ABR, Area Border Router, area border router, there may be many situations in the area border router:
1. Between multiple non-backbone areas, such as between Area 1 and Area 2, such as R4 in the figure;
2. It is between the backbone area and the non-backbone area, but there are no neighbors in the backbone area. For example, ABR has an interface in Area0 but no neighbors, as shown in AR2;
3. Between the backbone area and the non-backbone area, and there are neighbors in the backbone area, this is the real ABR , such as AR3 in the figure.
Definition and role of ABR (RFC3509):
Definition: ABR is located between regional boundaries and limits the scope of LSA flooding.
Function: This area advertises and describes the networks of other areas, and performs inter-area route announcements, filtering, and aggregation.
rule:
1. ABR1 and ABR2 are the two ABRs between the backbone area Area 0 and the common area Area 1. They enter the LSA3 route of the common area 1 through ABR1. If ABR2 has an OSPF neighbor in Area0, the LSA3 route does not enter the routing table of ABR2 ( That is, it will not be advertised to enter Area0 through Area1, and ABR2 can only learn the route through LSA1 or LSA2 of Area0);
2. If ABR2 has no neighbors in the backbone area Area 0, and only one network appears in the backbone area, the LSA3 advertised by ABR1 can enter the routing table of ABR2;
3. Routes that do not appear in the ABR routing table will not be advertised to other areas. This is the vector characteristic of the boundary .
The above rule is to avoid inter-area loops (a route that enters a common area through an ABR and then enters other areas through the ABR). This is the horizontal division rule between areas . The above rules apply to LSA3, LSA4, but not LSA5
As shown above:
1. Does the 10.2.0.0/24 route of AR1 appear in AR2?
Yes, according to Rule 2, 10.2.0.0/24 can appear in AR2 because AR2 is not a real ABR.
2. Does the 10.1.0.0/24 route of AR2 appear in AR1 and AR3?
Neither. According to rule 1, AR3 has neighbors in the backbone area Area 0, and only accepts LSA3 from the backbone area. It will not receive LSA3 10.1.0.0/24 routes from the non-backbone area AR2 that access the backbone area through the non-backbone area. , AR1 did not change the route. According to the vector characteristics of rule 3, AR3 does not have this route, and it will not be advertised to other areas as an ABR. Of course, the route will appear on AR4 .
3. AR4 is not a real ABR, so it can receive routes from other areas, but will not advertise routes to other areas.