Calculating Ospf Routes - 3Com Switch 7700 Configuration Manual

Calculating OSPF Routes

Fast convergence — Transmits the update packets instantly after the network
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topology changes so the change is synchronized in the AS
Loop-free — Calculates routes using the shortest path tree algorithm,
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according to the collected link states so that no loop routes are generated from
the algorithm itself
Area partition — Allows the network of AS to be divided into different areas
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for management convenience, so that the routing information that is
transmitted between the areas is further abstracted to reduce network
bandwidth consumption
Equal-cost multi-route — Supports multiple equal-cost routes to a destination
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Routing hierarchy — Supports a four-level routing hierarchy that prioritizes
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routes into intra-area, inter-area, external type-1, and external type-2 routes.
Authentication — Supports the interface-based packet authentication to
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guarantee the security of the route calculation
Multicast transmission — Uses multicast addresses to send updates.
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Configuring OSPF is described in the following sections:
Calculating OSPF Routes
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Configuring OSPF
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Troubleshooting OSPF
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The OSPF protocol calculates routes in the following way:
Each OSPF-capable router maintains a Link State Database (LSD), which
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describes the topology of the entire 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 themselves by transmitting
the protocol packets to each other. Thus, each router receives the LSAs of other
routers and all these LSAs constitute its LSD.
LSA describes the network topology around a router, so the LSD describes the
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network topology of the entire network. Routers can easily transform the LSD
to a weighted directed graph, which actually reflects the topology of the whole
network. All the routers have the same graph.
A router uses the SPF algorithm to calculate the shortest path tree which shows
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the routes to the nodes in the autonomous system. In this tree, the router is the
root. The external routing information is a leaf node. A router that advertises
the routes, also tags them and records the additional information of the
autonomous system. Therefore, the routing tables obtained from different
routers are different.
OSPF supports interface-based packet authentication to guarantee the security of
route calculation. OSPF also transmits and receives packets by IP multicast.
OSPF Packets
OSPF uses five types of packets:
Hello Packet
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OSPF 105