H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 236)

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Fast convergence: Transmits updates instantly after network topology changes for routing
information synchronization in the AS.
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Loop-free: Computes routes with the shortest path first (SPF) algorithm according to the collected
link states, so no route loops are generated.
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Area partition: Allows an AS to be split into different areas for ease of management and the routing
information transmitted between areas is summarized to reduce network bandwidth consumption.
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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 routes into intra-area,
inter-area, external Type-1, and external Type-2 routes.
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Authentication: Supports interface-based packet authentication to guarantee the security of packet
exchange.
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Multicast: Supports packet multicasting on some types of links.
Basic Concepts
Autonomous System
A set of routers using the same routing protocol to exchange routing information constitute an
Autonomous System (AS).
OSPF route computation
OSPF route computation is described as follows:
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Based on the network topology around itself, each router generates Link State Advertisements
(LSA) and sends them to other routers in update packets.
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Each OSPF router collects LSAs from other routers to compose a LSDB (Link State Database). An
LSA describes the network topology around a router, so the LSDB describes the entire network
topology of the AS.
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Each router transforms the LSDB to a weighted directed graph, which actually reflects the topology
architecture of the entire network. All the routers have the same graph.
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Each router uses the SPF algorithm to compute a Shortest Path Tree that shows the routes to the
nodes in the autonomous system. The router itself is the root of the tree.
Router ID
To run OSPF, a router must have a Router ID, which is a 32-bit unsigned integer, the unique identifier of
the router in the AS.
You may assign a Router ID to an OSPF router manually. If no Router ID is specified, the system
automatically selects one for the router as follows:
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If the loopback interfaces are configured, select the highest IP address among them.
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If no loopback interface is configured, select the highest IP address among addresses of active
interfaces on the router.
OSPF packets
OSPF uses five types of packets:
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Hello packet: Periodically sent to find and maintain neighbors, containing the values of some timers,
information about the DR, BDR and known neighbors.
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DD packet (database description packet): Describes the digest of each LSA in the LSDB,
exchanged between two routers for data synchronization.