Configuring ospf, Introduction to ospf, Basic concepts – H3C Technologies H3C SR8800 User Manual
Page 76: Autonomous system, Ospf route computation
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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). Now, OSPF version 2 (RFC 2328) is used.
NOTE:
Unless otherwise noted, OSPF refers to OSPFv2 throughout this document.
Introduction to OSPF
OSPF has the following features:
•
Wide scope—Supports various network sizes and up to several hundred routers in an OSPF routing
domain.
•
Fast convergence—Transmits routing updates instantly upon network topology changes.
•
Loop-free—Computes routes with the shortest path first (SPF) algorithm to avoid routing loops
•
Area-based network partition—Splits an AS into different areas to faciliate management. In
addition, routing information transmitted between areas is summarized to reduce traffic and routing
table sizes.
•
Equal-cost multi-path (ECMP) routing—Supports multiple equal-cost routes to a destination.
•
Routing hierarchy—Supports a four-level routing hierarchy that prioritizes routes into intra-area,
inter-area, external Type-1, and external Type-2 routes.
•
Authentication—Supports interface-based packet authentication to ensure the security of packet
exchange.
•
Support for multicast—Multicasts protocol packets on some types of links.
Basic concepts
Autonomous System
An Autonomous System (AS) comprises a group of routers that run the same routing protocol.
OSPF route computation
OSPF computes routes in an area as follows:
•
Based on the network topology around itself, each router generates Link State Advertisements (LSAs)
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 AS.
•
Each router transforms the LSDB to a weighted directed graph, which is the topology of the entire
network. All the routers 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 AS. The router itself is the root of the tree.