Lenovo Flex System Fabric CN4093, Application Manual

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CN4093 Application Guide for N/OS 8.4
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 CN4093 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 49 ), 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 49.
Injecting Default Routes
If the switch is in a transit area and has a configured default gateway, it can inject a
default route into rest of the OSPF domain. Use the following command to
configure the switch to inject OSPF default routes:
In the command above, <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.
When the switch is configured to inject a default route, an AS ‐ external LSA with
link state ID 0.0.0.0 is propagated throughout the OSPF routing domain. This LSA
is sent with the configured metric value and metric type.
The OSPF default route configuration can be removed with the command:
CN 4093(config-router-ospf)# default-information
<metric value> <metric type>
CN 4093(config-router-ospf)# no default-information
Stub Area
IF 1 IF 2
ASBR to
external networks
Default
route
IR
Area 1
Backbone
Area 0
Stub Area
Area 2
Priority default route
Priority
default route
ABR
IR
Metric:
900
Metric:
10
Metric:
201