VPP: Add comprehensive VPP documentation

This commit introduces extensive documentation for VyOS VPP (Vector Packet Processing)
Dataplane interfaces and features, including examples.

Features documented:
* VPP integration description, base requirements and limitations
* VPP dataplane core settings
* Features enabled in the dataplane: ACL, IPsec, NAT, sFlow
* VPP interface types: bonding, bridge, GRE, IPIP, kernel, loopback, VXLAN, XConnect

Note: This is an initial documentation version that will require ongoing improvements
based on user feedback and questions, real-world deployment testing and validation, additional
use cases and configuration scenarios discovered in production. Therefore, all pages are
marked as "need improvement".

1 files changed, 96 insertions, 0 deletions

diff --git a/docs/vpp/configuration/interfaces/ipip.rst b/docs/vpp/configuration/interfaces/ipip.rst
new file mode 100644
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+++ b/docs/vpp/configuration/interfaces/ipip.rst
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+:lastproofread: 2025-09-04
+
+.. _vpp_config_interfaces_ipip:
+
+.. include:: /_include/need_improvement.txt
+
+######################
+VPP IPIP Configuration
+######################
+
+VPP IPIP interfaces provide IP-in-IP tunneling with high-performance packet processing. IPIP tunnels encapsulate IP packets within IP packets, creating point-to-point connections across Layer 3 networks.
+
+Basic Configuration
+-------------------
+
+Creating an IPIP Interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. cfgcmd:: set vpp interfaces ipip <ipipN>
+
+   Create an IPIP interface where ``<ipipN>`` follows the naming convention ipip1, ipip2, etc.
+
+.. cfgcmd:: set vpp interfaces ipip <ipipN> remote <address>
+
+   Set the tunnel remote endpoint address. Supports both IPv4 and IPv6 addresses.
+
+.. cfgcmd:: set vpp interfaces ipip <ipipN> source-address <address>
+
+   Set the tunnel source address. Must match an address configured on the local system.
+
+**Basic Example:**
+
+.. code-block:: none
+
+   set vpp interfaces ipip ipip1
+   set vpp interfaces ipip ipip1 remote 203.0.113.2
+   set vpp interfaces ipip ipip1 source-address 192.168.1.1
+
+Interface Configuration
+-----------------------
+
+Description and Administrative Control
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. cfgcmd:: set vpp interfaces ipip <ipipN> description <description>
+
+   Set a descriptive name for the IPIP interface.
+
+.. cfgcmd:: set vpp interfaces ipip <ipipN> disable
+
+   Administratively disable the IPIP interface.
+
+Kernel Interface Integration
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. cfgcmd:: set vpp interfaces ipip <ipipN> kernel-interface <interface-name>
+
+   Bind a kernel interface to the IPIP tunnel for management and application compatibility.
+
+For detailed information about kernel interface integration, see :doc:`kernel`.
+
+Configuration Examples
+----------------------
+
+IPv4 IPIP Tunnel
+^^^^^^^^^^^^^^^^
+
+.. code-block:: none
+
+   # Basic IPv4 IPIP tunnel
+   set vpp interfaces ipip ipip1
+   set vpp interfaces ipip ipip1 description "Site-to-site IPIP tunnel"
+   set vpp interfaces ipip ipip1 remote 203.0.113.10
+   set vpp interfaces ipip ipip1 source-address 192.168.1.1
+
+IPv6 IPIP Tunnel
+^^^^^^^^^^^^^^^^
+
+.. code-block:: none
+
+   # IPv6 endpoints
+   set vpp interfaces ipip ipip2
+   set vpp interfaces ipip ipip2 remote 2001:db8::2
+   set vpp interfaces ipip ipip2 source-address 2001:db8::1
+
+IPIP with Kernel Interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: none
+
+   # IPIP tunnel with management interface
+   set vpp interfaces ipip ipip3
+   set vpp interfaces ipip ipip3 remote 203.0.113.30
+   set vpp interfaces ipip ipip3 source-address 192.168.1.1
+   set vpp interfaces ipip ipip3 kernel-interface vpptun3
+   set vpp kernel-interfaces vpptun3 address 10.0.2.1/30