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How does hybrid unicast/anycast work for global load balancing?

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bornfree

In the Dropbox Edge network write up, the author mentions that they have taken a hybrid approach by announcing both unicast and anycast ip of PoP.

One can enable this hybrid GSLB by announcing both PoP’s unicast subnet (e.g. /24) and one of its supernets (e.g. /19) from all of the PoPs (including itself). enter image description here

How does this exactly work?

  • When a user enters the dropbox website name, will DNS return both unicast and anycast ip to the user?
  • If the unicast ip doesn't work for the user because that PoP may not be available, does the browser automatically fallback to anycast ip and lands up to another PoP as shown in the image?
  • What is the reason for returning supernet(0/19) and not any other anycast ip?
  • How will this help in immediate fallback during an outage? How will it mitigate delay due to DNS ttl during an outage?
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Patrick Mevzek avatar
cn flag
Patrick Mevzek
Two generic points: consumers (DNS clients) can not know if an IP is anycast or not, they use whatever they get, and browsers do try multiple IP addresses until one succeeds, but this is dependent on each application.
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