11.1.4 LSA ads

<Structure of this section>

(1) LSA type

Routing updates for OSPF are called link state advertisements (LSA).

There are three main LSA types:

(a) Intra-area routing information

Announces the status of links to routers and networks for use by the SPF algorithm.

(b) Inter-area route information

Announces the status of routes to other areas.

(c) Out of AS routing data

When an OSPF router learns routing information for an external AS, the router can use OSPF to distribute these routes to all other OSPF routers. Any router that injects external AS routing information into an OSPF domain is called an AS boundary router.

(2) External AS route

If you configure route redistribution filtering in the switch configuration, the switch will advertise external AS routes. An AS boundary router adds the following information to LSAs that advertise external routes:

Metric

The metric is used by the learning router for path selection among LSAs. Use the default-metric command to set the default metric.
Metric types

There are two types of metric, type 1 and type 2. The route priority and the calculation method when using the metric for route selection differ between the types. The default metric type is 2.
Forwarding address (destination)

An address used as the destination that is reachable by OSPF. If the address is unreachable by OSPF, the router sets 0.0.0.0 as the forwarding address.
Tag

Additional information can be advertised as a tag.

(3) Advertising external AS routes between domains

The multiple OSPF domains connected via a single router operate as independent OSPF networks. For this reason, unless there are configuration settings that specify route redistribution, the router does not distribute the routes of one OSPF domain to the others. If you configure redistribution filtering in the configuration for OSPF routes learned from other domains, routes from other domains are advertised as external AS routes. The following table describes the default filter attributes.

Table 11-2: Filter attributes for redistributing routes in another domain
Attribute	Default
Attribute	External AS route	Intra-area/inter-area route
Metric	The value specified by the default-metric command. If no value is specified, 20 is used.	The value specified by the default-metric command. If no value is specified, 20 is used.
Metric types	Type 2 for external AS or NSSA routes.
Tag value	Inherits the tag value of the route.	0