feat: flip swap mechanism — MD as primary, RST as override (Phase 1)

This is the first of three phases inverting the per-page swap mechanism
so MD becomes the canonical primary and RST becomes the rare override.

Phase 1 — file renames + conf.py exclude_patterns flip only:
- Rename docs/**/md-<stem>.md to docs/**/<stem>.md (drop md- prefix)
  for all 254 stems previously listed in docs/_swap.txt
- Rename docs/**/<stem>.rst to docs/**/rst-<stem>.rst (add rst- prefix)
  for the same 254 stems
- Repurpose docs/_swap.txt as docs/_rst_overrides.txt; initially empty
  comment-only since no pages need the RST fallback right now
- conf.py exclude_patterns flipped: rst-*.rst is now excluded by default
  instead of md-*.md
- conf.py runtime-artifact references updated to _rst_override_state.json
  and _md_exclude.txt (Phase 2 will rewrite swap_sources.py to produce
  these names; for now no swap script runs because overrides list is empty)

Phase 2 (next commit on this branch) will rewrite scripts/swap_sources.py
with inverted rename direction, delete scripts/import_myst.py + tests, and
update tests/test_swap_sources.py for the new semantics.

Phase 3 will be the cleanup pass and ready-for-review flip.

Generated by robots https://vyos.io

1 files changed, 148 insertions, 0 deletions

diff --git a/docs/vpp/configuration/interfaces/loopback.md b/docs/vpp/configuration/interfaces/loopback.md
new file mode 100644
index 00000000..844892a3
--- /dev/null
+++ b/docs/vpp/configuration/interfaces/loopback.md
@@ -0,0 +1,148 @@
+---
+lastproofread: '2026-03-13'
+---
+
+(vpp-config-interfaces-loopback)=
+
+```{include} /_include/need_improvement.txt
+```
+
+
+# VPP Loopback Interface Configuration
+
+VPP loopback interfaces provide virtual interfaces that remain
+administratively up and are commonly used for stable addressing,
+routing protocols, and as Bridge Virtual Interfaces (BVI). Loopback
+interfaces in VPP offer high-performance virtual connectivity with optimized
+packet processing.
+
+## Basic Configuration
+
+### Creating a Loopback Interface
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\>
+
+Create a loopback interface where ``<vpploN>`` follows the naming
+convention ``vpplo1``, ``vpplo2``, etc.
+```
+
+**Basic Example:**
+
+```none
+set interfaces vpp loopback vpplo1
+```
+
+## Interface Configuration
+### Description and Administrative Control
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> description \<description\>
+
+Set a descriptive name for the loopback interface.
+```
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> disable
+
+Administratively disable the loopback interface.
+```
+
+### Kernel Interface Integration
+
+Kernel interface is bound to the VPP loopback interface for management
+and application compatibility.
+
+## IP Address Configuration
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> address \<ip-address/prefix\>
+
+Configure IPv4 or IPv6 addresses on the kernel interface. Multiple
+addresses can be assigned.
+```
+
+**Examples:**
+
+```none
+# IPv4 address
+set interfaces vpp loopback vpplo1 address 192.168.1.10/24
+
+# IPv6 address
+set interfaces vpp loopback vpplo1 address 2001:db8::10/64
+```
+
+## MTU Configuration
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> mtu \<size\>
+
+Set the Maximum Transmission Unit (MTU) for the kernel interface.
+The MTU must be compatible with the connected VPP interface.
+```
+
+## VLAN Configuration
+
+VPP kernel interfaces support VLAN (Virtual LAN) sub-interfaces for network
+segmentation.
+
+### Creating VLAN Sub-interfaces
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> vif \<vlan-id\>
+
+Create a VLAN sub-interface with the specified VLAN ID (0-4094).
+```
+
+### VLAN Sub-interface Configuration
+
+VLAN sub-interfaces support the same configuration options as the parent
+interface:
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> vif \<vlan-id\> address \<ip-address/prefix\>
+```
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> vif \<vlan-id\> description \<description\>
+```
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> vif \<vlan-id\> disable
+```
+
+```{cfgcmd} set interfaces vpp loopback \<vpploN\> vif \<vlan-id\> mtu \<size\>
+```
+
+**Examples:**
+
+```none
+# Configure VLAN 100
+set interfaces vpp loopback vpplo1 vif 100 address 192.168.100.1/24
+set interfaces vpp loopback vpplo1 vif 100 description "Management VLAN"
+set interfaces vpp loopback vpplo1 vif 100 mtu 1500
+
+# Configure VLAN 200
+set interfaces vpp loopback vpplo1 vif 200 address 192.168.200.1/24
+set interfaces vpp loopback vpplo1 vif 200 description "Guest VLAN"
+```
+
+## Configuration Examples
+### Basic Loopback Interface
+
+```none
+# Create simple loopback
+set interfaces vpp loopback vpplo1
+set interfaces vpp loopback vpplo1 description "Router ID interface"
+```
+
+### Loopback with Kernel Interface
+
+```none
+# Loopback with management access
+set interfaces vpp loopback vpplo2
+set interfaces vpp loopback vpplo2 description "Management loopback"
+set interfaces vpp loopback vpplo2 address 10.255.255.1/32
+```
+
+### Bridge Virtual Interface (BVI)
+
+```none
+# Loopback as BVI for bridge
+set interfaces vpp loopback vpplo3
+set interfaces vpp loopback vpplo3 description "Bridge gateway interface"
+set interfaces vpp bridge vppbr1
+set interfaces vpp bridge vppbr1 member interface vpplo3 bvi
+set interfaces vpp loopback vpplo3 address 192.168.100.1/24
+```