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232 Chapter 7: OSPF, EIGRP, and IGRP
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Determining the number of areas supported by a single router
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Determining how many routers will serve as the designated router
NOTE
When a single router is connected to more than three areas, router memory and CPU resources
might be overwhelmed. Limit the number of areas that connect to a single router to a maximum
of three. A router with a high-speed CPU and lots of memory might be able to handle
more areas.
Virtual Link
In an ideal design, all areas are physically connected to the backbone. If an area border router
goes down, an area might become physically isolated from Area 0. In this situation, a virtual
link can create a path through the transit area router to the backbone area. The ABRs at each
end of the virtual link treat the path as a point-to-point link. A virtual link can extend a backbone
area by joining two partitioned areas.
Figure 7-9 shows an example of an OSPF virtual link.
Figure 7-9
OSPF Virtual Links
NOTE
Virtual links are not recommended as part of a good CCDP OSPF design strategy. Virtual links
add a layer of complexity to the OSPF network. Use virtual links only as a last resort and only
as a temporary fix.
OSPF Area Addressing
To reemphasize a point made earlier, the processes of OSPF can put a drain on router resources.
Router memory and CPU usage are heavily utilized in large OSPF areas and domains. Using
Area 0
Area 1
Area 2
Area 0
Area 3
87200333.book Page 232 Wednesday, August 22, 2001 2:37 PM