Ospf review notes

The descriptions of link status information in OSPF are encapsulated and released in LSA. Common LSA has the following types:
Router LSA (Type1): This is generated by each vro. It describes the link status and overhead of the vro and transmits the information in the origin region.
Network LSA (Type2): generated by DR. It describes the link status of all routers in this segment and is transmitted in the originating region.
Network Summary LSA (Type3): it is generated by the ABR (Area Border Router, Regional Border Router). It describes the route of a certain Network segment in the region and is advertised to other regions.
ASBR Summary LSA (Type4): A route generated by the ABR that describes the route to the ASBR (Autonomous System Boundary Router, Autonomous System border Router) and is advertised to the relevant region.
AS External LSA (Type5): A route generated by the ASBR, which is described outside of the AS (Autonomous System) and advertised to all regions (except the Stub and NSSA regions ).
NSSA External LSA (Type7): generated by asbrs In The NSSA (Not-So-Stubby Area) region. routes described outside of the AS are transmitted only in The NSSA region.
Opaque LSA: it is a proposed LSA category. It consists of standard LSA headers that follow the information of special applications and can be directly used by OSPF, or other applications can distribute information to the entire OSPF domain for indirect use. Opaque LSA is divided into three types: Type9, Type10, and Type11, with different flood regions. Among them, Opaque LSA of Type9 only carries out flooding within the local link range, the Opaque LSA of Type10 only carries out flooding in the local region, and the LSA of Type11 can carry out flooding in an autonomous system.
 
Features of each region:
Totally Stub region: Type3 default route published by the ABR is allowed, and routes between regions and external routes of the autonomous system are not allowed;
Stub region: Unlike the Totally Stub region, this region allows routes between regions;
NSSA region: different from the Stub region, this region allows the introduction of external routes of autonomous systems. ASBR publishes the Type7 LSA announcement to this region;
Totally NSSA region: Unlike the NSSA region, routes between regions are not allowed in this region.
 
 
1. Router Classification
Based on different locations in AS, OSPF routers can be divided into the following four categories:
(1) Internal Router)
All interfaces of this type of router belong to the same OSPF region.
(2) Area Border Router (ABR)
This type of router can belong to more than two regions at the same time, but one of them must be a backbone region (for details about the backbone areas, see the next section ). An API is used to connect a backbone area and a non-backbone area. It can be physically or logically connected to a backbone area.
(3) Backbone Router)
This type of router has at least one interface in the backbone area. Therefore, all the backend routers (backend routers) and the internal routers (backend routers) located in Area0 are backbone routers.
(4) Autonomous System border router ASBR
The router that exchanges routing information with other AS instances is called ASBR. ASBR is not necessarily located at the border of AS. It may be a vro in the region or an ABR. As long as an OSPF router introduces external routing information, it becomes an ASBR.