TMS zl Management and Configuration Guide ST.1.0.090213

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Routing
OSPF
Route Computation
Routers use the information they receive from LSAs to assemble a topological
database of the AS (or, if configured, area). This database includes:
■ All routers in the AS or area
■ All networks in the AS or area
■ All links in the AS or area
■ The cost for all links
The topological database for all routers in an AS (or area) is the same.
Theoretically, any router could calculate a route to a destination for any
other router.
Depending on the type of LSAs that the router receives, the database can also
include:
■ Links to ranges of networks in other areas
■ Links to external networks
The router would use this information to generate inter-area and external
routes.
A router applies Dijkstra’s algorithm to its topological database to generate a
routing tree with itself as the root. This action is also called performing the
shortest path first (SPF) calculation. From the SPF tree, the router calculates
its own best routes. Each router’s routing table is unique. Each route in this
table contains the following information:
■ Type of route—OSPF (intra-area), inter-area, external 1, or external 2
■ Destination address and subnet mask
■ Administrative distance—by default, 110 for OSPF routes
■ Metric—the total cost for all links in between the router and the
destination
■ Next-hop address
Because OSPF routers can factor in the total cost for all links between the
router and the destination, they can use higher-speed connections, even when
they involve more hops.
A router performs SPF calculations when its OSPF database changes. Its SPF
calculation delay and hold timers prevent the router from performing the
calculation too often. These factors prevent router processes from being tied
up when, for example, a flapping VLAN causes continual topological changes.
(A flapping VLAN is a term used for VLANs that frequently change their status
from up to down and down to up.)