anycast in networking

Anycast vs Unicast.

• Unicast: Represents one destination/network location.

• Anycast, the same IP prefix is advertised from multiple locations.

                     203.0.113.10/32
                    SAME IP ADDRESS
                         |
          +--------------+--------------+
          |              |              |
       Mumbai         Singapore        London
       advertises     advertises      advertises
          |              |              |
        BGP            BGP            BGP

The Internet's routers use BGP routing information to determine which advertisement/path to use.

User in India
     |
     | 203.0.113.10
     ↓
Internet routers
     |
     ↓
Mumbai POP

while:

User in UK
     |
     | 203.0.113.10
     ↓
Internet routers
     |
     ↓
London POP

The user uses the same IP.

This is fundamentally different from DNS steering.

                         Internet
                            |
              Destination: 203.0.113.10
                            |
              +-------------+-------------+
              |             |             |
              ↓             ↓             ↓
          POP/Region A  POP/Region B  POP/Region C
           Mumbai       Singapore      London
              |             |             |
             BGP           BGP           BGP
              |             |             |
              ↓             ↓             ↓
        Load Balancer  Load Balancer  Load Balancer
              |             |             |
              ↓             ↓             ↓
           Service       Service       Service
              |             |             |
            Pods/VMs     Pods/VMs     Pods/VMs

The actual IP must be from address space you control for a real Internet deployment; 203.0.113.0/24 is documentation space and is useful only for diagrams/labs.

Anycast IP and DNS Steering:

DNS latency steering might do something conceptually like:

DNS
 |
 +-- Mumbai      30 ms
 +-- Singapore   70 ms
 +-- London     150 ms
 |
 ↓
Return Mumbai IP

Anycast gives you routing toward an advertised location; it isn't a DNS server performing a fresh latency test for every client.

Cloud providers can combine Anycast with health, capacity, proximity and other traffic-engineering mechanisms. For example, Google describes its global load balancing as using a single anycast IP and distributing traffic based on factors including proximity, availability and capacity.

---

Cloud Managed Anycast service

                    Anycast IP
                 203.x.x.x / static
                       |
          AWS Global Accelerator
                       |
            +----------+----------+
            |                     |
         Region A              Region B
         Mumbai                Singapore
            |                     |
          ALB/NLB               ALB/NLB
            |                     |
          Service               Service

---

build real Anycast yourself

                 Your ASN
               AS65000
                  |
        +---------+---------+
        |         |         |
        ↓         ↓         ↓
     Router A  Router B  Router C
      Mumbai   Singapore  London
        |         |         |
        ↓         ↓         ↓
      Server    Server    Server

Further quick notes:

• how really anycast multicast and floating ip works. Why and how the same IP prefix is advertised from multiple location. How this is different from DNS steering. How we can assign the same IP to three different system. what is : FRRouting (FRR),BIRD, GoBGP?