© 1999, Cisco Systems, Inc. 8-1 OSPF Overview RFC 2328, 2178, 1583

© 1999, Cisco Systems, Inc. www.cisco.com 8-1

OSPF Overview

RFC 2328, 2178, 1583

OSPF Overview

RFC 2328, 2178, 1583

• Has fast convergence

• Supports VLSM

• Has no hop count limitation

• Processes updates efficiently

• Selects paths based on bandwidth

What Is OSPF?

OSPF Terminology

Links

TokenRing

OSPF Terminology

Links

TokenRing

Cost=10

Cost=1785Cost=6

OSPF Terminology

Area 1Links

TokenRing

Cost=10

Cost=1785Cost=6

Area 0

OSPF Terminology

Area 1Links

TokenRing

Cost=10

Cost=1785Cost=6

Area 0DR

BDR

OSPF Terminology

Adjacencies Database

Lists Neighbors

Area 1Links

TokenRing

Cost=10

Cost=1785Cost=6

Area 0DR

BDR

OSPF Terminology

Topology Database

Lists All Routes

Area 1Links

TokenRing

Cost=10

Cost=1785Cost=6

Area 0DR

BDR


Lists Neighbors

Area 1Links

TokenRing

Cost=10

Cost=1785Cost=6

Area 0

OSPF Terminology

DR

BDR

RoutingTable

Lists Best Routes

Topology Database

Lists All Routes


Lists Neighbors


OSPF Operation within a Single Area

OSPF Operation within a Single Area

Establishing Adjacencies

Hello

A

D E

CB

Hello

afadjfjorqpoeru39547439070713

Router IDHello/Dead IntervalsNeighborsArea-IDRouter PriorityDR IP AddressBDR IP AddressAuthentication PasswordStub Area Flag

* *

* *

Establishing Adjacencies (cont.)

172.68.5.1/24

E0

172.68.5.2/24

E1A BDown State


172.68.5.1/24

E0

172.68.5.2/24

E1

Router BAdjacencies Database

172.68.5.1/24, int E1

I am router ID 172.68.5.1 and I see no one.

A BDown State

Init State


172.68.5.1/24

E0

I am router ID 172.68.5.2, and I see 172.68.5.1

172.68.5.2/24

E1


172.68.5.1/24, int E1


A BDown State

Init State


172.68.5.1/24

E0

I am router ID 172.68.5.2, and I see 172.68.5.1

Router AAdjacencies Database

172.68.5.2/24, int E0

172.68.5.2/24

E1


172.68.5.1/24, int E1


A BDown State

Init State

Two-Way State

• Hellos elect DR and BDR• Each router forms adjacency with DR and BDR

Electing the DR and BDR

DR BDR

• Hello packets exchanged via IP multicast • Router with highest OSPF priority elected

P=1 P=0P=1

Electing the DR and BDR (cont.)

P=3 P=2

Hello

DR BDR

Discovering Routes

E0

172.68.5.1

DRE0

172.68.5.3

No, I will start exchange because I have a higher router ID.

I will start exchange because I have router ID 172.68.5.1.Hello


Hello


Exstart State

Discovering Routes

E0

172.68.5.1

DRE0

172.68.5.3

No, I will start exchange because I have a higher router ID.

I will start exchange because I have router ID 172.68.5.1.

Here is a summary of my link-state database.

Hello


Hello


DBD


Exstart State

Exchange State

Here is a summary of my link-state database.DBD


Discovering Routes (cont.)

E0

172.68.5.1

E0

172.68.5.3

Thanks for the information!LSAck


LSAck


DR


E0

172.68.5.1

E0

172.68.5.3

I need the complete entry for network 172.68.6.0/24.

Here is the entry for network 172.68.6.0/24.

Thanks for the information!

LSR


LSAck


LSU


Loading State.



LSAck


DR


E0

172.68.5.1

E0

172.68.5.3

I need the complete entry for network 172.68.6.0/24.

Here is the entry for network 172.68.6.0/24.

Thanks for the information!

LSR


LSAck


LSU


Full State

Loading State.



LSAck


DR

Choosing Routes

Routing TableNet Cost Out Interface2.2.2.0 6 TR03.3.3.0 7 TR03.3.3.0 10 E0

TokenRing

Cost=10

Cost=6FDDI

Cost=1A B C

This is the best route to C.

1.1.1.0/24 2.2.2.0/24 3.3.3.0/24

4.4.4.0/24

Maintaining Routing Information

New Routerxx

Link-State Change

LSU1

DR

AB

• New router tells all OSPF DRs on 224.0.0.6


New Routerxx

Link-State Change

LSU

LSU

1

2

DR

A B


• DR tells others on 224.0.0.5



New Routerxx

Link-State Change

LSU

LSU

LSU

1

2

3


DR

A B




New Routerxx

Link-State Change

LSU

LSU

LSU

1

2

3

I need to update my routing table.

DR

AB

4

Maintaining Routing Information (cont.)

Is entry inlink-state

database?

No

Run SPF to calculate new routing table

Add to database

Flood LSA

End

LSA

LSU

A



database? Yes

No


Add to database

Flood LSA

Is seq. # the same?

No

Send LSUwith newer

information to source

EndEnd

LSA

LSU

Is seq. #newer?

No



database? Yes

No


Add to database

Flood LSA

Is seq. # the same?

Yes

No

Send LSUwith newer


EndEnd

LSA

LSU

Is seq. #newer?

No

Ignore LSA



database? Yes

No


Add to database

Flood LSA

Is seq. # the same?

Yes

No

Yes

Send LSUwith newer


EndEnd

LSA

LSU

Is seq. #newer?

No

A

Ignore LSA

Send LSAckto DR

Go to A

Configuring OSPF on Internal Routers

Can assign network or interface address.

Broadcast Network Point-to-Point Network

E0

10.64.0.1

10.64.0.2

E0

S0

10.2.1.210. 2.1.1

S1A B C

<Output Omitted>interface Ethernet0 ip address 10.64.0.1 255.255.255.0!<Output Omitted>router ospf 1 network 10.0.0.0 0.255.255.255 area 0

<Output Omitted>interface Ethernet0 ip address 10.64.0.2 255.255.255.0!interface Serial0 ip address 10.2.1.2 255.255.255.0<Output Omitted>router ospf 50 network 10.2.1.2 0.0.0.0 area 0 network 10.64.0.2 0.0.0.0 area 0

• Assigns a cost to an outgoing interface• May be required for interoperability• Use default cost between Cisco devices

Cisco Non-Cisco

Traffic

TokenRing

Router(config-if)#

ip ospf cost cost

Configuring Optional Commands (cont.)


Creating Multiple OSPF Areas

Creating Multiple OSPF Areas

Issues with Maintaining a Large OSPF Network

OSPF

OSPF

OSPF

OSPF

OSPF

My routing table is too big, I am running low on memory.

The SPF is running toooften for me to route.

I am only receivingLSAs, no data.

Area 0

Area 1 Area 2

Autonomous System

• Consists of areas and autonomous systems

• Minimizes routing update traffic

The Solution: OSPF Hierarchical Routing

OSPF Multiarea Components

Area 0I am a backbone.

Area 2I am a stub.

Area 1I am standard.

Areas Routers LSAs

Internal

ABR

ASBR

Backbone

Type 1


Type 2


Type 3/4


Type 5


Types of OSPF Routers

InternalRouters

Area 1 Area 2

ASBR andBackbone

Router

Backbone/InternalRouters

ABR and Backbone

Router

Backbone Area 0

ExternalAS

ABR and Backbone

Router

InternalRouters

Flooding LSUs to Multiple Areas (cont.)

Routing Table

Interarea routes

Intra-area routes

External (non-OSPF routes)

Area 1

Area 1 Area 0

Area 1 RIP

OSPF Area Configuration Example

192.168.15.2

Area 0

Stub Area 2

192.168.14.1 192.168.15.1S0

S0E0

ExternalAS

R4

R3

R3#

interface Ethernet 0ip address 192.168.14.1 255.255.255.0interface Serial 0 ip address 192.168.15.1 255.255.255.252

router ospf 100network 192.168.14.0 0.0.0.255 area 0network 192.168.15.0 0.0.0.255 area 2

R4#

interface Serial 0 ip address 192.168.15.2 255.255.255.252

router ospf 15network 192.168.15.0 0.0.0.255 area 2

Link-State Network Discovery

Routers calculate the shortest path to destinations in parallel

AA BB CC

X YZW

Routing TableRouting Table

WW 00

XX 00


XX 00

YY 00


YY 00

ZZ 00

E0

S1 S0

S1 S1

E1

E0 S1 S1 S0 S1 E1



SPF

AA BB CC

X YZ

TopologicalDatabase SPF

TopologicalDatabase SPF

TopologicalDatabase

W


WW 00

XX 00


XX 00

YY 00


YY 00

ZZ 00

E0

S1 S0

S1 S1

E1

E0 S1 S1 S0 S1 E1



SPF

SPF Tree SPF Tree SPF Tree

AA BB CC

X YZ

TopologicalDatabase

ARouting

Table

ARouting

Table

SPFTopological

Database

BRouting

Table

BRouting

Table

SPFTopological

Database

CRouting

Table

CRouting

Table

W


WW 00

XX 00


XX 00

YY 00


YY 00

ZZ 00

E0

S1 S0

S1 S1

E1

E0 S1 S1 S0 S1 E1

Link-State Topology Changes

Update processes proceed using the same link-state update

Process to update this

routingtable


routingtable


routingtable


routingtable

TopologyTopologychange change

ininlink-link-statestate

updateupdate


routingtable


routingtable

Link-State Concerns

Processing and memory required for link-state routing

SPF

SPF Tree

TopologicalDatabase

ARouting

Table

ARouting

Table

Link-State Concerns

Processing and memory required for link-state routing

Bandwidth consumed for initial link-state “flood”

SPF

SPF Tree

TopologicalDatabase

ARouting

Table

ARouting

Table

Problem: Link-State Updates

Unsynchronized updates, inconsistent path decisions

DD

BB

AACC

Network 1, UnreachableNetwork 1, Unreachable

Network 1, UnreachableNetwork 1, UnreachableNetwork 1goes down

then comes up

X, okX, ok

Slow path update



DD

BB

AACC

Network 1, UnreachableNetwork 1, Unreachable Network 1, UnreachableNetwork 1, Unreachable

Network 1, Back Up NowNetwork 1, Back Up Now


then comes up

X, okX, ok Fast path updates arrive first

Slow path update arrives lastSlow path update

Which SPF tree touse for routing?



DD

BB

AACC

Network 1, UnreachableNetwork 1, Unreachable Network 1, UnreachableNetwork 1, Unreachable

Network 1, Back Up NowNetwork 1, Back Up Now


then comes up

X, okX, ok Fast path updates arrive first

Slow path update arrives lastSlow path update

Which SPF tree touse for routing?

Link-State Update Problems

Synchronizing large networks—which network topology updates are correct?



Router startup—order of start alters the topology learned



Router startup—order of start alters the topology learned

Partitioned regions—slow updating part separated from fast updating part

Reduce the need for resources– “Dampen” update frequency

– Target link-state updates to multicast

–Use link-state area hierarchy for topology

–Exchange route summaries at area borders

Solution: Link-State Mechanisms

Reduce the need for resources–“Dampen” update frequency

–Target link-state updates to multicast

–Use link-state area hierarchy for topology

–Exchange route summaries at area borders

Coordinate link-state updates–Use time stamps

–Update numbering and counters

–Manage partitioning using an area hierarchy

Solution: Link-State Mechanisms

Documents

© 1999, Cisco Systems, Inc. 8-1 OSPF Overview RFC 2328, 2178, 1583