3Com 4007, Implementation Manual, Page: 467 / 684

Areas 467
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Because all routers connected to the backbone (ABRs) must
recompute routes whenever the topology changes for any link in the
AS, keeping the size of the backbone to a minimum is especially
important in an autonomous system that may contain unstable links.
At the very least, reducing the number of areas that connect a
backbone directly reduces the likelihood of link-state change.
■
Keep the maximum number of routers in the backbone area to about
50 or so, unless the link is of particularly high quality and the number
of routes is minimal.
■
Every ABR must connect to the backbone; this connection can be
physical or virtual. If a router has an OSPF neighbor that is physically
connected to the backbone, the router can use that neighbor to
establish a virtual link to the backbone. Do not use too many virtual
links to connect ABRs for the following reasons:
■
Stability of the virtual link depends on the stability of the
underlying area that it spans.
■
This dependency on underlying areas can make troubleshooting
difficult.
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Virtual links cannot run across stub areas.
■
Avoid placing hosts, such as workstations, servers, and other shared
resources, within the backbone area.
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Having more than one ABR per area reduces the chance that the area
will disconnect from the backbone.
■
A single ABR can connect one or more areas to the backbone. To
maximize stability, a single ABR should support no more than three
areas because the router must run the link-state algorithm for each
link-state change that occurs for every area to which the router
connects.