1. Preface
OSPF is a routing protocol for Internet Protocol (IP), widely used in enterprise networks, wide area networks (WAN) and large-scale Internet Service Provider (ISP) networks. Its widespread use demonstrates its crucial and irreplaceable role in routing protocols.
2. Overview
OSPF, namely, Open Shortest Path First. Its main function is to find the best path primarily between the source router and target router, regarding to the shortest path. It is an Interior Gateway Protocol (IGP). As a dynamic routing protocol, OSPF is designed to provide fast routing convergence, efficient bandwidth utilization and better scalability for large-scale networks, ensuring a secure data transmission in modern complex environment.
3. Working Principle
Through several mechanisms, OSPF guarantees the accurate routing information and timely updating. The following is a brief introduction to these parts.
(1) Link-State Database (LSDB)
Construction of LSDB. Each OSPF router maintains a LSDB, to record connection states between routers. Then, by exchanging Link-State Advertisements (LSA), achieves the updating and synchronization of LSDB, maintaining the consistency of the network topology across routers.
Updating and Synchronization of LSDB. OSPF exchanges LSAs periodically, keeping all routers’ LSDBs synchronized while allowing each router to calculate the same shortest path. Additionally, OSPF will send a new LSA to update LSDB, when the network topology changes.
(2) Dijkstra Algorithm
Calculate the Shortest Path. Dijkstra Algorithm is used to calculate the shortest path, as well as considering costs of links (such as bandwidth), and select the best path for the network.
Construction of SPF Tree. After calculating the shortest path, a “shortest path first tree” (SPF Tree) is formed, whose root is the current router itself, and the other nodes are other routers. In the SPF tree, OSPF can determine the best path. Therefore, OSPF will recalculate and update the SPF tree after LSDB update.
(3) Hello Protocol
Establish Neighbor Relationships. OSPF establishes and maintains neighbor relationships through Hello Protocol. When two OSPF routers want to exchange their router information, they will send Hello packets to discover each other, and verify if they can make a trusted communication.
Manage and Maintain the Neighbor Relationships. Once the Neighbor relationships are established, OSPF will send Hello packets periodically, to keep the relationship active while the changes in network topology can be reflected in the routing table. If a Hello packet is not received, the router will recalculate the route.
4. Network Design and Architecture
OSPF network is divided into multiple areas, with routers in different roles connecting the areas. Below is an overview of area division and router types in OSPF network.
(1) Area Division
OSPF divides the network into areas, utilizing router information exchange and management. This method effectively reduces the load on routers and enhances routing efficiency. The entire OSPF network is consisted of the following areas.
Area 0 (Backbone Area). Core of OSPF network, and all network connections have to pass through there. Responsible for forwarding routing information from different areas.
Common Area (Non-backbone Area). All areas except Area 0 are non-backbone areas, usually connecting with Area 0 via Area Border Routers (ABRs). Routers in these areas only maintain routing information in their own areas, reducing the routing costs.
Virtual Link. If Area0 cannot be connected with other areas directly, a virtual link can also be used for indirect connection. Typically, the virtual links will be used when physical network connection fails in area 0.
(2) Router Types
In OSPF network, there are three types of routers, internal router, area border router (ABR) and autonomous system boundary router (ASBR). Each router type serves a different role.
Internal Router. Belongs entirely to one area, and just knows the routing information in its own area.
Area Border Router. Connect two or more areas. Responsible for routing information’s exchanging and summarizing. Furthermore, an ABR maintains a routing table for each area it connects.
Autonomous System Boundary Router. Connects the OSPF network to external networks. The ASBR provides routing information for internal network through OSPF, meanwhile imports external routing information into the OSPF network.
(3) Considerations in Design
In large networks, proper area division and router role assignments are crucial. Too many areas may lead to complex management, however, too few areas also will affect the routing efficiency.
5. Advantages and Limitations
(1) Advantages
Scalability. OSPF supports large-scale networks, and improves management efficiency by lowering complexity through area division.
Efficiency. Uses link-state protocol, thus quickly converging and calculating the best path, and minimizing bandwidth consumption.
Ideal for Complex Design. Not only can handle complex network design, like multi-region and virtual links, but also supports fine-grained routing control.
(2) Limitations
Resource Consumption. Maintaining LSDB and exchanging information frequently consume much more CPU and memory resources.
Configuration Complexity. Its configuration is complex, especially making errors in large-scale and multi-area networks.
Large Network Problems. OSPF may face prolong convergence times and database synchronization problems in large-scale networks.
6. Conclusion
OSPF has a widespread use in enterprises, WAN and ISP networks, ensuring an efficient and secure transmission in networks. In the future, to better follow the changes in network demands, OSPF will pay more attention to scalability and flexibility, to support complex designs, such as multi-areas and virtual links.
