OSPF Basic Input

1. Administrative Distance
        * The administrative Distance for OSPF is 110
        * OSPF has 3 types of routes
                Intra-Area (same area route)
                Inter-Area (different area route)
                External Routes (Routes from other Protocols or from other OSPF processes)
        * OSPF uses 110 AD value default for all these 3 types of routes
        * Locally we can modify the AD value as per requirement for each type of route

2. Multicast address
        * OSPF uses 2 different Multicast addresses to forward the packet in the domain
        * 224.0.0.5 is used by
                all the routers to send hello packets
                DR router to send the database to the other routers (in the future we will talk about Router types)
        * 224.0.0.6 is used by
        * All the routers to send the database to the DR router (in the future we will talk about Router types)

3. Protocol Number
        * OSPF is a Layer 3 protocol
        * In the IP header, it uses Protocol number 89
        * Whenever we see protocol number 89 in any IP header so that packet is actually carrying the OSPF data

4. Timers
        * Hello(10 sec): after every 10-sec routers send hello packets to each other
        * Hold(40sec): if a router did not get any hello packet from the neighbor router till the next 40 sec it considers that neighbor is down and breaks the neighborship

5. Cost
        * OSPF uses Metric calculation called Cost
        * The route with the lowest cost will be installed in the routing table as the best path
        * By default router use (10*5/bandwidth) to find the cost for each route
        * The interfaces higher than 100mbps bandwidth will have the same cost
        * To calculate the cost for the higher link we use the OSPF reference cost mechanism(we will discuss in the future)

6. Link State Routing Protocol
        * OSPF is Link State Routing Protocol
        * It uses interfaces properties to find the best path

7. SPF Shortest Path First
        * OSPF uses SPF to do all the best path calculation
        * It uses SPF to find the best path between multiple Paths
        * SPF is triggered in an area when there is a change in topology or has added a new subnet in the network

8. Authentication
        * OSPFv2 supports cleartext, Message Digest 5 (MD5), and cryptographic SHA authentication. OSPFv3 relies on IPsec for authentication and encryption with AH and ESP, as well as authentication using cryptographic SHA