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Single Area OSPF

Single Area OSPF. Link State Routing Single Area OSPF Concepts Single Area OSPF Configuration. Identify Distance Vector & Link State Routing Characteristics. Updates contain entire routing table. Slow convergence. Updates consume significant bandwidth. Updates contain changes only.

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Single Area OSPF

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  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 Updates sent to all routers Topology changes trigger updates 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 Updates sent to all routers Topology changes trigger updates 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 it’s 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 ospfprocess-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 addressip-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 priority=DR • 2nd lowest priority=BDR Router(config-if)#ip ospfpriority number Router#show ip ospf interfacetype 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 costnumber

  16. Additional Configuration Network administrators can also configure • LSA update authentication • ‘Hello’ & ‘Dead’ interval timers • Default route to routers outside the area/autonomous system

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