OSPF basic Q &

1. Why is the OSPF convergence time always greater than 5 seconds?

Before performing the SPF algorithm, OSPF uses a five-second window to collect all LSA information about topology changes .. Once the algorithm recreates the topology database and creates a new route table, it cannot run the other database within 10 seconds. This delay helps prevent a "trashing (fragment ?)" Situation.

Trashing is manifested in the continuous running of algorithms due to unstable links. Due to the suspension of traffic during route computing, the traffic cannot be forwarded.

2. What do routers do when they receive an LSU?

If there are no identical entries, add the concurrent return LSAck to the DR, flood to the other routers, and update the route table. If there are identical entries, ignore them. If there are identical entries but new information exists, add the concurrent return LSAck to DR, flood to other routers, and update the route table. If the same entry but contains the old information, send back the updated LSU to the original sender.

3. When will the exchange Protocol and flooding process be run?

The exchange protocol is used to enable the neighbor router to enter the Full state. The first step is to establish the relationship between DR and BDR and DRother, during which all routers establish the master-slave relationship with DR/BDR. the master and slave routers exchange DBDs to synchronize their databases. once the neighboring router enters the Full status, the exchange Protocol process is no longer repeated unless the status changes. Flooding processes are used to change the link status at any time, such as link down or adding a new link to the network. the LSU packet contains changes to the status of all links and sends the changes to DR/BDR. DR forwards data to all DRother. if the router is connected to another network, LSU will also be sent to these networks.