Open Shortest Path First Protocol(OSPF)

Sudarshan Vasudevansvasu@cs.umass.edu

Overview

• Introduction

• Motivation

• OSPF Basics

• Hierarchical Routing in OSPF

• Summary

Introduction

• Development began in 1987

• OSPF Working Group (part of IETF)

• OSPFv2 first established in 1991

• Many new features added since then

• Updated OSPFv2 specification in RFC 2178

Motivation

• Original IGP used was RIP

• Based on Bellman-Ford Algorithm

• Worked well in small systems

• Suffered from problems of Distance Vector Protocol– Count to Infinity Problem– Slow Convergence

Motivation

• Problems with Distance Vector Protocol– Large update packets– Slow response to topological changes

• Need for a Link State Protocol

• A long list of functional requirements follows

Functional Requirements of OSPF

• Faster Convergence and less consumption of network resources

• A more descriptive routing metric– configurable – value ranges between 1 and 65,535– no restriction on network diameters

• Equal-cost multipath– a way to do load balancing

Functional Requirements(contd.)

• Routing Hierarchy– support large routing domains

• Separate internal and external routes• Support of flexible subnetting schemes

– route to arbitrary [address,mask] combinations using VLSMs

• Security• Type of Service Routing

OSPF Basicsthe essence

• Distributed, replicated database model– describes complete routing topology

• Link state advertisements– carry local piece of routing topology

• Distribution of LSAs using reliable flooding

• Link state database– identical for all the routers

Link State Advertisements(LSAs)

LS Age

Options LS Type

Link State ID

Advertising Router

LS Sequence Number

LS Checksum

Length

LSA Header

0 16

LSAs contd.

• Identifying LSAs– LS type field– Link State ID field

• mostly carries addressing information

• e.g. IP address of externally reachable network

– Advertising Router field• originating router’s OSPF router ID

LSAs contd.

• Identifying LSA instances– needed to update self-originated LSAs– LS Sequence Number field

• 32 bit values

• monotonically increasing until some max value

• 600 years to roll over!

• LSA checksum and LS Age guard against potential problems

LSAs contd.

• Verifying LSA contents– LS Checksum field

• computed by the originating router and left unchanged thereafter

• LS age field not included in checksum

• Removing LSAs from databases– LS Age field

• ranges from 0 to 30 min.• Max Age LSAs used to delete outdated LSAs

LSAs contd.

• Other LSA Header fields– Options field

• sometimes used to give special treatment during flooding or routing calculations

– Length field• includes LSA header and contents

• ranges from 20-65535 bytes

Sample Router LSA

10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6

10.1.1.3 10.1.1.5

Sample Router LSA contd..

LS Age

Options LS Type

Link State ID

Advertising Router

LS Sequence Number

LS Checksum

Length

Router Type 0

# of links

Link ID

Link Data

Link Type #TOS Metrics

Metric

0 seconds

E-bit,LS Type 1

10.1.1.1

10.1.1.1

0x80000006

0x9b47

60 bytes

0 (ordinary)

3

10.1.1.3

Ifindex 2 (unnumbered link)

1(point to point), 0

5

Link 1

Link State Database

• Collection of all OSPF LSAs

• databases exchanged between neighbors

• synchronization thru reliable flooding

• gives the complete routing topology

• each OSPF router has identical link-state database

Link State Database contd..

• Example of a link state database

LS Type Link State ID Adv Router LS Checksum LS Seq No LS Age

Router LSA 10.1.1.1 10.1.1.1 0x9b47 0x80000006 0

….. …... ….. ….. …. …...

Communication between OSPF Routers

• OSPF packets encapsulated in IP packets– standard 24 byte header

– OSPF packet type field

– OSPF router ID of sender

– Packet checksum

– Authentication fields

– OSPF Area ID

Neighbor Discovery and Maintenance

• OSPF Hello Protocol• Hello packets sent out every 10 seconds• helps to detect failed neighbors• RouterDeadInterval (default 40 seconds)• also ensures that link is bidirectional• neighboring routers agree on intervals

– hello interval set so that a link is not accidentally brought down

Database Synchronization

• Crucial to ensure correct and loop free routing• must be done before 2 neighbors start

communication• also whenever new LSAs are introduced

– uses reliable flooding

• each router sends LSA headers to its neighbor when connection comes up

• requests only those LSAs which are recent

Database Exchange

• Neighboring routers first exchange hellos• a database description packet packet establishes the

sequence number• the other router sends LSA headers• sequence number incremented for every pair od

database description packets – implicit acknowledgement for the previous pair

• after examining LSA headers explicit request sent for complete LSAs

Reliable Flooding

• Starts when a router wants to update self-originated LSAs

• Link State Update packets• Neighbor installs more recent LSAs into its

database• floods out on all interfaces except the one on

which it arrived• reliability-retransmissions until acks received

Reliable Flooding (contd..)

10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6

10.1.1.3 10.1.1.5

Time T1

uu

u

Reliable Flooding (contd..)

10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6

10.1.1.3 10.1.1.5

Time T2u

uu

u

u

Reliable Flooding (contd..)

10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6

10.1.1.3 10.1.1.5

Time T3

u

u

Reliable Flooding (contd..)

10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6

10.1.1.3 10.1.1.5

Time T3+

ack ack ack ack

ack

Reliable Flooding(contd..)

• Robustness– updates flooded over all the links , so failure of any link

doesn’t affect database synchronization

– LSAs refreshed every 30 minutes

– LSA checksum field detects corruption

– flooding loops avoided by LS Age field

– MinLSInterval limits rate of LSA origination

– Receivers can refuse to accept LSA updates if they received an update less than a second ago

Routing Calculations

• Link costs configurable by administrator• Smaller values for more preferred links• must make sense to add link costs• different costs for each link direction possible• Dijkstra’s shortest path algorithm

– incrementally calculates tree of shortest paths

– each link in the network examined once

– computes multiple shortest paths (equal-cost multipath)

Hierarchical Routing

• Technique used to build large networks• minimizes consumption of network resources such

as – router memory

– router computing resources

– link bandwidth

• with flat routing linear increase in routing table size

• with hierarchical, size increases logarithmically

an example

10.3.3

10.3.1 10.3.2

10.1.3

10.1.1 10.1.2

10.2.3

10.2.1 10.2.2

10.0.0.0/8

10.1

10.3

10.2

example contd..

• Consider a router in 10.1.1• assume 16 entries in each of the first level partitions• with flat routing, 9*16 = 144 entries/router• with 3 level hierarchy, the router has 16 entries

within 10.1.1.0/24 + entries for 10.1.2.0/24, 10.1.3.0/24,10.1.0.0/16 for a total of 19 entries.

• Marked reduction in routing table size• but might lead to suboptimal routing

OSPF Areas

• Two-level hierarchical routing scheme through the use of areas

• areas identified by 32-bit id• each area has its own link state database which is a

collection of network-LSAs and router-LSAs• area’s topology hidden from all other areas• interconnection of areas through area border

routers (ABRs) • ABR leaks IP addressing information to other

areas through summary LSAs

Sample Area Configuration

A

B C

D

G

H F

E

I J

AA 1

2 2

1 1

3 3

1 3 1

10.2.1.0/24 10.2.2.0/24

Area 0.0.0.1

10.1.2.0/24

10.1.1.0/24

Area 0.0.0.2

3

3

1

1

3

3

31

1

10.3.7.0/24

10.8.2.0/24

Area 0.0.0.3

Area 0.0.0.01

OSPF Areas contd..

• Example of Summary LSA(router B)

LS Age

Options LS Type

Link State ID

Advertising Router

LS Sequence Number

LS Checksum

Length

Network Mask

TOSMetric

0

0x2, Type 3(summary-LSA)

10.2.0.0

Router B’s router ID

0x80000001

28 bytes

255.255.0.0

TOS 0 (normal)Cost of 7

OSPF Areas contd..

• Reduction in link state databases of an area• reduction in amount of flooding traffic needed for

synchronization• reduction in the cost of the shortest path

calculations• increased robustness• routing protection• Hidden prefixes

Area Organization

• All the areas are connected to area 0.0.0.0 also called the backbone area

• need not have a direct physical connection though– virtual links provide logical link to backbone

– summary LSAs tunneled across non backbone areas

• exchange of routing information between areas using Distance Vector Protocol– absence of redundant paths between areas

– not subject to convergence problems

Incorporating external routing information

• Special routers called AS boundary routers at the edge of OSPF domain

• ASBRs originate AS-External LSAs

• only routes for which the choice of an ASBR makes sense are imported

• otherwise default routes are used

• AS external LSAs similar to Summary LSAs with 2 additional fields– Forwarding address

– external route tag

Interaction with areas

• AS-External LSAs flooded across borders• ASBR summary LSAs used to know the location

of the originator of AS-External LSA• Link State ID of ASBR Summary LSA set to the

OSPF router ID of the ASBR whose location is advertised

• similar to summary LSA in all other respects

OSPF Area Types

• Restrict the amount of external routing information within an area

• used when resources especially router memory is very limited

• two types of restricted areas– Stub Areas

– NSSAs or Not-So-Stubby-Areas

OSPF Area Types

• Stub Areas– don’t support ASBRs and hence no AS-External-LSAs– routing to external destinations based on default routes

originated by the area’s border routers– summary LSAs also made optional– must lie on the edge of OSPF routing domain– inter-area routing may also be based on default routes– improved scaling– but not preferred due to the possibility of suboptimal

routes

OSPF Area Types contd..

• NSSAs– import small amount of routing information

– this information flooded to other areas by the NSSA Border router

– Use Type-7 LSAs to import external routing information

– translated into AS-External-LSA at the NSSA Border

– one-way filter

Summary

• Why OSPF is needed in the Internet?• The basics of the protocol

– The Link state Advertisements

– Neighbor Discovery (Hello Protocol)

– Database Synchronization and reliable flooding

• Hierarchical Routing in OSPF– OSPF Areas and Area Organization

– Interaction with External Routing Information

– OSPF Area Types viz. Stub Areas and NSSAs

Issues not covered

• OSPF Network Types– Broadcast subnets

– NBMA Subnets

• OSPF Extensions• Multicast Routing using OSPF (MOSPF)• OSPF Management• and a whole lot of others!

Further Reading

• John T. Moy, OSPF - An Anatomy of an Internet Routing Protocol

• Christian Huitema, Routing in the Internet• RFC 2178

Concluding Remark

Thank You!