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Stub Default Metrics 487
Manual Memory
Allocation
You can manually control the OSPF current partition maximum size. You
can enter any value between 4096 and the maximum memory available
on your system, as shown in the
ip ospf partition modify
command
prompt.
You can also use manual memory allocation control to lower the OSPF
current maximum partition size to be less than the 4,200,000 default
minimum on extended memory systems. As noted previously, memory
reserved under the OSPF current maximum partition size is not available
to other protocols even if it is not allocated. If you must carefully
apportion memory among competing protocols, then you might want to
decrease the memory available to OSPF. A router located in a stub area
has no external link state advertisements (LSAs), for example, and might
require less memory.
System Memory
Allocation
You can also have OSPF use the system memory partition. There will be
no specific OSPF memory partition and no current maximum partition
size. OSPF will grow as it finds necessary, possibly encroaching upon the
space available to other protocols.
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 54 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.