Single Area OSPF

1
Single Area OSPF

• Link State Routing
• Single Area OSPF Concepts
• Single Area OSPF Configuration

2
Identify Distance Vector Link State Routing
Characteristics
Updates contain entire routing table
Slow convergence
Updates consume significant bandwidth
Updates contain changes only
Increased memory processing requirements
Topology changes trigger updates
Updates sent to all routers
Support CIDR/VLSM
Updates sent to neighbours
Rapid convergence
Periodic updates
3
Identify Distance Vector Link State Routing
Characteristics
Updates contain entire routing table
Slow convergence
Updates consume significant bandwidth
Updates contain changes only
Increased memory processing requirements
Topology changes trigger updates
Updates sent to all routers
Support CIDR/VLSM
Updates sent to neighbours
Rapid convergence
Periodic updates
4
Summary of Link State Features

• Responds quickly to network changes
• Use hellos to discover neighbours
• Send updates when a there has been a change in
  the network topology
• Updates contain changes not whole routing table
• Calculates shortest path to each route in the
  network

5
Link State Operation

• Routers are aware of directly connected networks
  known as links
• Routers send hellos to discover neighbours
• Routers send Link State Advertisements to other
  routers informing them of their links
• All routers add Link State Advertisements to
  their topological database
• Shortest Path algorithm calculates best route to
  each network
• When link states change LSA update sent to all
  routers which recalculate their routes

6
Topological Database

• Every router advertises directly connected
  networks via Link State Advertisements
• Every router has its own view of the network
  it builds a topological database
• Router A is aware of 2 paths to 192.168.157.0
  this provides redundancy should one of the
  routers fail

7
Evaluation of Link State Routing
8
Review Link State Distance Vector
9
OSPF Overview

• Preferred to RIP on larger networks
• Open Standard - IETF RFC 2328
• Link State routing protocol
• Interior Gateway Protocol for Autonomous systems
• Metric based on bandwidth
• Supports VLSM
• OSFP can use areas to allow hierarchical design

10
OSPF Key Words

• Adjacencies database
• Directly connected routers
• Topological Database
• Routes to every network
• Routing table
• Best path to each network
• Designated Router
• a router elected by all others to represent the
  network area
• Area 0
• backbone

11
Designated Router/Backup DR

• All LSA sent to DR/BDR instead of to every single
  router
• Reduces overhead of LSA updates
• Standard on multi-access networks
• DR is single point of failure solution is BDR

12
OSPF basic commands

• Router(config)router ospf process-id
• Router(config-router)network address
  wildcard-mask area area-id
• EXAMPLE
• Router(config)router ospf 2
• Router(config-router)network 172.16.10.0
  0.0.0.255 area 2
• NOTES
• process-id can be a value between 0 and 65,535
• Wildcard mask NOT subnet mask used with network
  command

13
OSPF Loopback Address

• For OSPF to function there must always be an
  active interface
• Physical interfaces e.g. serial/Ethernet may not
  always be active routing would fail
• Configure virtual loopback interface as
  solution
• Subnet mask will always be 255.255.255.255
• Router(config)interface loopback number
• Router(config-if)ip address ip-address
  subnet-mask

14
DR/BDR selection

• To suit the topology used the network
  administrator will want to choose DR/BDR
• DR/BDR election based on OSPF priority
• Lowest priorityDR
• 2nd lowest priorityBDR
• Router(config-if)ip ospf priority number
• Routershow ip ospf interface type number

15
OSPF Cost

• Cost is the OSPF metric used in path selection
• Cost is based on bandwidth
• Default bandwidth is 1.544Mbps cost is 64
• Cost can be 1 (100Mbps) to 65535
• Cost is 108 bandwidth
• Gigabit Ethernet will require change to default
  cost why?
• Router(config)interface serial 0/0
• Router(config-if)bandwidth 64
• Router(config-if)ip ospf cost number

16
Additional Configuration

• Network administrators can also configure
• LSA update authentication
• Hello Dead interval timers
• Default route to routers outside the
  area/autonomous system