H3C Technologies H3C S12500 Series Switches - Manual (Page 71)

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Configuring OSPF
Open Shortest Path First (OSPF) is a link state interior gateway protocol developed by the OSPF working
group of the Internet Engineering Task Force (IETF). OSPF version 2 is used for IPv4. Unless otherwise
noted, OSPF refers to OSPFv2 throughout this chapter.
Introduction to OSPF
OSPF has the following features:
• Wide scope—Supports various networks sizes and up to several hundred routers in an OSPF routing
domain.
• Fast convergence—Advertises routing updates instantly upon network topology changes.
• Loop free—Computes routes with the SPF algorithm to avoid routing loops.
• Area-based network partition—Splits an AS into multiple areas to facilitate management. This
feature reduces the lSDB size on routers to save memory and CPU resources, and reduces route
updates transmitted between areas to save bandwidth.
• Equal-cost multi-path (ECMP) routing—Supports multiple equal-cost routes to a destination.
• Routing hierarchy—Supports a 4-level routing hierarchy that prioritizes routes into intra-area,
inter-area, external Type-1, and external Type-2 routes.
• Authentication— Supports area- and interface-based packet authentication to ensure the security of
packet exchange.
• Support for multicasting—Multicasts protocol packets on some types of links to avoid impacting
other devices.
Basic concepts
Autonomous system
A group of routers running the same routing protocol constitute an autonomous system (AS).
Route calculation
OSPF computes routes as follows:
• Each router generates Link State Advertisements (LSAs) based on the network topology around itself,
and sends them to other routers in update packets.
• Each OSPF router collects LSAs from other routers to compose a link state database (LSDB). An LSA
describes the network topology around a router, and the LSDB describes the entire network
topology of the area.
• Each router transforms the LSDB to a weighted directed graph that shows the topology of the area.
All the routers within the area have the same graph.
• Each router uses the SPF algorithm to compute a shortest path tree that shows the routes to the nodes
in the area. The router itself is the root of the tree.