© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Configuring OSPF Introducing the OSPF Protocol

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Link-State Protocols

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Link-State Data Structures Neighbor table: –Also known as the adjacency database –Contains list of recognized neighbors Topology table: –Typically referred to as LSDB –Contains all routers and their attached links in the area or network –Identical LSDB for all routers within an area Routing table: –Commonly named a forwarding database –Contains list of best paths to destinations

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Link-State Routing Protocols Link-state routers recognize more information about the network than their distance vector counterparts. Each router has a full picture of the topology. Consequently, link-state routers tend to make more accurate decisions.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Link-State Data Structure: Network Hierarchy Link-state routing requires a hierachical network structure that is enforced by OSPF. This two-level hierarchy consists of the following: –Transit area (backbone or area 0) –Regular areas (nonbackbone areas)

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF area characteristics: Minimizes routing table entries Localizes impact of a topology change within an area Detailed LSA flooding stops at the area boundary Requires a hierarchical network design OSPF Areas

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Routers A and B are backbone routers. Backbone routers make up area 0. Routers C, D, and E are known as area border routers (ABRs). ABRs attach all other areas to area 0. Area Terminology

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF Adjacencies Routers discover neighbors by exchanging hello packets. Routers declare neighbors to be up after checking certain parameters or options in the hello packet.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Forming OSPF Adjacencies Point-to-point WAN links: –Both neighbors become fully adjacent. LAN links: –Neighbors form a full adjacency with the DR and BDR. –Routers maintain two-way state with the other routers (DROTHERs). Routing updates and topology information are passed only between adjacent routers. Once an adjacency is formed, LSDBs are synchronized by exchanging LSAs. LSAs are flooded reliably throughout the area (or network).

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF Calculation Routers find the best paths to destinations by applying Dijkstras SPF algorithm to the link-state database as follows: Every router in an area has the identical link-state database. Each router in the area places itself into the root of the tree that is built. The best path is calculated with respect to the lowest total cost of links to a specific destination. Best routes are put into the forwarding database (routing table).

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v SPF Calculation Assume all links are Ethernet, with an OSPF cost of 10.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Link-State Data Structures: LSA Operation

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary Link-state routing protocols respond quickly to changes, send triggered updates when changes occur, and send periodic updates every 30 minutes. A two-tier hierarchical network structure is used by OSPF in which the network is divided into areas. This area structure is used to separate the LSDB into more manageable sizes. Adjacencies are built by OSPF routers using the Hello protocol. Over these logical adjacencies, LSUs are sent to exchange database information between adjacent OSPF routers.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary (Cont.) Dijkstras SPF algorithm is used to calculate best paths for all destinations. SPF is run against the LSDB, and the outcome is a table of best paths, known as the routing table. After an LSA entry ages, the router that originated the entry sends an LSU about the network to verify that the link is still active. The LSU can contain one or more LSAs.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v