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Module 1: Single-Area OSPFv2 Concepts, The solution is selecta DR to…

- - - - Reduced frequency of SPF calculations - Multiarea OSPF localize the impact of a topology change within an area. For instance, it minimizes routing update impact because LSA flooding stops at the area boundary.
        
        Reduced link-state update overhead - Designing multiarea OSPF with smaller areas minimizes processing and memory requirements.
        
        Smaller routing tables - Tables are smaller because there are fewer routing table entries. This is because network addresses can be summarized between areas. Route summarization is not enabled by default.
- - - - In the Exchange state, the two routers exchange one or more DBD packets. A DBD packet includes information about the LSA entry header that appears in the LSDB of the router. The entries can be about a link or about a network. Each LSA entry header includes information about the link-state type, the address of the advertising router, the cost of the link, and the sequence number. The router uses the sequence number to determine the newness of the received link-state information.
    - - The router compares the information received with the information it has in its own LSDB. If the DBD packet has a more current link-state entry, the router transitions to the Loading state.
    - - In the ExStart state, the two routers decide which router will send the DBD packets first. The router with the higher router ID will be the first router to send DBD packets during the Exchange state.
  - - - The 7 states os OSPF operation
  - - - R2 receives the Hello packet from R1 and adds the R1 router ID to its neighbor list. R2 then sends a Hello packet to R1. The packet contains the R2 Router ID and the R1 Router ID in its list of neighbors on the same interface.
    - - R1 receives the Hello and adds the R2 Router ID to its list of OSPF neighbors. It also notices its own Router ID in the list of neighbors of the Hello packet. When a router receives a Hello packet with its Router ID listed in the list of neighbors, the router transitions from the Init state to the Two-Way state.
        
        If the two adjacent neighbors are interconnected over a point-to-point link, then they immediately transition from the Two-Way state to the ExStart state.
        
        If the routers are interconnected over a common Ethernet network, then a designated router DR and a BDR must be elected.
    - - When OSPFv2 is enabled, the enabled interface transitions from the Down state to the Init state. The Router 1 (R1) starts sending Hello packets out all OSPF-enabled interfaces to discover OSPF neighbors to develop adjacencies with.
    - - Because R1 and R2 are interconnected over an Ethernet network, a DR and BDR election takes place. R2 becomes the DR and R1 is the BDR. This process only occurs on multiaccess networks such as Ethernet LANs. Hello packets are continually exchanged to maintain router information.
- - - - Discover OSPF neighbors and establish neighbor adjacencies.
        
        Advertise parameters on which two routers must agree to become neighbors.
        
        Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks like Ethernet. Point-to-point links do not require DR or BDR.