Nortel Networks WEB OS 212777 - Manual (Page 78)

Web OS 10.0 Application Guide
78 n Chapter 4: OSPF
212777-A, February 2002
Default Routes
When an OSPF routing device encounters traffic for a destination address it does not recog-
nize, it forwards that traffic along the default route. Typically, the default route leads upstream
toward the backbone until it reaches the intended area or an external router.
Each Web switch acting as an ABR automatically inserts a default route into each attached
area. In simple OSPF stub areas or NSSAs with only one ABR leading upstream (see Area 1 in
Figure 4-3 ), any traffic for IP address destinations outside the area is forwarded to the switch’s
IP interface, and then into the connected transit area (usually the backbone). Since this is auto-
matic, no further configuration is required for such areas.
Figure 4-3 Injecting Default Routes
In more complex OSPF areas with multiple ABRs or ASBRs (such as area 0 and area 2 in Fig-
ure 4-3 ), there are multiple routes leading from the area. In such areas, traffic for unrecognized
destinations cannot tell which route leads upstream without further configuration.
To resolve the situation and select one default route among multiple choices in an area, you can
manually configure a metric value on each ABR. The metric assigns a priority to the ABR for
its selection as the priority default route in an area. The following command is used for setting
the metric value:
where sets the priority for choosing this switch for default route. The value
none sets no default and 1 sets the highest priority for default route. Metric type determines
the method for influencing routing decisions for external routes.
To clear a default route metric from the switch, use the following command:
>> # /cfg/ip/ospf/default
>> # /cfg/ip/ospf/default none
IF 2 IF 1
IF 1 IF 2
IF 1 IF 2
Backbone Stub Area Stub Area
Area 2 Area 1 Area 0
ABR
ABR
ASBR to
External Networks
IR
ABR
Metric:
201
Metric:
10
Default
Route
Priority
Default
Route
Priority
Default
Route
IR
Metric:
900
Metric:
201
Metric:
200