Cisco Systems OL-8241-02 Manual De Usuario

ASBR routes can be advertised as type1 or type2 ASE. The difference between 
type1 and type2 is how the cost is calculated. For a type2 ASE, only the external 
cost (metric) is used when comparing multiple paths to the same destination. For 
type1 ASE, the combination of the external cost and the cost to reach the ASBR 
is used.

OSPF routers advertise routes using LSAs. The link-state database stores the 
LSAs from routers throughout the area. The advertisements depict the topology 
of the autonomous system. They could include:

Router links that describe the state and cost of each router’s interface to an 
area

Network links from the designated router (see the 

 section) that describe all routes on a segment for multi-access segments 

with more than one attached router

Summarized links from ABRs that describe networks in the AS but outside 
an area

External links from ASBRs that describe destinations external to the AS

All routers that are connected to an area maintain identical routing databases 
about the area. Routers that are connected to multiple areas maintain a separate 
routing database for each attached area. 

Instead of each router sending routing information to every other router on the 
network, OSPF routers establish adjacencies among neighboring routers. When 
the link-state databases of two neighboring routers are synchronized, they are 
considered adjacent. 

OSPF routers collect raw topological data from the LSAs that they receive. Each 
router then prunes this data down to a tree of the shortest network paths centered 
on itself. The router examines the total cost to reach each router or network node 
in its domain. By discarding all but the lowest-cost path to each destination, the 
router builds a shortest-path tree to each destination, which it uses until the 
network topology changes. It is possible to have multiple lowest-cost paths to a 
destination.