What is OSPF…? Open Shortest Path First (OSPF) is an adaptive routing protocol for Internet Protocol (IP) networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system (AS). OSPF is perhaps the most widely-used interior gateway protocol (IGP) in large enterprise networks. IS-IS, another link-state dynamic routing protocol, is more common in large service provider networks.
Count… OSPF was designed to support variable-length subnet masking (VLSM) or Classless Inter-Domain Routing (CIDR) addressing models. OSPF detects changes in the topology, such as link failures, very quickly and converges on a new loop- free routing structure within seconds. An OSPF network may be structured, or subdivided, into routing areas to simplify administration and optimize traffic and resource utilization.
Count… OSPF does not use a TCP/IP transport protocol (UDP, TCP), but is encapsulated directly in IP datagrams with protocol number 89. OSPF uses both unicast and multicast to send packets and link state updates. The OSPF protocol, when running on IPv4, can operate securely between routers. OSPF version 3 introduces modifications to the IPv4 implementation of the protocol.
Neighbor Relationships As a link state routing protocol, OSPF establishes and maintains neighbor relationships in order to exchange routing updates with other routers. The neighbor relationship table is called an adjacency database in OSPF. Provided that OSPF is configured correctly, OSPF forms neighbor relationships only with the routers directly connected to it. In order to form a neighbor relationship between two routers, the interfaces used to form the relationship must be in the same area.
Area Types Areas are logical groupings of hosts and networks,including their routers having interfaces connected to anyof the included networks.Several special area types are defined:1. Backbone Area2. Stub Area3. Not-so-Stubby Area4. Proprietary Extensions5. Transit Area
Path Preference OSPF uses path cost as its basic routing metric, which was definedby the standard not to equate to any standard value such as speed,so the network designer could pick a metric important to thedesign.Four types of metrics are recognized:1. Intra-area2. Inter-area3. External Type 1, which includes both the external path cost and the sum of internal path costs to the ASBR that advertises the route,4. External Type 2, the value of which is solely that of the external path cost
OSPF Router Types The router type is an attribute of an OSPF process. Agiven physical router may have one or more OSPFprocesses.OSPF defines the following router types:1. Area Border Router (ABR)2. Autonomous System Boundary Router (ASBR)3. Internal Router (IR)4. Backbone Router (BR)
OSPF Packet Header Every OSPF packet starts with a common 24 byteheader. This header contains all the necessaryinformation to determine whether the packet should beaccepted for further processing.
OSPF in Broadcast and Non- Broadcast Networks In broadcast multiple-access networks, neighboradjacency is formed dynamically using multicast hellopackets to 126.96.36.199.For non-broadcast multiple-access networks(NBMA), RFC 2328 defined the following two officialmodes for OSPF:1. No broadcast2. Point-to-multipoint
Implementations 1. BIRD implements both OSPFv2 and OSPFv32. GNU Zebra, a GPL routing suite for Unix-like systems supporting OSPF3. Netware implements OSPF in its Multi Protocol Routing module.4. OpenBSD includes an OpenOSPFD implementation within the OpenBGPD protocol.5. XORP, a routing suite implementing RFC2328 (OSPFv2) and RFC2740 (OSPFv3) for both IPv4 and IPv6.6. Windows NT 4.0 Server, Windows 2000 Server and Windows Server 2003 implement OSPFv2 in the Routing and Remote Access Service, although the functionality was removed in Windows Server 2008.
Applications OSPF was the first widely deployed routing protocol that could converge a network in the low seconds, and guarantee loop-free paths. OSPF can provide better load-sharing on external links than other IGPs. It has many features that allow the imposition of policies about the propagation of routes that it may be appropriate to keep local, for load sharing, and for selective route importing more than IS-IS (Intermediate System To Intermediate System).