Configuring Ospf - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

IP Routing—Configuring RIP, OSPF, BGP, and PBR

Configuring OSPF

Configuring OSPF
OSPF was designed to cope with several of RIP's shortcomings. For example,
OSPF provides quicker convergence and more sophisticated methods of
computing best routes. Instead of sending routing table entries, routers send
link state advertisements (LSAs) that allow peers to construct a more com-
prehensive, accurate, moment-to-moment topology of the network.
An LSA advertises the state and cost of each of the router's connections—to
an OSPF network or to another router. In a WAN divided into areas, special
LSAs can advertise the state and cost of a connection to the entire range of
networks in an area. Other special LSAs advertise external routes.
OSPF routes are typically more reliable than RIP routes. RIP only takes the
number of hops into account when computing a route's cost, but OSPF also
considers the relative cost of each link used in the route. OSPF usually
computes the cost of a link relative to that link's inverse bandwidth.
In addition, RIP networks are limited to 15 hops. OSPF allows networks to
expand beyond this limit.
OSPF can also provide increased security. You can configure a router to
require a clear-text or a message digest 5 (MD5) encrypted key from a peer
before exchanging LSAs with it.
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 interfaces only send updates
on their own link states rather than sending all routes discovered by the
protocol, as RIP interfaces do.
You can also divide an OSPF network into areas, each of which deals with its
own routing. After you partition the AS into areas, routers take on differenti-
ated 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 13-31.
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