(module-srv6)=
# Segment Routing over IPv6 (srv6) Configuration Module

This configuration module configures SRv6 over IS-IS or BGP and can use the advertised SIDs for global and L3VPN connectivity services.

Supported SRv6 features:

* Micro SIDs in F3216 format
* IS-IS routing with SRv6 TLVs
* Advertising SRv6 SIDs with BGP
* SRv6 next hops for global IPv4/IPv6 BGP routes (Layer-3 services with SRv6)
* BGP IPv4/IPv6 L3VPN over SRv6

You can use IS-IS or EBGP to advertise SRv6 SIDs[^CRM], and use SRv6 for layer-3 services (IPv4 islands over SRv6, BGP-free core with IPv6) and IPv4/IPv6 layer-3 VPNs.

[^CRM]: The module checks whether the corresponding routing module is enabled and will trigger a configuration error if it's missing.

(module-srv6-platform)=
## Platform Support

The following table describes the per-platform support of SRv6 features:

```{features}
- title: SRv6<br>with IS-IS
  enabled: srv6.isis
- title: SRv6<br>with EBGP
  enabled: srv6.ebgp
- title: Global BGP<br>over SRv6[^GB]
  enabled: srv6.bgp
- title: L3VPN<br>over SRv6[^L3V]
  enabled: srv6.vpn
```

```{note}
* The minimum Cisco IOS/XE release with working SRv6 is release 17.16.01a.
* Cisco 8000v does not support the *shift-only* microsegment behavior needed to interoperate with other SRv6 implementations. 
* When in doubt, check the [SRv6 integration test results](https://release.netlab.tools/_html/coverage.srv6).
```

[^GB]: Using SRv6 next hops for global IPv4/IPv6 BGP routes.

[^L3V]: Using SRv6 next hops for L3VPN routes. VPNv4 and VPNv6 address families are enabled on IPv6 IBGP sessions.

## Configurable Global and Node Parameters

* **addressing.srv6_locator** -- global address pool[^poolname] for allocation of SRv6 locator prefixes, the default prefix is defined in `topology.defaults.srv6.locator_pool` (5F00::/16, the IANA reserved range defined by [RFC9602](https://datatracker.ietf.org/doc/rfc9602/)
* **srv6.allocate_loopback** -- global flag (default: `False`) to replace the IPv6 loopback address of each SRv6-enabled node with an IPv6 address allocated from the locator range
* **srv6.bgp** -- enable BGP with IPv4 and IPv6 address families over SRv6 (disabled by default, [more details](srv6-services)).
* **srv6.vpn** -- enable BGP with VPNv4 and VPNv6 address families over SRv6 ([more details](srv6-services)). BGP/SRv6 L3VPN is disabled by default.
* **srv6.igp** -- list of IGP protocols for which to enable SRv6, default `[isis]`. To exchange SRv6 SIDs between autonomous systems, add `ebgp` to **srv6.igp**

[^poolname]: You can change the name of the default SRv6 locator pool with the `topology.defaults.const.srv6.locator_pool.name` parameter

(srv6-node-parameters)=
## Node Parameters

* **srv6.locator**: an optional IPv6 address prefix to allocate to a given SRv6 node; by default, each node is assigned a unique /48 prefix from the global pool
* **srv6.transit_only**: an optional Boolean flag to optimize resource usage and only allocate transit behaviors, not endpoint behaviors

(srv6-services)=
## Configurable Layer-3 Services and L3VPN Parameters

This module provides 2 parameters that are identical in structure, controlling different BGP address families:

* **srv6.bgp** -- Controls SRv6 next hops for global IPv4 and IPv6 BGP routes
* **srv6.vpn** -- Controls VPNv4 and VPNv6 address families

Each parameter could be a boolean (*True* to enable both IP address families on IBGP sessions), or a dictionary of parameters:

* **ipv4** -- enable IPv4/VPNv4 address family
* **ipv6** -- enable IPv6/VPNv6 address family

**ipv4** and **ipv6** parameters could be:

* Boolean value *False* to disable the address family within node data (overwriting global defaults)
* Boolean value *True* to enable the address family on IBGP sessions
* A string or a list of *ibgp/ebgp* keywords[^NE]

For tested examples, see the [SRv6 integration tests](https://github.com/ipspace/netlab/tree/dev/tests/integration/srv6).

[^NE]: The only SRv6 service _netlab_ supports over EBGP are global BGPv4 routes with SRv6 SID next hops.