Add .github/copilot-instructions.md for AI-assisted development

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+# Copilot Instructions for sonic-frr
+
+## Project Overview
+
+sonic-frr is SONiC's fork of FRRouting (FRR), the open-source IP routing protocol suite. It provides BGP, OSPF, IS-IS, PIM, BFD, LDP, PBR, Babel, EIGRP, NHRP, and other routing protocol implementations. SONiC uses FRR as its routing stack, with FPM (Forwarding Plane Manager) integration to push routes into the SONiC data plane via fpmsyncd.
+
+## Architecture
+
+```
+sonic-frr/
+├── bgpd/               # BGP daemon implementation
+├── ospfd/               # OSPFv2 daemon
+├── ospf6d/              # OSPFv3 daemon
+├── isisd/               # IS-IS daemon
+├── pimd/                # PIM-SM/SSM daemon
+├── bfdd/                # BFD daemon
+├── ldpd/                # LDP daemon
+├── pbrd/                # PBR daemon
+├── staticd/             # Static route daemon
+├── babeld/              # Babel daemon
+├── eigrpd/              # EIGRP daemon (alpha)
+├── nhrpd/               # NHRP daemon (alpha)
+├── zebra/               # Core routing daemon (RIB, FPM, kernel integration)
+├── lib/                 # Shared libraries (vty, thread, prefix, etc.)
+├── vtysh/               # VTY shell (CLI)
+├── yang/                # YANG models for routing protocols
+├── tests/               # Test suite
+├── doc/                 # Documentation (Sphinx)
+├── debian/              # Debian packaging
+├── bootstrap.sh         # Autotools bootstrap
+├── configure.ac         # Autoconf configuration
+└── Makefile.am          # Automake build rules
+```
+
+### Key Concepts
+- **Zebra**: Central daemon managing the RIB (Routing Information Base) and kernel route installation
+- **FPM (Forwarding Plane Manager)**: Interface to push routes to external consumers (SONiC's fpmsyncd)
+- **VTY**: Virtual terminal interface — the CLI for configuring routing protocols
+- **Zserv**: Internal protocol for communication between routing daemons and zebra
+- **YANG models**: Data models for protocol configuration and state
+
+## Language & Style
+
+- **Primary language**: C (core daemons), Python (tests, scripts)
+- **C standard**: C11
+- **Indentation**: Tabs (kernel style) in C code
+- **Naming conventions**:
+  - Functions: `module_action_object` (e.g., `bgp_route_add`, `ospf_lsa_install`)
+  - Structs: `struct bgp_info`, `struct ospf_area`
+  - Macros: `UPPER_CASE`
+  - Enums: `UPPER_CASE` values
+- **Code formatting**: Follow existing FRR style — see `.clang-format`
+- **Memory management**: Use FRR's memory tracking macros (`XMALLOC`, `XFREE`, `MTYPE_*`)
+- **Threading**: FRR uses an event-driven model with `struct thread_master` — avoid blocking calls
+- **Logging**: Use `zlog_*` macros (e.g., `zlog_info`, `zlog_warn`, `zlog_err`)
+
+## Build Instructions
+
+```bash
+# Bootstrap autotools
+./bootstrap.sh
+
+# Configure (typical SONiC build configuration)
+./configure \
+  --prefix=/usr \
+  --sysconfdir=/etc/frr \
+  --localstatedir=/var/run/frr \
+  --enable-fpm \
+  --enable-multipath=64
+
+# Build
+make -j$(nproc)
+
+# Install
+sudo make install
+
+# Build Debian package
+dpkg-buildpackage -rfakeroot -b -us -uc
+```
+
+## Testing
+
+```bash
+# Run unit tests
+make check
+
+# Run topotests (topology-based integration tests)
+cd tests/topotests
+sudo pytest -v
+
+# Individual protocol tests
+sudo pytest tests/topotests/bgp_* -v
+```
+
+### Test Types
+- **Unit tests**: In `tests/` — test individual functions and data structures
+- **Topotests**: Topology-based integration tests using Mininet-like environments
+- **VTY tests**: Test CLI command parsing and configuration
+
+## PR Guidelines
+
+- **Signed-off-by**: Required on all commits
+- **CLA**: Sign Linux Foundation EasyCLA (for SONiC changes)
+- **Upstream FRR**: Consider whether changes should also be submitted upstream to FRRouting/frr
+- **SONiC-specific patches**: Clearly separate SONiC-specific patches from upstream FRR code
+- **Commit message**: Reference the FRR module: `bgpd:`, `zebra:`, `ospfd:`, etc.
+- **Testing**: Include topotests for new protocol features
+
+## Dependencies
+
+- **libyang**: YANG data modeling library
+- **json-c**: JSON parsing
+- **readline**: CLI line editing
+- **libcap**: Linux capabilities
+- **protobuf-c**: Protocol Buffers for FPM
+- **libnl**: Netlink communication
+
+## Gotchas
+
+- **FPM integration**: SONiC relies heavily on FPM — changes to zebra's FPM code directly affect route programming
+- **SONiC fork divergence**: This fork carries SONiC-specific patches on top of upstream FRR; rebase carefully
+- **Event loop**: All daemons use FRR's event loop — never block in callbacks
+- **Memory types**: Register new memory types with `MTYPE_*` macros for leak detection
+- **VTY commands**: Follow the `DEFUN`/`DEFPY` macro pattern for new CLI commands
+- **Thread safety**: FRR daemons are largely single-threaded per process — the event-driven model handles concurrency
+- **YANG models**: New configuration should include YANG model definitions
+- **Route redistribution**: Changes to route redistribution affect the entire routing stack