3Com CoreBuilder 3500, Implementation Manual

Page: 400 / 592

400 C
HAPTER
14: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF)
Stub Default
Metrics
Generally, a stub area is a network that is connected to an OSPF routing
domain by a single area border router (ABR). External link state
advertisements are not advertised into stub areas. Instead, the ABR injects
a Type 3 summary link state advertisement that contains a single external
default route into the stub area. The routers within the stub area use this
single external route to reach all destinations outside the stub area. This
arrangement saves routing table space and system resources because
stub area routers do not have to learn a multitude of external routes for
the greater network; they need only store a single external route.
The stub default metric determines whether an ABR generates the
default route into the stub area to which it is connected, and the cost
associated with that route.
For example, in Figure 70 earlier in this chapter, you would configure area
border router 1 to generate a default route into stub area 2. If you define
a stub default metric of 4, area border router 1 will generate a default
route with an associated cost of 4 into stub area 2.
If you remove the stub default metric, the ABR does not advertise a
default route into the stub area.
A stub area can have multiple ABRs and multiple exit points. However, all
of the exit points and routers must contain the same external routing
data so that the choice of an exit point does not need to be made for
each external destination.
Important
Considerations
Consider the following guidelines when you define stub default metrics:
■
By default, area border routers advertise a stub default metric of 1.
■
Stub default metrics are relevant only for area border routers (ABRs)
that are attached to stub networks.
■
If your network does not have external routes, you do not need to
configure the stub default metric; and you do not need a stub area.
■
If you remove the stub default metric, the ABR does not advertise a
default route into the stub area.