ProLiant BL p-Class GbE2 Interconnect Switch Application Guide

OSPF 117
Default routes
When an OSPF routing device encounters traffic for a destination address it does not recognize, 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 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 the figure below), 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 automatic, no further configuration is required for such areas.
Figure 19 Injecting default routes
In more complex OSPF areas with multiple ABRs or ASBRs (such as area 0 and area 2 in the figure), 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:
>> # /cfg/l3/ospf/default <metric value> <metric type (1 or 2)>
where <metric value> 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/l3/ospf/default none
Virtual links
Usually, all areas in an OSPF AS are physically connected to the backbone. In some cases where this is not
possible, you can use a virtual link. Virtual links are created to connect one area to the backbone through
another non-backbone area.
The area which contains a virtual link must be a transit area and have full routing information. Virtual links cannot
be configured inside a stub area or NSSA. The area type must be defined as transit using the following
command:
>> # /cfg/l3/ospf/aindex <area index>/type transit