3Com 8800 Configuration Manual page 202

3Com Switch 8800 Configuration Guide
As shown in Figure 19-5: RTA and RTD are configured to belong to only one area,
whereas RTB (area0 and area1) and RTC (area1 and area 2) are configured to belong
to two areas. In which, RTB also belongs to area0, which is compliant with the
requirement. However, none of the areas to which RTC belongs is area0. Therefore, a
virtual link should be set up between RTC and RTB. Ensure that area2 and area0
(backbone area) is connected.
RTA RTA
Figure 19-5 OSPF areas
The backbone area (area 0) cannot be configured as the stub area and the virtual
link cannot pass through the stub area. That is, if a virtual link has been set up
between RTB and RTC, neither area1 nor area0 can be configured as a stub area.
In the above figure, only area 2 can be configured as the stub area.
Routers in the stub area cannot receive external routes.
The backbone area must guarantee the connectivity of all nodes.
area0 area0 area1 area1
RTB RTB
19-34
Chapter 19 OSPF Configuration
area2 area2
RTC RTC RTD RTD