Configuring OSPF to Advertise the Default Route to an OSPF Area
Context
On an OSPF network, multiple switches often reside on the area border and AS border for next-hop backup or traffic load balancing. To reduce routing entries and improve resource usage on the OSPF network, a default route can be configured in the following scenarios:
- An ABR advertises a Type 3 LSA carrying the default route for guiding switches to forward inter-area packets.
- An ASBR advertises Type 5 or Type 7 LSAs carrying the default route for guiding switches to forward AS external packets.
When no matching route is discovered, the switch can forward packets through the default route.
The preference of the default route in Type 3 LSAs is higher than that of the route in Type 5 or Type 7 LSAs.
Default route advertisement is determined by the type of the area to which the default route is imported. Table 5-20 lists the default route advertisement by area type.
Area Type |
Generated By |
Advertised By |
LSA Type |
Flooding Area |
|---|---|---|---|---|
Common area |
The default-route-advertise command |
ASBR |
Type 5 LSA |
Common area |
Stub area |
Automatically |
ABR |
Type 3 LSA |
Stub area |
NSSA |
The nssa [ default-route-advertise ] command |
ASBR |
Type 7 LSA |
NSSA |
Totally NSSA |
Automatically |
ABR |
Type 3 LSA |
NSSA |
Perform the following steps on the ASBR running OSPF.
Procedure
- Configure OSPF to advertise the default route to the OSPF
area.
- Run the following commands as required:
Run:
default-route-advertise [ [ always | permit-calculate-other ] | cost cost | type type | route-policy route-policy-name [ match-any ] ] *
OSPF is configured to advertise the default route to the OSPF area.
- always indicates that an LSA describing the default route is generated and then advertised regardless of whether there are active default routes of other OSPF processes in the routing table of the local device.
- permit-calculate-other indicates that the local router is allowed to calculate the default routes advertised by other switches after adverting its default route.
- route-policy route-policy-name indicates that the local device advertises default routes according to the parameters of the configured routing policy when there are matched default routing entries generated by other OSPF processes.
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NOTE: - An ASE LSA that describes the default route is generated and then advertised only when there are active default routes of other OSPF processes in the routing table of the local device.
- Before advertising a default route, OSPF compares the preferences of default routes. Therefore, if a static default route is configured on an OSPF switch, to add the default route advertised by OSPF to the current routing table, ensure that the preference of the configured static default route is lower than that of the default route advertised by OSPF.
- Run the following commands as required:
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