What is IP Anycast? Where it is used in networking?

What is IP Anycast? Is IP Anycast a routing protocol? Where IP Anycast is used in Networking?. In this post, I will answer these questions. I discuss these questions and often ask them in my training as well. I always receive many different answers but you will see how it is easy to understand the idea behind anycast after reading this post.

IP Anycast is a way of assigning IP addresses. The same IP address is assigned to multiple nodes. It is not a routing protocol, switching protocol, or a special network design. Just a way of assigning an IP address. There are many use cases for it. Internally in the networks, Multicast uses IP Anycast for load balancing and redundancy.

Specifically, PIM ASM (Protocol Independent Multicast - Any Source Multicast) uses IP Anycast for the RP (Rendezvous Point) address assignment. The same IP address is assigned on multiple nodes in the network and the underlying IGP protocol shortest path is used to determine the closest RP in a topology.


IP Anycast for  CDN (Content Delivery Networks)

IP Anycast is a special method for request routing in CDN architecture. Let's have a look at what is request routing and how Anycast is used in request routing. Request routing or also known as server redirection is a method to bring the customer to the optimal server in a CDN architecture.  


Figure - IP Anycast vs. Unicast

 
In this approach, the same IP address is assigned to multiple servers located in a distributed manner.

When the client sends requests to the IP address, the requests will be routed to the nearest server defined by the routing policy. With this approach content providers may lose some server selection flexibility. Consider a scenario in which Anycast forwards requests to the nearest (yet overloaded) server, by simply respecting a distance-based routing policy.

CDN service providers who configure their platform with Anycast set a single IP address for all their nodes!
Unlike a DNS Based CDN Redirection, where every node has a unique IP address and recursive DNS routes the client to the closest node, it uses the Border Gateway Protocol (BGP) to route clients using the natural network flow of the Internet
BGP is a network-level protocol that is used by Internet edge routers to exchange routing and reachability information so that every node on the network, even though it is autonomous, knows the state of its closest network neighbors.

Anycast uses this information to efficiently route traffic based on hop count ensuring the shortest traveling distance between the client and its final destination.

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Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

Today, with his carefully selected instructors, OrhanErgun.Net is providing IT courses to tens of thousands of IT engineers. 

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