TMS zl Management and Configuration Guide ST.1.0.090213

9-26
Routing
OSPF
Because OSPF routers send each other more messages than RIP routers send,
OSPF can consume more bandwidth. However, OSPF minimizes the number
of packets routers must send in several ways. In point-to-point networks, only
neighboring routers fully exchange their databases. In multicast networks,
only one router (the DR) floods LSAs. Also, OSPF VLANs only send updates
on their own link states rather than sending all routes discovered by the
protocol, as RIP VLANs do.
You can also divide an OSPF network into areas, each of which deals with its
own routing. After you partition the autonomous system (AS) into areas,
routers take on differentiated roles and only learn about their own area,
further reducing the strain on individual routers. If you design your network
carefully, OSPF should not consume more bandwidth than RIP. You will learn
more about designing OSPF areas in “Areas” on page 9-27.
You will recall that a routing protocol must dictate options such as:
■ How routers compute a route’s metric and select the best route for their
routing table
■ What information routers include in routing updates
■ Which routers and router VLANs send and receive updates
■ When routers send and receive updates
The most important concepts to understand are how:
■ Routers use the information in LSAs to synchronize their topological
databases
■ Routers compute best routes from their topological database
■ Areas divide networks into separate routing domains
■ Internal routers handle intra-area routing for stub areas
■ ABRs handle inter-area routing through the network backbone
LSAs
OSPF is a link-state protocol; routers send each other LSAs to distribute
information about their connections to networks and to other routers. LSAs
help routers synchronize their databases. All routers in an AS (or area) must
use the same database in order to generate accurate routes.
OSPF defines several types of LSAs. Some of these LSAs are flooded to all
routers or DRs in an area, and some are sent to routers throughout the entire
AS. VLANs in stub areas do not listen for certain LSAs. (You can read more
about different types of LSAs in “LSA Types” on page 9-30.)