What are an ABR and an ASBR, and can one router be both?
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An ABR has interfaces in area 0 and at least one other area. It keeps a separate LSDB per area and generates Type 3 and Type 4 LSAs between them. An ASBR redistributes routes from outside OSPF, such as static, connected or BGP, and generates Type 5 (or Type 7 in an NSSA). One router can be both.
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Roles
| Role | Defined by | Originates |
|---|---|---|
| Internal router | All interfaces in one area | Type 1, Type 2 if DR |
| Backbone router | At least one interface in area 0 | Type 1, Type 2 if DR |
| ABR | Interfaces in area 0 and another area | Type 3, Type 4, Type 1 per area |
| ASBR | Redistributes into OSPF | Type 5, or Type 7 inside an NSSA |
How to see them
R2# show ip ospf border-routers
OSPF Router with ID (2.2.2.2) (Process ID 1)
Base Topology (MTID 0)
Internal Router Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 3.3.3.3 [1] via 10.0.23.3, GigabitEthernet0/0/1, ASBR, Area 1, SPF 7
i 1.1.1.1 [1] via 10.0.12.1, GigabitEthernet0/0/0, ABR, Area 0, SPF 9show ip ospf also prints "It is an area border router" and "It is an autonomous system boundary router".
A gotcha
A router with interfaces in two non-backbone areas and none in area 0 is not a true ABR. It does not pass summaries between those areas.
They will ask next
How does a router in area 2 know the path to an ASBR that lives in area 1?
Through the Type 4 LSA. The ABR advertises the ASBR's router ID and its cost to reach it, so routers outside the ASBR's area can compute the cost to the external routes.
Traps
- Saying an ASBR must sit in area 0. It can be in any area except a stub or totally stubby area.