Lucent Technologies Stinger IP2000, Руководство по настройке

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6-4 Stinger® IP2000 Configuration Guide
OSPF Configuration
Overview of OSPF features supported by the IP2000
To reduce the number of adjacencies each router must form, OSPF calls one of the
routers the designated router. As routers begin to form adjacencies, they elect a
designated router and then all other routers on the network establish adjacencies,
primarily with the designated router. This process simplifies the routing table update
procedure and reduces the number of link-state records in the database. The
designated router plays other important roles as well to reduce the overhead of OSPF
link-state procedures.
To prevent the designated router from becoming a serious liability to the network if it
fails, OSPF routers also elect a backup designated router at the same time. Other
routers maintain adjacencies with both the designated router and its backup, but the
backup router leaves as many of the processing tasks as possible to the designated
router. If the designated router fails, the backup immediately becomes the designated
router and a new backup is elected.
You choose the designated router on the basis of the processing power, speed, and
memory of the system, then assign priorities to other routers on the network, in case
both the designated and backup designated routers fail.
Note
The Stinger unit can function as a designated router or backup designated
router. However, many sites choose to assign a LAN-based router for these roles to
dedicate the Stinger unit to WAN processing.
Routing across NBMA interfaces
An OSPF nonbroadcast multiaccess (NBMA) network is any network that has
multiple points of access (more than two routers) and does not support broadcast
capability. OSPF routers operate on an NBMA network much as they do on a
broadcast network, by using the Hello protocol to form adjacencies and identify the
designated router. However, because the routers cannot discover their neighboring
routers dynamically by means of broadcasts, you must specify some additional
parameters. For Stinger units, a WAN link is always point-to-point. However, you can
configure the system to interact with NBMA routers via a virtual circuit across an
ATM trunk interface.
Configurable cost metrics
You assign a cost to the output side of each router interface. The lower the cost, the
more likely the interface is to be used to forward data traffic. Costs can also be
associated with the externally derived routing data.
The OSPF cost can also be used for preferred-path selection. If two paths to a
destination have equal costs, you can assign a higher cost to one of the paths so it can
be a backup to be used only when the primary path is not available.
Figure 6-2 shows how costs are used to direct traffic over high-speed links. For
example, if Router-2 in Figure 6-2 receives packets destined for Host B, it routes
them through Router-1 across two T1 links (
cost=20 ) rather than across one 56Kbps
B channel to Router-3 ( cost=240 ).