OSPF all skill points:
1. OSPF is a link state protocol, its AD=110, and the COST algorithm defaults to 100M/bandwidth.
2. How to elect the RID of OSPF: the largest IP of the loopback port, without loopback, the largest IP of the physical interface, if the administrator specifies it, it means what it means.
3. How to elect OSPF DR and BDR: The highest priority is DR, and the default priority is 1. Therefore, by default, the highest RID is DR, and the second highest is BDR. If the priority is 0, it means to give up the qualification. Could be DRother. Note: Affected by DEAD time.
4. Five types of OSPF messages: HELLO, used to discover and maintain neighbor relationships, and elect DR and BDR. DBD: LSA header information, used to send link digests to neighbors. LSR: used to request the LSA needed by the neighbor. LSU: Used to send the requested LSA to the neighbor. LSACK: Used to confirm the received LSU.
5. The 7 states of OSPF: DOWN, INIT, TWOWAY, EXSTART, EXCHANGE, LOADING, FULL.
6. OSPF's 4 network types: P-to-P P-to-MP BMA NBMA
7. OSPF's 6 types of LSA:
Type 1, called router-LSA, which is generated by all routers and used to send LSA in the domain.
Type 2 is called network-LSA, which is used to send LSA by DR in the domain.
Type 3 is called network-sumary-LSA, which is used by ABR to transfer LSAs summarized by each area between two adjacent domains.
Type 4 is called ASBR-SUMARY-LSA, which is sent by the ABR to members that are not in the same area as the ASBR to indicate who is the ASBR.
Type 5 called ASBR-EXT-LSA is sent by ASBR to the entire OSPF protocol domain to describe external protocol routing, such as RIP.
Type 7 of the NSSA area is used by the ASBR to send external routes to the NSSA area.
8. Special area of OSPF: STUB, no 4/5/7. Complete STUB, no, 4/5/7, and optimize the complete 3 categories into a default 3 categories. For NSSA, there is no 4/5, and there is one more category 7. Complete NSSA, there is no 4/5, and there is only one default category 3.
9. The E1 route refers to the external type 1 route. The metric value of this route is 20 by default, and it is accumulated hop by hop in the OSPF domain.
The E2 route is an external type 2 route. The metric of this route is fixed at 20 by default and does not accumulate. N1 and N2 are the same as E1 and E2. The difference is that E1 and E2 are obtained through 5 types of LSA learning, and N1 and N2 are obtained through 7 types.
10. Network summary: used on ABR, area 1 range net mask used summary-address net mask on ASBR
11. When an area is not directly connected to area 0, a virtual link is needed to exchange link state information normally. The method is to configure area 1 virtual-link 4.4.4.4 on the two adjacent ABRs. Zone 1 refers to the middle zone number.