Ospf Overview - 3Com 8807 Configuration Manual

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OSPF Overview

Introduction to OSPF
Process of OSPF Route
Calculation
OSPF C
ONFIGURATION
Open Shortest Path First (OSPF) is an Interior Gateway Protocol based on the link
state developed by IETF. At present, OSPF version 2 (RFC2328) is used, which is
available with the following features:
Applicable scope: It can support networks in various sizes and can support
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several hundreds of routers at maximum.
Fast convergence: It can transmit the update packets instantly after the
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network topology changes so that the change is synchronized in the AS.
Loop-free: Since the OSPF calculates routes with the shortest path tree
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algorithm according to the collected link states, it is guaranteed that no loop
routes will be generated from the algorithm itself.
Area partition: It allows the network of AS to be divided into different areas for
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the convenience of management so that the routing information transmitted
between the areas is abstracted further, hence to reduce the network
bandwidth consumption.
Equal-cost multi-route: Support multiple equal-cost routes to a destination.
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Routing hierarchy: OSPF has a four-level routing hierarchy. It prioritizes the
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routes to be intra-area, inter-area, external type-1, and external type-2 routes.
Authentication: It supports the interface-based packet authentication so as to
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guarantee the security of the route calculation.
Multicast transmission: Support multicast address to receive and send packets.
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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). The
routers on the network transmit the LSAs among them by transmitting the
protocol packets to each others. Thus, each router receives the LSAs of other
routers and all these LSAs compose its LSDB.
LSA describes the network topology around a router, so the LSDB describes the
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network topology of the whole network. Routers can easily transform the LSDB
to a weighted directed graph, which actually reflects the topology architecture
of the whole network. Obviously, all the routers get a graph exactly the same.
A router uses the SPF algorithm to calculate the shortest path tree with itself as
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the root, which shows the routes to the nodes in the autonomous system. The
external routing information is the leave node. A router, which advertises the
routes, also tags them and records the additional information of the