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Open Shortest Path First (OSPF) is a link status based interior gateway protocol developed by IETF.
The current version is version 2 (RFC2328) and its characteristics are as follows:
Wide scope of application – It supports networks of all sizes and can support up to hundreds of
routers.
Fast convergence – In case of change in topological structure of network, it will immediately
send update package to let the change synchronize in the autonomous system.
Loop-free – Since OSPF calculates routing with the shortest path tree algorithm based on the
collected link status, the algorithm itself ensures that no loops will be generated.
Zoning – It allows dividing the network of autonomous system into regions for management.
The routing information transmitted between the regions is further abstracted, thereby reducing
the occupied network bandwidth.
Equivalent routing – It supports multiple equivalent routings to the same destination address.
Hierarchical routing – It uses four different routings, which, in the order of priority, are
intra-area routing, inter-area routing, the first type of external routing and the second type of
external routing.
Support authentication – it supports interface-based package authentication to ensure the safety
of routing calculation.
Multicast sending – The protocol package supports sending in the form of multicast.
9.2.3.2 Basic Principles of OSPF Protocol
Regardless of zoning, the routing calculation process of OSPF protocol can be briefly described as
follows:
Each router that supports OSPF protocol maintains a Link State Database (LSDB) that
describes the topological structure of the entire autonomous system. Each router generates a
Link State Advertisement (LSA) based on its surrounding network's topological structure and
sends LSA to other routers in the network through mutual transmission of protocol package.
Thus, each router can receive the LSA of other routers and all the LSAs compose a LSDB.
LSA is a description of the topological structure of the router’s surrounding network, while
LSDB is a description of the entire network’s topological structure. The router will convert
LSDB into another digraph with weight as the true reflection of the entire network’s
topological structure. The digraphs obtained by all routers are completely the same.
Each router will use SPF algorithm to calculate the shortest path with itself as the root based
on the digraph and this trees shows all routings to the nodes in the autonomous system with
external routing information as the leaf nodes. External routing can be marked by the router
that broadcasts it, so as to record additional information on the autonomous system. The routing
table obtained by each router is different. In addition, in order to allow each router broadcast
the local status information (e.g. information on available interface, information on reachable
neighbors, etc.) in the entire autonomous system, it is required to establish multiple adjacency
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