OSPF - ABR has at least one interface in Area 0
By definition in OSPF, an Area Border Router (ABR) must have at least one interface in the backbone area 0. This is a requirement of OSPF design to ensure loop prevention.
This requirement has several implications:
- A router with all of its interfaces in a single area is not an ABR.
- A router may have interfaces in multiple non-backbone OSPF areas only if at least one of its interfaces is in Area 0.
- All communication between non-backbone areas must take place via the backbone area, thus such communication will take place over a ABR.
- Direct communication between non-backbone areas is not allowed, even if an ABR has a direct link to each of those non-backbone areas.
- If an ABR has no interfaces on Area 0, it may require the use of a virtual link.