OSPF Tutorial

There are 3 type of tables

+ Neighbor

+ Topology

+ Routing

Neighbor table

+ Contain information about the neighbors

+ Neighbor is a router which shares a link on same network

+ Another relationship is adjacency

+ Not necessarily all neighbors

+ LSA updates are only when adjacency is established

Topology table

+ Contain information about all network and path to reach any network

+ All LSA’s are entered into the topology table

+ When topology changes LSA’s are generated and send new LSA’s

+ On topology table an algorithm is run to create a shortest path, this algorithm is known as SPF or dijkstra algorithm

Routing Table

+ Also knows as forwarding database

+ Generated when an algorithm is run on the topology database

+ Routing table for each router is unique

D: Exchange LSDB’s list

Neighbors use DD (Data Description) to exchange their LSDB catalogs.  In this scenario, R1 sends DD to R2 first. It says: I have a Route LSA from R1. R2 also sends DD to R1: I have a Route LSA from R2.

Note: DD works like table fo content. It lists what LSDB has, but not details. By  reading DD, the receiving router can determine what it is missing and them ask the sender to transmit required LSAs..

R1 Request, R2 Update

R1 has learned that R2 has a R2 Router LSA that it does not have.

R1 sends a LS Request to R2. When R2 receives this request, it sends an Update to transmit this LSA to R1.

R2 Request, R1 Update

R2 also sends request to R1. R1 replies an Update. Upon receiving Update, R2 adds R1 Router LSA to its LSDB, calculates its routes, and add a new entry (192.168.1.0, S1/0) to its routing tabe.

Note: OSPF works distributely. After routers have synchronized their LSDB, they use the same data (LSDB) to calculate shortest paths, and updates their routing tables independently.

Ack update : LSAs are received

In order to assure reliable transmission, when a router receives an Update, it sends an Ack to the Update sender. If the sender does not receivie Ack within a specific peried, it times out and retransmits Update.

Note: OSPF uses Update-Ack to implemnet relaible transmission. It does not use TCP.

H1 ping H2: succeeded.

Each OSPF router creates a Router LSA to describe its interfaces’ IP addresses and floods its Router LSA to its neighbors. After a few rounds of flooding, all OSPF routers have the same set of Router LSAs in their LSDBs. Now routers can use the same LSDB to calculate routes and update routing tables.

From LSDB, a router learns the entire topology: the number of routers being connected. Router interfaces and their IP addresses, interface link costs (OSPF metric). With such detail information, routers are able to calculate routing paths to reach all destinations found in LSDB. For example, in the OSPF basic simulation (see External links), R1’s LSDB contains two Router LSAs: – A Router LSA from R1. R1 has two links. Their IP addresses are 192.168.1.0/24,192.168.3.0/30. – A Router LSA from R2. R2 has two links. Their IP addresses are 192.168.2.0/24,192.168.3.0/30. From these LSA, R1 can calculate the routing path to reach remote destination 192.11.68.2.2 and adds an entry (192.168.2.0/24, S1/0) to its routing table.