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Overview of OSPF

OSPF protocol is a link-state protocol. Each router is responsible for discovering and maintaining the relationship with its neighbors, describing the known neighbor list and link cost LSU (Link State Update) packets, learning the network topology of the whole autonomous system through reliable flooding with other routers in the autonomous system AS (Autonomous System), and injecting routing information of other AS through the routers at the boundary of the autonomous system to get the routing information of the whole Internet. Every other specific time or when the link state changes, the LSA is regenerated, and the router advertises the new LSA through the flooding mechanism in order to achieve real-time routing updates.

OSPF (Open shortest path first)

Internal Gateway Protocol based on Link State Information (IGP Protocol)

Based on IP protocol, protocol number: 89

SPF algorithm: all routers in the OSPF area will get LSA from neighboring routers, store these LSA in LSDB, calculate the optimal path to each place, and then store the optimal path in the global routing table. In the process of calculation, the loop has been eliminated.

OSPF features can adapt to large-scale network routing changes, fast convergence speed, no routing ring support variable length subnet mask VLSM support area partition support multicast address to send protocol packets

1. OSPF four routing types describe DR when the multi-access network changes, DR is responsible for updating all other routers BDRBDR will monitor the status of DR and take over its role when the current DR fails. ABRABR is used to connect backbone areas and non-backbone areas, which can be physically connected to backbone areas. It can also be logically connected to the ASBRASBR router, which is used to inject routes learned by other routing protocols into the OSPF domain by route selection redistribution.

II. Five regions of OSPF

1. Backbone area (Area0)

2. Non-backbone areas-distinguish according to the types of roads that can be learned.

(1) Standard area

(2) Terminal area

(3) complete endings

(4) impure terminal region

Third, OSPF packet type packet type packet function Hello packet periodic transmission, used to discover, establish and maintain neighbor relational database description package (DBD) summary information link-state request packet (LSR) for requesting required LSA from neighbors link-state update packet (LSU) acknowledges receipt of LSU with details link-state acknowledgement packet (LSACK) that recovers the requested LSA with the other party Each LSA needs to be identified separately

4. OSPF network type network type hello time Dead time neighbor and adjacency broadcast 10s40s automatically establish neighbors, elect DR/BDRP2P10s40s automatically establish neighbors, do not need to vote DR/BDRP2MP30s120s manually refer to neighbors, do not need to vote DR/BDRNBMA30s120s manually refer to neighbors, elect DR/BDR

5. OSPF adjacency establishment process state process Down (invalid state) receives hello activation to int state Init (initial state) receives hello message, does not find that its own R-id2-Way (two-way communication state) receives hello message, and understands its own R-IDexstart (initial state of information exchange) interactive first DBD message in the active neighbor field, determines the master-slave relationship, and the larger R-ID is masterexchange (information exchange state) interactive DBD message. Header information with LSA loading (message loading status) when the M bit in the local DBD message is 0, the full (full adjacency state) routers establish a full adjacency state, and the LSDB synchronization completes the sixth OSPF link state.

1. The composition of link-state database

(1) each router creates a database of each interface, corresponding neighboring nodes and interface speed.

(2) each entry in the link-state database is called LSA (link-state advertisement), and there are six common LSA types.

2. Link-state advertisement (LSA) type

Type1: router LSA issued by routers in the area (all routes)

Type2: the network LSA is issued by the DR in the area (announces the network status information)

Type3: summary link advertisements from other regions issued by the network summary LSA ABR (summary switching)

Type4: ASBR summary sent by LSA ABR to advertise ASBR information (tell ASBR location)

Type5: issued by AS external LSA ABR to advertise external routes (diplomat)

Type6: an external route issued by ASBR within the external LSA NSSA area of the NSSA and used to connect to this area

VII. Working process of OSPF

8. The measure value of OSPF COST= 10 ^ 8 / BW

The shortest path strength is calculated based on the interface specified time (cost).

Interface type cost (108/BW) Fast Ethernet1Ethernet1056K1785 IX OSPF configuration commands description commands start OSPF routing process rout ospf process-id specify the interface and area where the OSPF protocol is running network address invers-mask area area-ad view routing table information (direct connection / learning) show ip route only view routes learned by OSPF show ip route ospf view OSPF protocol configuration information show ip protocol view how OSPF is configured and ABR information show ip ospf view all LSA data information show within LSDB Ip ospf database view OSPF configuration information on interfaces show ip ospf interface view status of OSPF neighbors and adjacencies show ip ospf neighbor view details of OSPF neighbors (including DR/BDR) show ip ospf neighbor detail

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