Calculating Ospf Routes - 3Com 7700 Configuration Manual

76 C 5: R
HAPTER OUTING

Calculating OSPF Routes

P O
ROTOCOL PERATION
Scope — Supports networks in 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 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 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 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 — Supports multicast addresses to receive and send
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packets.
The OSPF protocol calculates routes in the following way:
Each OSPF-capable router maintains a Link State Database (LSDB), 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 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. Obviously, all the routers have a graph that is 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 leave node. A router, which advertises the
routes, also tags them and records the additional information of the
autonomous system. Therefore, the routing tables obtained by 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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The Hello packet is the most common packet sent by the OSPF protocol. A
router periodically sends it to its neighbor. It contains the values of some
timers, DR, BDR and the known neighbor.
Database Description (DD) Packet
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