HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 58

Configuring OSPF, Overview, Basic concepts, Autonomous System, OSPF route computation

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Configuring OSPF Overview Open Shortest Path First (OSPF) is a link state interior gateway protocol developed by the OSPF working group of the IETF. Now, OSPF version 2 (RFC 2328) is used. Unless otherwise noted, OSPF refers to OSPFv2 throughout this chapter. The term "router" in this chapter refers to both routers and Layer 3 switches. 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 facilitate 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 area- and 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 in the area to a weighted directed graph, which is the topology of the entire network. All the routers of the area have the same graph. • Each router uses the SPF algorithm to compute a shortest path tree showing the routes to the nodes in the AS. The router itself is the root of the tree. 48

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48
Configuring OSPF
Overview
Open Shortest Path First (OSPF) is a link state interior gateway protocol developed by the OSPF working
group of the IETF. Now, OSPF version 2 (RFC 2328) is used. Unless otherwise noted, OSPF refers to
OSPFv2 throughout this chapter. The term "router" in this chapter refers to both routers and Layer 3
switches.
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 facilitate 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 area- and 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 in the area to a weighted directed graph, which is the topology of
the entire network. All the routers of the area have the same graph.
•
Each router uses the SPF algorithm to compute a shortest path tree showing the routes to the nodes
in the AS. The router itself is the root of the tree.