3Com 8807 Configuration Manual page 337

Figure 86 OSPF areas
area0 area0
RTA RTA RTB RTB
The backbone area (Area 0) cannot be configured as the stub area and the
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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 figure above, only Area 2 can be configured as the stub area.
Routers in the stub area cannot receive external routes.
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The backbone area must guarantee the connectivity of all nodes.
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Troubleshooting OSPF Faults
area1 area1 area2 area2
RTC RTC
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RTD RTD