3Com 3C13636 Configuration Manual page 639

3Com Router 3000 Ethernet Family
Configuration Guide
As is shown in the following figure, only an area is configured in RTA and RTD, but two
areas are configured in RTB (area0, area1) and RTC (area1, area2) respectively. In
which, RTB has an area with the ID of 0, so it meets the requirement; while both areas
of RTC are not 0, and accordingly it is necessary to create a virtual connection between
RTC and RTB. Ensure that Area 2 and Area 0 (the backbone area) be interconnected.
RTA
Figure 4-11 OSPF area
A virtual connection cannot go across a STUB area. The backbone area (Area 0)
cannot be configured into a STUB area either. In other words, if a virtual
connection is configured between RTB and RTC, neither Area 1 nor Area 0 can be
configured into a STUB area. In the above figure, only Area 2 can be configured
into a STUB area.
The router in the STUB area cannot receive external routes.
The backbone area should guarantee the connection between various nodes.
Area 0
RTB
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Chapter 4 OSPF Configuration
Area 1 Area 2
RTC RTD