3Com corebuilder 3500 Implementation Manual page 378

378 C 14: O
HAPTER PEN
S P F (OSPF)
HORTEST ATH IRST
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.
Virtual links cannot run across stub areas.
Avoid placing hosts, such as workstations, servers, and other shared
resources, within the backbone area.
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.