IP Link-State Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 376

Major LSA Types (Continued) Table 8-2

Type Code Type Description

4 Summary LSA for ASBRs Originated by ABRs. Sent into an area by the ABR to

advertise the AS boundary routers (ASBRs).

5 AS External LSA Originated by ASBRs. Advertises destinations external to

the OSPF AS, flooded throughout the entire OSPF AS.

7 Not-so-stubby area

External LSA

Originated by ASBRs in a not-so-stubby area (NSSA). It
is not flooded throughout the OSPF AS, only to the
NSSA.

External Path Types
Two types of External paths exist. External paths can be Type 1 or Type 2. Do not confuse these
with the router and network LSAs. By default, external routes Type 2. The metric of Type 2
external routes is the metric of the route at the point of redistribution into the OSPF network.
Type 1 external routes have a metric that is the sum of the redistribution cost plus the cost of
the path to reach the ASBR. To set the external metric type for external routes, use the metric-
type keyword in the redistribution statement, as shown in Example 8-1.

Setting the External Route Metric Type Example 8-1

Router9(config)#router ospf 100
Router9(config-router)#redistribute static metric-type 1

The keyword metric-type 1 causes the redistributed routes to be external Type-1 routes. The
keyword metric-type 2 makes the routes external Type-2 routes.

OSPF Areas
As a network grows, the initial flooding and database maintenance of LSAs can burden the CPU
of a router. OSPF uses areas to reduce these effects. An area is a logical grouping of routers and
links that divides the network. Routers share link-state information with only those routers in
their area. This reduces the size of the database.

Each area is assigned a 32-bit integer number. Area 0 (or 0.0.0.0) is reserved for the backbone
area. The backbone area is responsible for distributing routing information between areas. If
more than one area exists in a network, one of the areas must be a backbone area. Every other
area must have at least one connection to the backbone. As you can see in Figure 8-1, commu-
nication between area 1 and area 2 must flow through area 0, or at least flow through a router
that is directly connected to area 0.