Single Area OSPF

  • A link is an interface on a router
  • Router sends a Link state advertisement (LSA) packet every 30 minutes and immediately when a router changes state.
  • A link state database (topology database) provides an overall picture of the network.
  • AS – Autonomous System
  • Area – Grouping of contiguous networks
  • All areas communicate through the backbone area (area 0)
  • Neighbor OSPF routers must recognise each other on the network before they can share information
  • The Hello protocol  establishes and maintains neighbor relationships

Hello Packets Contain

Router ID: 32 bit unique number, the highest IP address on an active interface by default eg: is higher than

Hello and Dead Intervals: The frequency in seconds that the router sends hello packets, dead timer is the time a router waits before declaring a router dead the default is 4 times the hello interval and the hello interval is usually 10 seconds on a multiaccess network.

Neighbors: Communications is established when a router recognises itself in a hello packet from a neighbor router.

Area ID: Two routers must share a common segment and their interfaces must belong to the same area on that segment.

Router Priority: 8 bit number indicating a priority, used to select a designated router (DR) and a backup router (BDR).

Authentication Password: If auth is enabled 2 routers must exchange the same password.

Stub Area Flag: Router must agree on stub area flag (for stub areas)

OSPF Cost = Reference Bandwidth/Interface Bandwidth (in bps)

SPF Algorithm

Each router is the root of a tree and calculates the shortest path to each node, based on the cost to reach each destination.  LSA’s are flooded throughout an area to build the topology database on each router.

The cost or metric of an interface is an indication of the overhead required to send packets across an interface.

There is more overhead/cost/time delays sending over a T1 line than a 10Mpbs link.

Loopback Interface

  • Router ID uniquely identifies each router in the OSPF network
  • You can override the default IP being advertised by configuring an IP address on a loopback interface
  • OSPF is more reliable if a loopback interface is configured because the interface is always active and cannot be in a down state like a real interface
  • Advertised loopback address can be used for management an unadvertised loopback interface cannot

Troubleshooting OSPF

  • Troubleshoot Neighbour Adjacencies
  • Troubleshoot Routing Table Issues
  • Troubleshoot Authentication

Troubleshooting Neighbour Adjacencies

  • Are all interfaces up?
  • Do local and neighbor MTU sizes match?
  • Are all OSPF neighbors configured?
  • Does all Hello packet info match the neighbour?

A healthy neighbour state is FULL

The four items in a hello packet that must match are:

  1. Area ID
  2. Hello and Dead Intervals
  3. Authentication Password
  4. Stub Area Flag

Troubleshooting OSPF Routing Tables

  • Are all OSPF networks advertised?
  • Do route filters block OSPF networks?

Route Table Codes

O – OSPF Intra area route, within same area

OIA – OSPF Inter Area, from another router

OE1 or E2 – External OSPF route from another AS

Debugging IP OSPF Adjacencies will show any mismatched authentication types:

  • Null = Type 0
  • Simple Pass – Type 1
  • MD5 – Type 2

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