OSPF Configuration 235
OSPF Configuration Open Shortest Path First (OSPF) is an Interior Gateway Protocol based on the link state
developed by IETF.
Only the Switch 5500-EI supports the OSPF protocol.
The Switch 5500 uses OSPF version 2 (RFC2328), which has the following features:
■Scope—Supports networks of various sizes and can support several hundred
routers.
■Fast convergence—Transmits the update packets instantly after the network
topology changes so that the change is synchronized in the AS.
■Loop-free—Calculates routes with the shortest path tree algorithm, 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 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.
■Routing hierarchy—Supports a four-level routing hierarchy that prioritizes the
routes into intra-area, inter-area, external type-1, and external type-2 routes.
■Authentication—Supports interface-based packet authentication to guarantee the
security of the route calculation.
■Multicast transmission—Uses multicast address to receive and send packets.
Calculating OSPF Routes The OSPF protocol calculates routes as follows:
■Each OSPF-capable router maintains a Link State Database (LSDB), which 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
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
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.