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368 C
HAPTER
14: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF)
Routing Packets
A packet’s source and destination determine the routers that move it:
■
Intraarea — When a packet’s source and destination are in the same
area, the packet is routed using internal router databases. No routers
are used outside the area.
■
Interarea — When a packet’s source and destination are in different
areas, the topology databases in the backbone area dictate the paths
that are taken between areas.
You can use virtual links to influence the routes that are taken for
interarea traffic. See “Virtual Links” later in this chapter.
■
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 70. 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.