 TOS x metric

TOS=y 0 TOS y metric

……. …… ………..

The OSPF Database - The External Links State Record

Network Mask

E,TOS=0 0 TOS 0 metric

E,TOS=x 0 TOS x metric

External route tag (0)

External route tag (x)

Calculation of the Routing Table

The present routing table is invalidated.The intra-area routes are calculated by building the

shortest path tree into each attached area.The inter-area routes are calculated through the

examination of summary LSAs.In ABRs connected to one or more transit areas, the

transit area’s summary LSAs are examined to see if better paths exist using transit areas than were found in steps 2 and 3 above.

Routes to external destinations are calculated, through the examination of AS external LSAs.

Advantages of OSPF

OSPF is a standard protocol that all vendors can implement interoperability.

It provides rapid, deterministic calculation of internet routes. It uses Link State Advertisements.

It facilitate separate administration of differing parts of the internet.

It facilitate hiding of detailed information about the internet.

It provides a more advanced use of metrics. With OSPF one can isolate misconfigured or

malfunctioning routers in the internet and route around them.

OSPF provides for the effective use of information derived from other routing protocols

Disadvantages of OSPF

Link-state protocols use large amounts of router memory to store topological databases, as each router keeps a map of the entire network.

When a network experiences frequent changes, link-state routers use a large portion of network bandwidth by sending out LSAs at each network change.