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Chapter 3 Configuring Open Shortest Path First (OSPF)
OSPF Overview
3-6
Cisco Content Services Switch Routing and Bridging Configuration Guide
OL-4580-01
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.
Link-State Databases
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 “Setting the Priority of the
CSS” 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.