Last Updated: Saturday, 15 February 2014 17:41
The fourth article in our Anycast DNS series covers Anycast DNS using Open Shortest Path First or OSPF routing protocol. OSPF is a dynamic routing protocol used to build larger scale IP networks. It differs from RIP, because it is a link-state routing protocol and falls into the group of Interior Gateway Protocols that operate within a single Autonomous System or AS.
OSPF is a link-state routing protocol that runs Dijkstra's algorithm to calculate the shortest path to other networks. Taking the bandwidth of the network links into account, it uses cost as its metric. OSPF works by developing adjacencies with its neighbors, periodically sending hello packets to neighbors, flooding changes to neighbors when a link's status changes, updating its neighbors of all recent link state changes every 30 minutes. The use of this algorithm makes OSPF much faster at network convergence, which is why it makes a better selection for building Anycast DNS solutions for enterprise environments.
Our OSPF network will consist of a two different OSPF areas 0.0.0.51 and 0.0.0.52 that are connected to a thrid area, backbone area 0.0.0.0. This is shown in the figure below:
Each router, will have one interface directly connected to backbone area 0.0.0.0. Their other interface(s) will connect into their own respective OSPF routing areas. We do not dive into the details of OSPF networking because it is beyond the scope of this article. Our goal is to provide a working recipe for using OSPF, Cisco routers, and Quagga Open Source host-based routing software.