Route filtering in OSPF

Hello!

Below you will wild the procedure for OSPF route filtering

Run system-view

The system view is displayed.

Run ospf [ process-id ]

The OSPF process view is displayed.

Run filter-policy { acl-number | acl-name acl-name | ip-prefix ip-prefix-name | route-policy route-policy-name [ secondary ] } import

OSPF is configured to filter received routes.

The parameter acl-number specifies the number of a basic ACL.

The parameter acl-name acl-name specifies the name of an ACL.

The parameter ip-prefix ip-prefix-name specifies the name of an IP prefix list.

OSPF is a link-state dynamic routing protocol, with routing information carried in LSAs. Therefore, the filter-policy import command cannot be used to filter advertised or received LSAs.

The filter-policy import command is used to filter the routes calculated by OSPF. Only routes that pass the filtering criteria are added to the routing table. Routes that do not pass the filtering criteria are not added to the OSPF routing table but can be advertised.

Run commit

The configuration is committed.

Best regards!

Hi Samu90, 

The filter-policy import command configures a filtering policy to filter routes received by OSPF.\

Usage Scenario

The filter-policy import command is used to set a filtering policy for received routes. Only the routes that pass the filtering can be added to the routing table. The routes that fail to pass the filtering cannot be added to the routing table but can be advertised.

The OSPF routing information is recorded in the LSDB. Instead of filtering the received or sent LSAs, the device filters routes calculated by OSPF using the filter-policy import command.

Precautions

For an ACL configured using the acl command, when the rule command is used to configure a filtering rule, the filtering rule takes effect only when the source address range is specified by the source parameter and the time period is specified by the time-range parameter.

Creating an ACL before it is referenced is recommended. If a nonexistent ACL is referenced using the command, all routes received by OSPF are delivered to the IP routing table.

Creating an IP prefix list or route-policy before it is referenced is recommended. By default, nonexistent IP prefix lists or route-policies cannot be referenced using the command. If the route-policy nonexistent-config-check disable command is run in the system view and a nonexistent IP prefix list or route-policy is referenced using the current command, all routes received by OSPF are delivered to the IP routing table.

Hi,

OSPF route filtering is divided into two aspects: One is pure routing entries, and the other is LSA routing based on SPF calculation. Therefore, the most thorough filtering is to filter LSAs. In the following six types of filtering, only the first type of filter-policy import is used to filter routes, and the latter is used to filter LSAs.

OSPF filtering

1. filter-policy import (configured in the OSPF process)

This command can only suppress the generation of routes. Therefore, you can filter the routes regardless of the routes to be generated. The LSA flooding, however, cannot be prevented.

[R3-ospf-1]undo filter-policy ip-prefix 1 import

2. filter-policy export (configured in the OSPF process)

This command is used to filter the ASE/NSSA on the ASBR. This command is used to suppress the generation of LSAs. Once the LSA is generated, this command is useless. In addition, this command can be configured only on the ASBR and is not used on other routes, the LSA has been generated.

[R1-ospf-1]undo filter-policy ip-prefix 1 export

3. asbr-summary not-advertise (configured in the OSPF process)

This command is used to filter the ASE/NSSA generated by itself on the ASBR. In fact, the is suppressed. The local LSDB does not contain the related LSA..

[R1-ospf-1]undo asbr-summary 11.11.11.11 255.255.255.255

4. Filter import/export (configured in the area)

This command is used to filter the three types of LSAs on an ABR, regardless of whether the three types of LSAs are generated by the ABR or received by other routers. It is invalid for Type 5 LSAs.

[R3-ospf-1-area-0.0.0.0]undo filter ip-prefix 1 export

[R3-ospf-1-area-0.0.0.200]undo filter ip-prefix 2 import

5. abr-summary not-advertise (configured in the area)

This command is used to filter the three types of LSAs on an ABR. Only the three types of LSAs generated by the ABR can be filtered. The three types of LSAs received from other devices cannot be filtered. (The reason is that this command is an aggregation command. Currently, the aggregation commands of the VRP and IOS only aggregate the routes of the local area but not the inter-area routes. Therefore, the abr-summary not-advertise command is used to filter the intra-area routes of the local area.)

6. ospf filter-lsa-out is configured on the interface. 

You can filter out any type of LSA. After the configuration is complete, you need to clear the process.