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458 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
■
To a stub area — When a packet’s destination is in a stub area (an
area that does not accept external route advertisements), OSPF uses
the area’s predefined default route. You configure default routing in
area border routers that serve an OSPF stub area, such as area border
router 1 in Figure 54. For more information, see “Stub Default
Metrics” later in this chapter.
■
To a different autonomous system — When a packet’s source and
destination are in different autonomous systems, ASBRs compute the
routing paths using data obtained from another protocol, such as the
Border Gateway Protocol. The boundary routers flood these external
routes throughout all areas in the autonomous system except stub
areas.
Key Guidelines for
Implementing OSPF
Consider the following guidelines when you design a scalable and
dependable OSPF internetwork:
These parameters
must be consistent
across all routers
The following OSPF interface parameters must be consistent across all
routers on an attached network:
■
Hello interval
■
Dead interval
■
Password
Addressing scheme
The addressing structure that you implement can affect both the
scalability and the performance of your OSPF internetwork. Consider the
following guidelines when you define an addressing structure to use for
your OSPF internetwork:
■
Make the range of subnets that are assigned within each OSPF area
should be contiguous to allow optimal summarization by area border
routers (ABRs).
■
Define the address space so that you can easily add new areas,
restructure existing ones, or add additional routers as your network
grows.