OSPF (Open Shortest Path First), is like the on-steroids version of RIP (Routing Information Protocol). Most companies and network managers choose OSPF after determining RIP either simply can’t handle the larger network, or they’re in need of faster route convergence time. OSPF is sometimes used to connect different subnets of branch offices, however, that is a less than ideal use case. For those types of remote office connectivity, technologies such as VLL (Virtual Leased Line) are better suited.
To better understand how OSPF acts as a routing algorithm, we’ll begin with a conceptual overview and then look a bit deeper at its protocol and area configurations.
OSPF consists of a unique 32-bit router ID number. When a router utilizes OSPF, it sends welcome packets to neighboring routers that includes link-state information and other linked IDs. This information enables communication to be bi-directional with an ACK based protocol. When, on a point-to-point link, OSPF acknowledges the link as connected, the link is classified as ‘up’. When on a broadcast link, the router needs to wait for an election and only then can identify if the link is connected.
You can inject a Priority ID into the election ballot so that either the DR (Designated Router) or largest IP address comes out on top. The key concept behind having a DR is that they are the first to generate LSA (Link State Advertisements) that describe varying routes. They also initiate database exchanges with any other available routers within a given subnet. This enables the user to keep the protocol scalable so that resolving the routes becomes linear with the number of routes as opposed to higher orders.
OSPF has three different types of routers:
- ABR (Area Border Router) defines a router in the backbone area, also sometimes called ‘area zero’, as well as in another or more areas. A good way to think of area zero is the root node of a hub-and-spoke where the leafs need to visit the root – same concept applies for area zero.
- DR and BDR (Designated Router and Backup Designated Router), keeps the subnet database. The DR routers can send and receive updates from other routers within the same network and the database is accordingly updated.
- ASBR (Autonomous System Boundary Router) connects one or more Autonomous Systems and exchanges route information with the purpose of disseminating route information that is received from other autonomous systems within its own autonomous system.
In regard to redistribution, if there is an internal router that isn’t a BR that needs to be connected to a new network outside the user’s control, a non-IGP (Interior Gateway Protocol) routing protocol should be used to exchange routes once the initial connection is made. The user may create a static route within the router to the new network. Miscellaneous information is then ‘redistributed’ into the OSPF so that it becomes an ASBR and the entire user’s network will then be able to connect to the new network.
However, as our brief overview suggests, OSPF can get fairly complicated and will not be suitable for connecting branch offices in most cases. However, within the IGP family OSPF has desirable characteristics in terms of quick convergence.
Cahit Akin, CEO, Mushroom Networks, Inc.
Mushroom Networks is the provider of SD-WAN (Software Defined WAN) and NFV solutions capable of Broadband Bonding that enables self-healing WAN networks that route around network problems such as latency, jitter and packet loss.