OSPF

OSPF Concept

Why OSPF so popular?

• FAST CONVERGENCE

• VLSM SUPPORT

• LARGE NETWORK SUPPORT

• STUB CAPABILITIES

• AUTHENTICATION

• EFFICIENCY

• ROUTE TAGGING CAPABILITIES

Technical Overview Of OSPF

• OSPF sends hello packets to 244.0.0.5 (all OSpF routers)

  • DR and BDR communicate on 244.0.0.6

• Neighbor EVALUATES hello packets to form or decline neighbor relation

• Link-State data exchanged (DBD)

• New info RECORDED and FLOODED

• When database is completes SPF ALGORITHM runs

• Valid routes enter the routing table

THE OSPF hello packet

• Router ID

• Area ID  (Must match)

  • Address of Originating interface

• Authentication information (Must Match)

• Hello Interval (Must Match)

• Dead Interval (Must Match)

• Priority

• DR/BDR ids

• Neighbor Toiuter IDs

OSPF Networks

Broadcast

Eg: Ethernet, Token ring

Single Operational Mode

DR/BDR Election, 10 sec hello ,Dual Multicast add

Point-to-Point

Eg, T1, CAS, ISDN BRI/PRI

Single Operational mode

No DR/BDR, 10 sec hello,Single Multicast add

NBMA Networks (Non Broadcast Multi-access)

• Eg FrameRealy, ATM

• Five Operational Modes

  • • Non Broadcast (NDMA) --------------------------------------RFC Standard

      • Default mode

      • Neighbor are statically configured ( neighbor command is used)

      • one subnet

      • Act like a LAN environment (DR/DBR Elected)

      • DR/BDR elected (must have full connectivity with other routers)

• • Point-to-Multipoint ---------------------------------------------RFC Standard

    • Like a series of point-to-point (no DR/BDR Elected)

    • Everything is sent at 244.0.0.5 

    • Requires single subnet

    • Neighbor Automatically form ( network command is used)

    • Point-to-Point - ------------------------------------------------Cisco Proprietary

      • Default on FR point-to-point interface

      • Timers: Hello= 10 sec , Dead = 40 sec

      • Multicast Support = Neighbors Discovers

      • Compatible with Point-to- Multipoint

      • No DR/BDR,

    • Broadcast -----------------------------------------------------Cisco Proprietary 

      • Ethernet/Broadcast Style communication on NBMA network (DR/DBR Elected)

    • Point-to-Multipoint, Non Broadcast - -------------------Cisco Proprietary

      • Same as Point-to-Multipoint RFC Standard

      • Neighbor are statically configure

  • All Neighbor are statically assigned

• Routers use UNI-CAST instead of multicast

• DR must have fully connectivity to all routers

Point-to-multipoint

• There is no DR election

• MULTICAST is used for messages 

• The Framrellay cloud must allow Multicast and 

Neighbor States

Hello sent

init state

2-way state

EX state

Exchange state

Full state

OSPF Areas

Routers Types

LSAs