3Com 4007, Руководство по внедрению, Страница: 470 / 684

470 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
When a router interface within an area first comes online, it determines if
a designated router exists for the area. If one exists, the new router
accepts the designated router regardless of its own router priority.
Therefore, if you want to change the designated router for an area,
configure the router that you want to serve as the new designated router
to have a higher priority than other routers in the same area. The next
time that the election process is initiated, the router that has the highest
router priority is elected as the designated router for the area.
Area ID The area ID associates the router interface with an OSPF area. By default,
all OSPF interfaces that you create on the system belong to the backbone
area (0.0.0.0). If you want to change this association, specify a different
area ID for any or all interfaces on the system.
Cost The interface cost parameter reflects the line speed of the port. Although
the system calculates a default cost based on the module media type, you
can set the cost manually to a different value. In most cases, you can
accept the default value that the system sets.
Specifying Cost Metrics for Preferred Paths
In OSPF, the best path is the one that offers the least-cost metric. A cost is
associated with each router output interface and each route as follows:
■
Each output interface is assigned a default cost by the system based
on the media bandwidth to which it is attached.
■
Each route is assigned a metric that is equal to the sum of all metrics
for all the links in the route.
You can configure area routers to use preferred paths by manually setting
higher cost metrics for those paths that are not preferred.
For example, the fastest default media for OSPF is FDDI. All interfaces that
are attached to a Fiber Distributed Data Interface (FDDI) media have a
default cost metric of 1. All interfaces attached to faster media types,
such as ATM or Gigabit Ethernet, are also assigned a default cost of 1. To
ensure that the Gigabit Ethernet interface is the preferred route, leave the
Gigabit Ethernet link with a metric cost of 1, and manually configure the
slower links, such as FDDI, with a higher cost metric, for example, 2.