You can use the following features to control the advertising and receiving of OSPF routes:
Route summarization enables a device to summarize routes with the same prefix into a single route and to advertise only the summarized route to other areas. Route summarization reduces the size of a routing table and improves device performance.
OSPF can use routing policies to filter routes. By default, OSPF does not filter routes.
Set the maximum number of external routes supported by the LSDB to dynamically limit the OSPF LSDB's size.
When a large OSPF network is deployed, an OSPF routing table includes a large number of routing entries. To accelerate route lookup and simplify management, configure route summarization to reduce the size of the OSPF routing table. If a link frequently alternates between Up and Down, the links not involved in the route summarization are not affected. This prevents route flapping and improves network stability.
OSPF supports two route summarization modes.
When an ABR transmits routing information to other areas, it generates Type 3 LSAs by network segment. If consecutive network segments exist in this area, you can summarize these network segments into a single network segment. The ABR generates one LSA for the summary network segment and advertises only that LSA.
If route summarization has been configured and the local router is an ASBR, the local router summarizes imported Type 5 LSAs within the summary address range. If an NSSA has been configured, the local device also summarizes imported Type 7 LSAs within the summary address range.
If the local router is both an ASBR and an ABR, it summarizes Type 5 LSAs translated from Type 7 LSAs.
OSPF routing policies include access control lists (ACLs), IP prefix lists, and route-policies. For details about these policies, see the section "Routing Policy" in the NetEngine 8000 FFeature Description - IP Routing.
OSPF route filtering applies in the following aspects:
OSPF can import the routes learned by other routing protocols. A device uses a configured routing policy to filter routes and imports only the routes matching the routing policy. Only an ASBR can import routes, and therefore a routing policy for importing routes must be configured on the ASBR.
Advertising of imported routes
After importing routes, OSPF advertises them to its neighbors. Only an ASBR can import routes, and therefore a routing policy for the advertising of imported routes must be configured on the ASBR.
If OSPF imports a large number of external routes and advertises them to a device with a smaller routing table capacity, the device may restart unexpectedly. To address this problem, configure a limit on the number of LSAs generated when an OSPF process imports external routes.
A device uses a routing policy to filter received intra-area, inter-area, and AS external routes. The device adds only the routes matching the routing policy to its routing table. All routes can still be advertised from an OSPF routing table.
The device filters only routes calculated based on LSAs, and therefore learned LSAs are complete.
Inter-area LSA learning
An ABR in an area can be configured to filter Summary LSAs advertised to the area. The ABR can advertise only Summary LSAs, and therefore inter-area LSA learning is valid only on the ABR.
During inter-area LSA learning, the LSAs advertised to the area are directly filtered.
Inter-area LSA advertising
An ABR in an area can be configured to filter Summary LSAs advertised out of the local area. The ABR can advertise only Summary LSAs, and therefore inter-area LSA advertisement is valid only on the ABR.
OSPF requires that devices in the same area have the same LSDB. As the number of routes increase continually, some devices cannot carry excess routing information due to limited system resources. This situation is called an OSPF database overflow.
You can configure stub areas or NSSAs to prevent resource exhaustion caused by continually increasing routing information. However, configuring stub areas or NSSAs cannot prevent an OSPF database overflow caused by a sharp increase in dynamic routes. To resolve this issue, set the maximum number of external routes supported by the OSPF LSDB to dynamically limit the LSDB's size.
The maximum numbers set for all devices in the OSPF AS must be the same.
If the number of external routes in the LSDB reaches the preset maximum number, the device enters the overflow state and initiates the overflow timer. The device automatically exits the overflow state after the overflow timer expires. Table 1 lists the operations performed by the device after it enters or exits the overflow state.
OSPF Processing Procedure
Staying at overload state
Deletes self-generated non-default external routes and stops advertising non-default external routes.
Discards newly received non-default external routes and does not reply with a Link State Acknowledgment (LSAck) packet.
Checks whether the number of external routes is still greater than the configured maximum number when the overflow timer expires.
Exiting from the overflow state
Ends the overflow timer.
Advertises non-default external routes.
Accepts newly received non-default external routes and replies with LSAck packets.