One address, many sites
Routing can choose among identical destinations.
With anycast, several locations provide the same service while announcing the same address or prefix. A client sends traffic to one address, and the routing system selects one of the available announcements.
The selected location is often topologically close, but “closest†means best according to routing policy and path attributes, not necessarily the shortest physical distance. A route withdrawal can also move traffic to another location.
Build the model
Four anycast pieces
Shared address
The same service address or prefix is reachable from several sites.
Announcements
Each location makes the shared destination reachable through routing.
Path selection
The network chooses one available route according to its rules.
Withdrawal
A site can stop announcing so traffic can move elsewhere.
Mini experiment
Choose a service location
Select the route state and see which location can answer the request.
The North location announces the shared service prefix and is preferred for this client path.
One shared address can lead to this available location.
Knowledge check
Test your anycast model
Answer five questions. The answer order changes each time.