IPv4 Anycast with Linux and Quagga

 in
Ease configuration headaches and improve availability with anycast.
Monitoring and Automatic Route Withdrawal

As I mentioned previously, the fact that a host is up does not mean that the service that host provides is up. When a host running Quagga goes down, any routes that host inserted into OSPF will be withdrawn. We need to do the same thing when a service does down. Any piece of monitoring software that can run a handler script in response to a monitoring event can be used for this task. The basic idea is to execute a test against the anycast IP from each anycast endpoint. If a test fails, you need to run ifdown lo:0 on the failed endpoint. Quagga will detect the downed interface and withdraw the route to that interface from OSPF. Administrators then can fix the box at their leisure and place the box back into service with a simple ifup lo.

Conclusion

Anycast is a great technique to enhance the reliability and fault tolerance of applications and services on your network. When designing your anycast topology, keep several rules and guidelines in mind. I've shown a very basic use case and deployment of anycast here. You can take the same concepts covered in this article, along with a fair bit of networking knowledge, and scale them to a worldwide deployment. If you do it right, you can have redundancy without nearly as many idle machines sitting around.

Philip Martin has been working and playing with Linux for about ten years and is currently a Systems Engineer for a large on-line retailer. When he is not working with computers, he spends his days trying to be more like Alton Brown and in an ongoing quest to get invited to an Iron Chef America filming. He can be reached at phillip.martin@gmail.com.

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