An external route leads to a destination outside the OSPF autonomous system. An autonomous system boundary router (ASBR) advertises that route into OSPF, allowing internal routers to reach external networks. section 18.1.2
Classify OSPF routes by destination location:
- Intra-area routes reach destinations inside the router’s own OSPF area. section 18.1.2
- Inter-area routes reach destinations in an OSPF area the router does not connect to. section 18.1.2
- External routes reach destinations beyond the OSPF autonomous system and are advertised by an ASBR. section 18.5
ASBR advertising an external network
A router connected to OSPF and an external network, such as the internet, acts as an ASBR. When it advertises a route to that external destination, other OSPF routers treat the destination as an external route. section 18.1.2
Figure 18.2 A multi-area OSPF network consisting of four areas. Area 0 is the backbone area to which other areas must connect. Areas 1, 2, and 3 are non-backbone areas, and traffic between them must pass through area 0.
Use the multi-area topology to identify the ASBR and distinguish its external-network boundary from internal, backbone, and area-border roles. section 18.1.2
