3Com SuperStack 4 Configuration Manual page 118

130 C 6: IP R
HAPTER OUTING
OSPF Configuration
Calculating OSPF Routes
P O
ROTOCOL PERATION
Open Shortest Path First (OSPF) is an Interior Gateway Protocol based on the link
state developed by IETF.
The Switch 5500G-EI uses OSPF version 2 (RFC2328), which has the following
features:
Scope — Supports networks of various sizes and can support several hundred
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routers.
Fast convergence — Transmits the update packets instantly after the network
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topology changes so that the change is synchronized in the AS.
Loop-free — Calculates routes with the shortest path tree algorithm, according
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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 the
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routes into intra-area, inter-area, external type-1, and external type-2 routes.
Authentication — Supports interface-based packet authentication to
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guarantee the security of the route calculation.
Multicast transmission — Uses multicast address to receive and send packets.
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The OSPF protocol calculates routes as follows:
Each OSPF-capable router maintains a Link State Database (LSDB), which
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describes the topology of the entire AS. Depending on the surrounding
network topology, 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. In this way, each router receives the LSAs of
other routers and all these LSAs constitute its LSDB.
LSA describes the network topology around a router, so the LSDB describes the
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network topology of the entire network. Routers can easily transform the LSDB
to a weighted directed graph, which actually reflects the topology of the whole
network. All 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. 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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