Shulou(Shulou.com)06/01 Report--

Large-scale network construction-OSPF details II (special areas and virtual links) what is ospf? OSPF (Open Shortest Path First Open shortest path first) is an internal gateway protocol (Interior Gateway Protocol, referred to as IGP), which is used to make routing decisions within a single autonomous system (autonomous system,AS). It is an implementation of the link-state routing protocol, which belongs to the Internal Gateway Protocol (IGP), so it operates within the autonomous system. The famous Dixar algorithm (Dijkstra) is used to calculate the shortest path tree. OSPF is divided into two versions: OSPFv2 and OSPFv3, in which OSPFv2 is used in IPv4 network and OSPFv3 is used in IPv6 network. OSPFv2 is defined by RFC 2328 and OSPFv3 is defined by RFC 5340. Compared to RIP, OSPF is a link-state protocol and RIP is a distance vector protocol. Router Typ

Internal Router: intra-domain router

ABR (Area Border Router): area Border Router

Backbone Router (BR): backbone router

ASBR (Autonomous System Boundary Router): autonomous system boundary router.

Type of route for OSPF:

-Internal routing-anything announced / introduced through network

# routing within the area

# routing between areas

-external routes-any announced / introduced via import-route

Entry information of OSPF's database-LSA

Type 1 LSA (router)-all intra-region

Type 2 LSA (network)

Type 3 LSA (sum-net)-all between regions

Type 4 LSA-this type of LSA must be combined with type 5 LSA to calculate routes; type 5 LSA-are external and ASE routes; type 7 LSA-are external and NSSA routes; the database organization form of OSPF: 1. It is organized with "area" as the smallest unit, which leads to: each type of LSA has a certain transmission range limit; 2. The databases of all routers in the same area are fully synchronized. # virtual link (Virtual Link) the following 2 cases require the use of a virtual link: connect to a backbone area through a non-backbone area. Connect the two sides of a segmented backbone area through a non-backbone area. Virtual link is a logical Tunnel, some rules for configuring virtual link: virtual link must be configured between 2 ABR. The area through which the virtual link passes is called Transit Area, and it must have complete routing information. Transit Area cannot be Stub Area. Avoid using virtual links as much as possible, which increases the complexity of the network and makes it more difficult to troubleshoot. # OSPF Special area-definition: if an area does not receive external routes, then this area is called a special area -function: because external routing is unstable, we configure an area as a special area only to protect it from the instability of external links. -Type 1: stub area-stub area / stub area this region does not support type 4, type 5 LSA; support type 1, type 2, type 3 LSA Note: the ABR of the stub area automatically generates a default route to the stub area represented by type 3 LSA-type 2: Totally Stub: complete stub area in order to protect this area from the instability of external links and other internal areas, we configure this area as a complete stub area. The database of all routers in this area does not contain type 3, 4, and 5 LSA; only type 1, type 2 LSA and a special type 3 LSA. These three types of LSA are automatically generated by the ABR in the Totally Stub area and are used to represent the "default route". Type 3: NSSA: not so stub area. This area can introduce external routes, but it can only exist in the form of 7 types of LSA. Type 4 and 5 LSA are not supported in this region; type 1, 2, 3, 7 LSA are supported. Therefore, this area can not be affected by the instability of "external link". We can do this when we want to "introduce external routing" in an area without being affected by other external links. Note: class 1.7 LSA can only exist in the NSSA area and will not be transmitted to the ABR in the 2.NSSA area of other areas. It will automatically convert class 7 LSA to class 5 LSA and can only do one-way translation, that is, it cannot convert class 5 to class 7. The ABR of 3.NSSA area will automatically generate a default route represented by type 7 LSA to the NSSA area. -Type 4: totally NSSA: this type of area cannot contain 3, 4, 5 can contain 1, 2, 7, and a special class 3, which represents the default route; is automatically generated by the ABR of the NSSA area

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