Anycast works by announcing the same IP address from multiple locations simultaneously. Unlike unicast (traditional single-server routing) or multicast, anycast routes traffic to the nearest available node based on BGP routing metrics. This requires:

• A public Autonomous System Number (ASN)
• BGP-enabled network infrastructure
• Control over IP address space (either through ownership or provider delegation)

# Basic BGP configuration example (Cisco IOS)
router bgp 64512
 network 192.0.2.0/24
 neighbor 203.0.113.1 remote-as 64500
 neighbor 203.0.113.1 route-map SET-LOCAL-PREF in
!
route-map SET-LOCAL-PREF permit 10
 set local-preference 200

Key technical considerations:

• IP Address Ownership: You'll need a /24 IPv4 block (or larger) registered with RIR
• BGP Peering: Each location must establish BGP sessions with upstream providers
• Traffic Engineering:
  • Local Preference settings for primary/backup nodes
  • AS Path prepending for traffic balancing

While many companies offer anycast DNS (like Cloudflare or NS1), few provide generic anycast IP services. Notable exceptions include:

• Bare Metal Providers:

  # Example using Packet (now Equinix Metal) API
  curl -X POST -H "Content-Type: application/json" \
    -H "X-Auth-Token: $API_KEY" \
    -d '{"anycast_ip": "192.0.2.1", "facilities": ["dfw2", "sjc1", "ams1"]}' \
    https://api.packet.net/projects/{project_id}/anycast-ips

• Cloud Providers: AWS Global Accelerator, Google Cloud Premium Tier
• Specialized Networks: Fastly, StackPath

TCP Session Persistence: Anycast breaks TCP connections when routes change. Solutions:

// Node.js example using session stickiness via geohash
const geohash = require('ngeohash');
const region = geohash.encode(lat, lon, 3); // 3-character precision

app.use((req, res, next) => {
  if(req.cookies.region && req.cookies.region !== region) {
    // Redirect to proper region
    res.redirect(https://${region}.anycast.example.com${req.path});
  }
  res.cookie('region', region);
  next();
});

Route Flapping Prevention:

• Set appropriate BGP timers (holdtime > 3x keepalive)
• Implement dampening with care (default: suppress route for 15-30 minutes after 3 flaps)

Monitoring Considerations:

# Prometheus blackbox exporter config for anycast monitoring
modules:
  anycast_http:
    prober: http
    http:
      preferred_ip_protocol: "ip4"
      ip_protocol_fallback: false
    timeout: 5s

Typical anycast deployment costs include:

Alternative approaches:

# Using AWS Global Accelerator CLI commands
aws globalaccelerator create-accelerator \
  --name "MyAnycastService" \
  --ip-address-type IPV4 \
  --enabled

Anycast is a network addressing and routing method where incoming requests are routed to the topologically nearest server in a group of potential receivers. Unlike unicast (one-to-one) or multicast (one-to-many), anycast enables one-to-nearest communication - perfect for geographically distributed web services.

To implement anycast for your web service, you'll need:

• BGP (Border Gateway Protocol) capability
• Multiple geographically distributed data centers
• Identical server configurations across locations
• Stateless application architecture (or session synchronization)

Here's a simple Python script to verify anycast routing is working as expected:

import requests
from collections import Counter

locations = [
    "nyc.yourservice.com",
    "lon.yourservice.com",
    "tok.yourservice.com",
    "syd.yourservice.com"
]

def test_anycast(target_url, iterations=10):
    results = []
    for _ in range(iterations):
        try:
            response = requests.get(target_url, timeout=2)
            results.append(response.headers.get('X-Server-Location', 'unknown'))
        except:
            results.append('error')
    return Counter(results)

for location in locations:
    print(f"Testing {location}:")
    print(test_anycast(f"http://{location}/ping"))

When evaluating anycast providers, consider these technical aspects:

• BGP Peering Relationships: More peers mean better routing
• POP (Point of Presence) Distribution: Geographic coverage matters
• Anycast Prefix Length: /24 for IPv4 is standard
• Traffic Engineering Capabilities: Can they adjust routing based on load?

Be aware of these technical challenges:

// Example of problematic session handling in anycast
app.use(session({
  secret: 'your-secret-key',
  resave: false,
  saveUninitialized: true,
  store: new MemoryStore() // This won't work across locations!
}));

Instead, use distributed session stores like Redis or database-backed sessions.

If full anycast isn't feasible, consider:

• DNS-based geo-routing
• CDN edge computing
• Global load balancers

Essential metrics to track:

# Sample Prometheus query for anycast performance
sum(rate(http_requests_total{job="web-service"}[5m])) by (location)