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Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF). Agenda. OSPF Basics OSPF Terminology SPF Tree Calculation Verifying OSPF. OSPF Basics. OSPF Basics (1). Open standards. Use Dijkstra algorithm. It only supports IP routing.

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Open Shortest Path First (OSPF)

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  1. Open Shortest Path First (OSPF)

  2. Agenda • OSPF Basics • OSPF Terminology • SPF Tree Calculation • Verifying OSPF

  3. OSPF Basics

  4. OSPF Basics (1) • Open standards. • Use Dijkstra algorithm. • It only supports IP routing. • OSPF is supposed to be designed in a hierarchical fashion  separate the larger internetwork into smaller internetworks called areas. • To decrease routing overhead • To speed up convergence • To confine network instability to single area • Area 0  backbone area.

  5. OSPF Basics (2)

  6. OSPF Terminology

  7. OSPF Terminology (1) • Link  a network or router interface. • Router ID  an IP address used to identify the router  highest loopback interfaces. • Neighbors  routers that have an interface on a common network. • Adjacency  relationship between two OSPF routers that permits the direct exchange of route updates. • Neighborship database  a list of all OSPF routers for which Hello packets have been seen. • LSA (Link State Advertisement)  an OSPF data packet containing link-state and routing information that’s shared among OSPF routers.

  8. OSPF Terminology (2) • Topology database  information from all of the LSA packets. • Designated Router (DR) • All routers on the multi-access network will establish adjacencies with the DR and BDR. • Highest priority  router ID is used as a tiebreaker. • Backup Designated Router (BDR) • A hot standby for the DR.

  9. Type of Networks • Broadcast multi-access • A DR and a BDR must be elected. • Ethernet • Nonbroadcast multi-access (NBMA) • A DR and a BDR must be elected. • Frame relay, X.25, ATM • Point-to-point • Direct connection between two routers  physical or logical. • Point-to-multipoint • A series of connections between a single interface on one router and multiple destination routers. • All of the interfaces belong to the same network.

  10. SPF Tree Calculation

  11. SPF Tree Calculation • Shortest Path First (SPF) algorithm. • Each router constructing a tree  the router is the root and all other networks are arranged along the branches and leaves. • Separate trees will be constructed for each area. • OSPF metric  cost. • Cisco bases link cost on bandwidth  108/bandwidth

  12. Verifying OSPF

  13. OSPF Troubleshooting Commands • show ip route  shows the entire routing table • show ip ospf  displays OSPF information • show ip ospf database  displays the topology database • show ip ospf interface  displays all interface-related OSPF information • show ip ospf neighbor  shows all OSPF neighbors

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