Plan IPv6 anycast service addresses across sites and estimate the routing spread each instance serves.
Anycast reuses one address at many locations and lets routing pick the nearest. The subnet-router anycast address is the all-zeros interface identifier inside a prefix, which every router on the link must answer. Sizing each instance for the post-failure load, not the average, is the difference between anycast absorbing a site outage and cascading into a full outage.
IPv6 Anycast
Every instance advertises the same address, so load per instance = total requests ÷ instances, and after one failure = total ÷ (instances − 1).
Every instance advertises the same address, so load per instance = total requests ÷ instances, and after one failure = total ÷ (instances − 1). Anycast reuses one address at many locations and lets routing pick the nearest. The subnet-router anycast address is the all-zeros interface identifier inside a prefix, which every router on the link must answer.
Sizing each instance for the post-failure load, not the average, is the difference between anycast absorbing a site outage and cascading into a full outage.
This calculator takes 3 inputs: Service prefix, Anycast instances advertising it, Total requests per second. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.