Ospf Overview - 3Com 7757 Configuration Manual

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OSPF Overview

Introduction to OSPF
OSPF Route Calculation
OSPF C
ONFIGURATION
Open shortest path first (OSPF) is a link state-based interior gateway protocol
developed by IETF. At present, OSPF version 2 (RFC 2328) is used, which has the
following features:
High applicability: OSPF supports networks of various sizes and can support up
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to several hundred routers.
Fast convergence: OSPF can transmit update packets immediately after the
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network topology changes so that the change can be synchronized in the
autonomous system (AS).
Loop-free: Since OSPF calculates routes with the shortest path tree algorithm
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according to the collected link states, it guarantees that no loop routes will be
generated from the algorithm basis.
Area partition: OSPF allows an autonomous system network to be divided into
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different areas for convenient management so that routing information
transmitted between the areas is abstracted further, thereby reducing network
bandwidth consumption.
Equivalent route: OSPF supports multiple equivalent routes to the same
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destination.
Routing hierarchy: OSPF has a four-level routing hierarchy. It prioritizes the
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routes as intra-area, inter-area, external type-1, and external type-2 routes.
Authentication: OSPF supports interface-based packet authentication to
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guarantee the security of route calculation.
Multicast transmission: OSPF supports transmitting protocol packets in
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multicast mode.
Taking no account of area partition, the routing calculation process of the OSPF
protocol is as follows:
Each OSPF-capable router maintains a link state database (LSDB), which
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describes the topology of the whole AS. According to the network topology
around itself, each router generates a link state advertisement (LSA). Routers
on the network exchange LSAs with each other by transmitting protocol
packets. Thus, each router receives the LSAs of other routers and all these LSAs
form the LSDB of the router.
An LSA describes the network topology around a router, whereas an LSDB
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describes the network topology of the whole network. Routers can easily
transform the LSDB to a weighted directed map, which actually reflects the