OSPFv2 type 3 LSA filtering

OSPFv2 type 3 LSA filtering provides an ABR that is running the OSPFv2 protocol with the ability to filter type 3 link-state advertisements (LSAs) that are sent between different OSPFv2 areas. Filtering of routes can be defined using prefix-list filters to either permit or deny certain prefixes. Only specified prefixes can be sent from one area to another area and all other prefixes are prohibited. OSPFv2 type 3 LSA filtering can be applied for the LSAs coming into a specific OSPFv2 area or going out of a specific OSPFv2 area, or into and out of the same OSPFv2 areas concurrently. Any change in the prefix-list used for type 3 filtering may result in the advertisement of new summary LSAs or the withdrawal of previously advertised summary LSAs.

Type 3 LSAs refer to summary links and are sent by ABRs to advertise destinations outside the area. OSPFv2 type 3 LSA filtering gives the administrator improved control of route distribution between OSPFv2 areas.

In certain situations, reachability for some IP network prefixes from outside of an area or to a specific area should be restricted. Such a situation is illustrated in the figure below where a device called R3 is advertising stub networks that belong to the non-backbone area (Area1). An ABR, R2, will generate summary routes for these prefixes. If OSPFv2 type 3 LSA filtering is not configured, all the stub networks are seen as summary LSAs in the backbone area (Area 0) and are flooded to all applicable OSPFv2 devices. These type 3 LSAs can be filtered out using the OSPFv2 type3 LSA Filter.

Usage and configuration guidelines

OSPFv2 type 3 LSA filtering is only applicable to ABRs. Configurations are accepted prior to the device becoming an ABR but OSPFv2 type 3 LSA filtering only occurs once the device becomes an ABR.
When OSPFv2 type 3 LSA filtering is enabled in the “in” direction, all type 3 LSAs originated by the ABR to this area are filtered by the prefix list, based on information from all other areas. Type 3 LSAs, originated as a result of the area range command in a different area, are treated like any other individuallly originated type 3 LSA. Any prefix that does not match an entry in the prefix list is implicitly denied.
When OSPFv2 type 3 LSA filtering is enabled in the “out” direction, all type 3 LSAs advertised by the ABR are filtered by the prefix list, based on information from this area to all other areas. If the area range command has been used to configure type 3 LSAs for this area, these type 3 LSAs that correspond to these configurations are treated like any other type 3 LSA. Prefixes that do not match are implicitly denied.
Type 3 LSAs received from other devices are not filtered out. The OSPFv2 type 3 LSA filter is only applied when the ABR generates summary routes to advertise.
If a prefix-list used in type 3 LSA filtering is not created or defined, all summary LSAs are discarded.
OSPFv2 type 3 LSA filtering is supported for both default VRFs and non-default VRFs.

Table 1. Behavior for prefix list configurations
IP prefix list	OSPF area prefix list	Event	Filtering done
XXX	Not defined	None	No (permit all)
Not defined	Defined	None	Yes (deny all)
Not defined	Defined	IP prefix list defined	Recalculation
Defined (no rules configured)	Defined	None	Implicit deny (deny all)
Defined (rules configured)	Defined	IP prefix list deleted	Recalculation and deny all
Defined (rules configured)	Defined	IP prefix list rule added or modified or deleted	Recalculation
Defined (rules configured)	Defined	Area prefix list deleted	Recalculation and permit all