3Com 4007, Implementation Manual, Page: 466 / 684

466 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
■
Whenever there is a change in network topology (such as when a link
is lost or comes online), routers in all affected areas must converge on
the new topology. If your internetwork consists of unstable links, you
can partition the AS into smaller areas to minimize the number of
areas that affected when the topology changes as a result of those
unstable links.
Stub areas
■
Because area border routers do not advertise external routes into stub
areas, configuring an area as a stub area helps to minimize router
table sizes, and therefore the memory requirements, of the routers
within the area. Routers within a stub area need to store only a single
default external route for destinations outside the autonomous
system, as well as for intra-area and inter-area routes.
■
If your network has no external routes, there is no advantage to
configuring a stub area.
■
Stub areas cannot contain autonomous system boundary routers
(ASBRs)
Backbone area
A stable, fault-tolerant backbone is vital to your OSPF internetwork. It
ensures communication between all areas within the AS. Consider the
following guidelines when you design the backbone area:
■
If you have only one area in your autonomous system, then you do
not need to configure a backbone area (0.0.0.0).
■
A backbone area must have the area ID of 0.0.0.0. Routers in the
backbone area must be able to communicate with each other through
interfaces that are configured in area 0.
■
Connect all area border routers to the backbone area with either
physical or virtual links.
■
Backbones must be contiguous, meaning all area border routers
(ABRs) that comprise the backbone must have a direct path to all
other ABRs that are attached to the backbone.
■
Configure ABRs with high redundancy so that no single link failure can
cause a partition in the backbone.
■
The more areas that the backbone interconnects, the greater the
volume of traffic that must travel over the backbone. To increase
stability, keep the size of the backbone reasonable.