15.2 OSPF
OSPF (Open Shortest Path First) is a dynamic routing protocol that is frequently used in
larger networks as an alternative to RIP. It was originally developed to avoid a number of
limitations of RIP (when used in larger networks).
The problems (with RIP) avoided by OSPF include:
• Reduced network load: After a short initialization phase, routing information is not sent
periodically as with RIP, but only changed routing information.
• Authentication: Gateway authentication can be configured to increase the security when
exchanging routing information.
• Routing Traffic Control: Gateways can be combined to form areas to limit the traffic cre-
ated by exchanging routing information.
• Connection costs: OSPF differs from RIP in that the connection costs are not calculated
from the number of next hops, but from the bandwidth of the respective transport medi-
um.
• No limitation of the number of hops: The limitation of the maximum number of 16 hops for
RIP does not exist for OSPF.
Although the OSPF protocol is considerably more complex than RIP, the basic concept is
the same, i.e. OSPF also determines the best path for forwarding the packets in each case.
OSPF is an Interior Gateway Protocol that is used to distribute routing information within an
autonomous system (AS). The Link State Updates are exchanged between the gateways
by flooding. Each change of routing information is passed to all gateways in the network.
OSPF areas are defined to limit the number of Link State Updates. All gateways of an area
have an identical Link State database.
An area is interface-specific. Gateways whose interfaces belong to several areas and con-
nect these to the backbone are called Area Border Routers (ABR). ABRs therefore contain
the information of the backbone area and all areas connected. A gateway whose interfaces
are all incorporated in one area are called Internal Routers (IR).
There are four types of Link State packets: Router links show the state of the interfaces of
a gateway that belong to a certain area. Summary links are generated by the ABR to define
how the information on reachability in the network is exchanged between areas. Usually all
information is sent to the backbone area, which then passes the information to the other
areas. Network links are sent by Designated Routers (DS) within a segment and propagate
all gateways that are connected to a certain multi-access segment like Ethernet, Token
Ring and FDDI (also NBMA). External links point to networks outside the AS. These net-
works are incorporated in OSPF using redistribution. In this case, an Autonomous System
Border Router (ASBR) incorporates these external routes in the AS.
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