Web OS 10.0 Application Guide78 n Chapter 4: OSPF 212777-A, February 2002Default RoutesWhen 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 upstreamtoward 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 attachedarea. In simple OSPF stub areas or NSSAs with only one ABR leading upstream (see Area 1 inFigure 4-3), any traffic for IP address destinations outside the area is forwarded to the switch’sIP 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 RoutesIn 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 unrecognizeddestinations 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 canmanually configure a metric value on each ABR. The metric assigns a priority to the ABR forits selection as the priority default route in an area. The following command is used for settingthe metric value:where sets the priority for choosing this switch for default route. The valuenone sets no default and 1 sets the highest priority for default route. Metric type determinesthe 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 noneIF 2 IF 1IF 1 IF 2IF 1 IF 2Backbone Stub AreaStub AreaArea 2Area 1 Area 0ABRABRASBR toExternal NetworksIRABRMetric:201Metric:10DefaultRoutePriorityDefaultRoute PriorityDefaultRouteIRMetric:900Metric:201Metric:200
