# Next hops

> Which next hop ansage sends, IPv6 routes over an IPv4 session, and link-local addresses.

Every route ansage announces carries a next hop: the address the router forwards the traffic to.
By default it is ansage's own address of the session.

| Session over          | IPv4 routes                           | IPv6 routes                           |
| --------------------- | ------------------------------------- | ------------------------------------- |
| IPv4                  | our local IPv4 address of the session | **none**: set `next_hop_ipv6`         |
| IPv6 (global address) | **none**: set `next_hop_ipv4`         | our local IPv6 address of the session |

## IPv6 routes over an IPv4 session

The most common case: one session over IPv4 that carries both families. Without an IPv6 next
hop, ansage cannot announce the IPv6 prefixes on it, so set the server's IPv6 address on that
link:

```toml
[[neighbor]]
address = "10.0.0.1"
asn = 64501
next_hop_ipv6 = "2001:db8:0:1::10"
```

Without it, ansage logs once per session
`no next hop for this family (session runs over the other family; set next_hop_ipv4/next_hop_ipv6), its routes are not announced`,
and `ansage show summary` shows `0` sent IPv6 prefixes for that neighbor.

The router needs the same for the routes it sends back (BIRD: `next hop address` in the `ipv6`
channel), see [Router configs](/docs/guides/router-config.md).

## IPv4 routes over an IPv6 session

The other way round: set `next_hop_ipv4`.

```toml
[[neighbor]]
address = "2001:db8:0:1::1"
asn = 64501
next_hop_ipv4 = "10.0.0.10"
```

## Link-local next hops

On a link shared with the neighbor, IPv6 routes may also carry our link-local address next to
the global one (RFC 2545):

```toml
[[neighbor]]
address = "2001:db8:0:1::1"
asn = 64501
next_hop_ipv6_link_local = "fe80::10"
```

`next_hop_ipv6_link_local` must be in `fe80::/10`, and `next_hop_ipv6` must be a global address.

## Rules ansage checks

- A next hop is never the neighbor's own address (RFC 4271); `ansage check` refuses that.
- `next_hop_ipv6` is global, `next_hop_ipv6_link_local` is link-local.

## Routes ansage learns

With `[kernel]`, the next hops of learned routes matter too: ansage resolves each through the
kernel's routing table and uses link-local next hops on a shared link. See
[Kernel routes](/docs/guides/kernel-routes.md#next-hops).