# Run an anycast service

> Announce one prefix from several servers, take a site out when its service fails, and drain it for maintenance.

In anycast, several servers announce the same prefix; routers send each client to the nearest one.
ansage runs on every server and peers with that site's router. This is the setup ansage's
[end-to-end tests](/docs/development/e2e.md) run against FRR and BIRD.

```txt
 clients --- r1 (FRR) ------ r2 (BIRD) --- clients
              |                  |
            srv-a              srv-b
     announces 192.0.2.0/24   announces 192.0.2.0/24
```

### Put the service address on every server

Every server answers on an address from the anycast prefix, usually on the loopback:

```sh
ip addr add 192.0.2.1/32 dev lo
ip addr add 2001:db8:1::1/128 dev lo
```

Make it permanent with your network config (systemd-networkd, netplan, `/etc/network/interfaces`).

### Configure ansage on every server

The same prefixes everywhere; `router_id` differs per server.

```toml title="/etc/ansage/ansage.toml (srv-a)"
asn = 64500
router_id = "203.0.113.21"
announce = ["192.0.2.0/24", "2001:db8:1::/48"]
graceful_restart = true      # upgrades without dropping routes
drain_time = 30              # drain 30 s on every stop

[[neighbor]]
address = "10.201.11.2"
asn = 64510
description = "r1"
next_hop_ipv6 = "2001:db8:201:11::10"   # IPv6 routes over an IPv4 session
```

`next_hop_ipv6` is needed because this session runs over IPv4, see [Next hops](/docs/guides/next-hops.md).

### Configure the routers

Each router peers with its server(s) and passes the prefix on. See
[Router configs](/docs/guides/router-config.md) for FRR and BIRD.

### Install and check

```sh
curl -fsSL https://github.com/miggish/ansage/releases/latest/download/install.sh \
  | sudo sh -s -- --config ./ansage.toml --operator "$USER"
ansage show summary
```

On the router, the prefix now has one path per site.

### Add a health check

A server whose service fails must stop announcing. A small loop calls `ansage withdraw` and
`ansage announce`, see [Health checks](/docs/guides/health-checks.md).

## Maintenance

Take a site out before working on it, so clients move without a single failed connection:

```sh
ansage drain --wait 30     # routers move traffic away, the service still answers
systemctl stop ansage      # or work on the box while drained
...
systemctl start ansage     # or ansage undrain
```

With `drain_time = 30`, `systemctl stop ansage` drains by itself. See [Drain a site](/docs/guides/drain.md).

## Upgrading ansage itself

With `graceful_restart = true` on ansage and the router, `ansage upgrade` restarts ansage while the
router keeps forwarding to this site. See [Upgrades](/docs/guides/upgrades.md).

> **One AS for all sites, or one each:** Both work. With one shared AS, ansage on one site drops the routes for the same prefix that come
> back from another site (its own AS is in the path); with `import = "none"`, the default, it
> stores nothing learned anyway.