The goal is to simplify routing information in the network.
Config#
- It’s a router-level configuration. (under
router ospf <process_id>) - Within the same area, different routers can have mixed and matched area types.
- Networks with an ASBR cannot be configured as Stub or Totally Stubby because these area types block external routes.
- Totally Stubby Areas have a single ABR, which serves as the sole entry/exit point to the network.
Type of Roles#
- ABR: Connects to Area 0 with at least one interface in another area.
- ASBR: Redistributes external routes into OSPF.
Area Types:
- Stub Area: Allows summary routes but denies external routes.
- Totally Stubby Area: Denies both summary and external routes.
- Not-So-Stubby Area (NSSA): Denies summary routes but allows external routes using Type 7 LSAs.
Type of Routes#
Summary Routes (Type 3):
- Generated by ABRs.
- Provide an entry point for remote areas into the network.
External Routes (Type 5 / 7):
- Generated by ASBRs.
- Provide an entry point for remote or local areas into external networks.
Routing#
- By default, summary and external routes propagate to other areas for inter-area communication.
- Summary routes from other areas are used to reach internal destinations.
- External routes from other areas are used to reach external networks.
In Stub,
- External Routes: Type 5 LSAs (external routes) are not advertised within stub areas.
- Default Routes: ABRs inject a default route (
0.0.0.0/0) into the stub area to handle routing.
Multiple ABRs#
- Each ABR advertises a default route into the stub area.
- Routers within the stub area select the best ABR to forward external traffic based on OSPF cost.
- If one ABR becomes unavailable, another ABR serves as the next hop for traffic.
Routing to External Networks#
- Routers in the stub area send traffic to ABRs using the default route.
- The ABRs forward this traffic to the appropriate external network using their complete routing tables.
OIA#
- OIA (IA) refers to routes for communication between different OSPF areas, allowing for inter-area traffic to be efficiently routed.