Introduction to routing OSPF, the difference between OSPF and other routes, the concept of OSPF area, and the concepts of OSPF routing pronouns IR, BR, ABR, and ASBR.

4.2.0 Routing OSPF (Introduction, OSPF and static, OSPF area concept, OSPF routing synonym)

Introduction to OSPF

OSPF Open Shortest Path First (Open Shortest Path First)

The OSPF dynamic routing protocol belongs to the interior gateway protocol (IGP) 链路状态路由协议and works inside a single autonomous system (AS).

The difference between OSPF and other routes

1. Support VLSM variable length subnet mask.

2. Network reachability . RIP is limited to 15 hops, so RIP cannot support larger networks. OSPF has no limit by transmitting link state information.

3. More route selection methods , RIP selects routes based on the number of hops may not necessarily select the optimal route. OSPF is determined by the cost of the link, and this method can select the optimal route.

4. Generate less routing information . RIP will broadcast all its routing table information every 30 seconds, so it will consume a lot of bandwidth and affect user traffic. OSPF only transmits the changed routing part, and only when the network changes, it saves a lot of bandwidth.

5. Dynamic routing generation . Compared with static routing, OSPF is a dynamic routing protocol that can automatically generate routing entries. Dynamically generated routes do not require manual protocol configuration again due to link changes.

OSPF area concept

When OSPF configures routing protocols, it needs to divide the logical network topology into areas, which mainly fall into 骨干区域、非骨干区域、特殊区域three categories .

So what is the role of dividing the area?

1. The multi-area design enables OSPF to support larger-scale networks .

2. The area divides OSPF routes into different groups , and different groups are identified by area IDs to reduce the number of devices in the area (the size of the LSDB database). When a device fails, the range of influence is within the area and will not affect devices outside the area. .

A macroscopic understanding is to divide a large network into small regional networks for management. When each small regional network fails, it will not affect other regional networks, which greatly improves the stability of the network.

OSPF routing synonym concept

In an OSPF network, each router has its own pronoun: IR, BR, ABR, and ASBR.

[A router can have multiple code names, ABR can also be BR, ASBR...]

• IR - (Internal router)Intra-area router , an OSPF router with all OSPF-enabled interfaces in the same area

• BR - (Backbone router) backbone router , as long as there is one interface belonging to the router in area 0

• ABR - (Area border router)Area Border Router , a router that connects multiple areas but has at least one interface belonging to area 0

  • Not all routers at area borders are ABRs:
  • 1. Pseudo-ABR , the device exists in multiple areas, and one area is the device in the Area0 backbone area.
  • 2. True ABR , multiple interfaces link multiple areas, and there is one 活跃接口位于Area0骨干区域内device

• ASBR—— (Autonomous system border router)Autonomous System Boundary Router , which imports other protocol routes/introduces routes of other OSPF processes to the router of this AS** (router bordering other routing protocols that are not in the same process)**