AS: Autonomous system, a collection of routers running the same protocol in a common administrative domain environment
Interior Gateway Protocol: A protocol for communication between routers within a single autonomous system (IGP)
Exterior Gateway Protocol: Communication Protocol (EGP) for routers between multiple Autonomous Systems (AS)
The three tables of OSPF:
Neighbor List: List all neighbor routers that each router has established adjacency relationship with
Link State Database: Displays the topology of the entire network
Routing table: routing table derived from the shortest path tree algorithm
Backbone area: represented by Area 0, used for routing information transfer between areas, responsible for connecting the core of other areas
Non-backbone area: Except for Area 0, all others are non-backbone areas (be sure to ensure that there must be a backbone area in an AS)
Standard area: can receive various link state information and summary route advertisements
Stub area: do not learn autonomous system external routes
Completely stubby area: not only does not learn autonomous system external routers, but also does not learn according to meet routers
Impure peripheral region: ASBR has a special aspect of the peripheral region
Router ID: The unique identifier of the router running the OSPF routing protocol
DR: A designated router elected on the network segment. Other routers only need to establish adjacency with the designated router and transmit link state information.
BDR: When the designated router fails, a new router is selected to immediately take over the work of the designated router to ensure the normal operation of the network.
OSPF packet types (there are five)
Hello package: used to discover and maintain neighbor relationships, elect DR and BDR
Database Description Packet (DBD): used to send summary information to neighbors to synchronize the link state database
Link state request packet (LSR): After receiving the database description packet, it is necessary to request detailed information
Link Status Update Packet: Sends detailed link status (mainly LSA)
Link Status Acknowledge Packet: Acknowledges that a link status update has been received
Seven states of OSPF
1. Down state: OSPF has not been enabled, and the router has not exchanged information with its neighbors
2. Initial state (init): the process in which each interface of the router sends Hello packets through the 224.0.0.5 multicast address
3. Two-way state (2-way): conduct DR and BDR elections
4. Quasi-start state (Exstart): DR and BDR establish adjacency with other routers and send database descriptions
5. Exchange state (Exchange): In this state, ensure that all routers receive the database description message
6. Loading: Send link status request and link status update message
7. Full adjacency (Full): The router will complete the synchronization of the database, and the link state database has reached full consistency
Three kinds of traffic of OSPF
Intra-area communication: traffic consisting of packets exchanged by routers within a single area
Inter-domain traffic: refers to the traffic composed of data packets exchanged between routers in different areas
External traffic: traffic consisting of packets exchanged between routers in one autonomous system and routers in another autonomous system
Three OSPF routers
Internal router: all connections belong to only one area
Area Border Router (ABR): A router responsible for connecting one or more areas to the backbone area
Autonomous System Border Router (ASBR): A router responsible for connecting inside an autonomous system to another autonomous system
Six common LSAs
1. LSA1: Router LSA: Each router in the OSPF area will generate
2. LSA2: Network LSA: generated by the DR in the area
3. LSA3: Network summary LSA: generated by ABR
4, LSA4: ASBR summary LSA: generated by ABR, responsible for advertising ASBR information
5. LSA5: AS external LSA: generated by ASBR, responsible for advertising external routes
6. LSA7: NSSA (non-pure stub area) external LSA: generated by ASBR in the NSSA area, advertising NSA external routes
NSSA area: a special stub area, in which there is an ASBR router, and a special link state advertisement LSA7 appears in this area (the external LSA of the NSSA area can only exist in a non-pure stub area)
Priority of OSPF route types: Intra-area path (O) > Inter-area path (OIA) > E1's external path (OE1) > E2's external path (OE2)