Add comprehensive host interface configuration documentation
Document the persistent interface file approach using binds, explaining: - Dual-homing architecture with LACP bonding - VLAN tagging configuration on hosts - Interface file format and parameters - Deployment process and requirements - Testing and troubleshooting procedures
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docs/HOST_INTERFACE_CONFIGURATION.md
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docs/HOST_INTERFACE_CONFIGURATION.md
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# Host Interface Configuration Guide
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## Overview
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All four hosts in the lab use **persistent interface configuration files** mounted via ContainerLab's `binds` feature. This approach provides cleaner, more maintainable configuration compared to using `exec` commands.
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## Architecture
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### Dual-Homing with LACP Bonding
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Each host is dual-homed to an MLAG pair of leaf switches:
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- **host1**: dual-homed to leaf1 + leaf2 (VTEP1)
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- **host2**: dual-homed to leaf3 + leaf4 (VTEP2)
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- **host3**: dual-homed to leaf5 + leaf6 (VTEP3)
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- **host4**: dual-homed to leaf7 + leaf8 (VTEP4)
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### VLAN Configuration
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Hosts handle VLAN tagging using sub-interfaces on the bond:
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| Host | VLAN | IP Address | Purpose | VRF |
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|------|------|------------|---------|-----|
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| host1 | 40 | 10.40.40.101/24 | L2 VXLAN test | default |
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| host2 | 34 | 10.34.34.102/24 | L3 VXLAN test | gold |
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| host3 | 40 | 10.40.40.103/24 | L2 VXLAN test | default |
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| host4 | 78 | 10.78.78.104/24 | L3 VXLAN test | gold |
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## Interface Files Structure
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Each host has a configuration file in `hosts/` directory:
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- `hosts/host1_interfaces` → mounted to `/etc/network/interfaces` in host1
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- `hosts/host2_interfaces` → mounted to `/etc/network/interfaces` in host2
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- `hosts/host3_interfaces` → mounted to `/etc/network/interfaces` in host3
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- `hosts/host4_interfaces` → mounted to `/etc/network/interfaces` in host4
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## Interface Configuration Format
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### Example: host1_interfaces
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```
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auto lo
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iface lo inet loopback
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# Bond interface with LACP (802.3ad)
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auto bond0
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iface bond0 inet manual
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bond-mode 4
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bond-miimon 100
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bond-lacp-rate 1
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bond-slaves eth1 eth2
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# VLAN 40 on bond0
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auto bond0.40
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iface bond0.40 inet static
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address 10.40.40.101
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netmask 255.255.255.0
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vlan-raw-device bond0
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```
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### Key Parameters Explained
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**Bond Configuration:**
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- `bond-mode 4`: LACP (802.3ad) mode - requires LACP on switch side
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- `bond-miimon 100`: Link monitoring interval (100ms)
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- `bond-lacp-rate 1`: Fast LACP (1 second intervals)
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- `bond-slaves eth1 eth2`: Physical interfaces in the bond
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**VLAN Sub-interface:**
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- `bond0.40`: VLAN interface notation (bond0.VLAN_ID)
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- `vlan-raw-device bond0`: Parent interface for VLAN
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- Static IP configuration with address/netmask
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## Deployment Process
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When ContainerLab starts a host:
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1. **Mount interface file** via binds
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2. **Install packages**: `apk add ifupdown bonding vlan`
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3. **Load kernel modules**:
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- `modprobe bonding` - enables LACP bonding
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- `modprobe 8021q` - enables VLAN tagging
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4. **Bring up interfaces**: `ifup -a` reads `/etc/network/interfaces`
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## Switch Configuration Requirements
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For proper LACP operation, leaf switches must have:
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```
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interface Port-Channel1
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description host-X
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switchport mode trunk
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switchport trunk allowed vlan <vlan-id>
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mlag 1
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port-channel lacp fallback timeout 5
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port-channel lacp fallback individual
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no shutdown
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interface Ethernet1
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description host-X-link1
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channel-group 1 mode active
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lacp timer fast
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no shutdown
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```
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**Critical settings:**
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- `port-channel lacp fallback`: Required for ContainerLab timing
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- `lacp timer fast`: Matches host's fast LACP rate
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- `no shutdown`: Must explicitly enable Port-Channel interface
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## Advantages of This Approach
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1. **Persistence**: Configuration survives container restarts
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2. **Clarity**: Single file shows complete network config
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3. **Maintainability**: Easy to modify VLAN assignments
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4. **Production-like**: Mirrors real-world dual-homing scenarios
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5. **Clean deployment**: No manual post-deployment fixes needed
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## Testing Connectivity
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### L2 VXLAN (same VLAN)
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```bash
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# host1 (VLAN 40) → host3 (VLAN 40)
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docker exec clab-arista-evpn-fabric-host1 ping -c 4 10.40.40.103
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```
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### L3 VXLAN (inter-VRF)
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```bash
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# host2 (VLAN 34, VRF gold) → host4 (VLAN 78, VRF gold)
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docker exec clab-arista-evpn-fabric-host2 ping -c 4 10.78.78.104
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```
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## Troubleshooting
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### Verify bond status on host
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```bash
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docker exec clab-arista-evpn-fabric-host1 cat /proc/net/bonding/bond0
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```
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### Check VLAN interface
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```bash
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docker exec clab-arista-evpn-fabric-host1 ip addr show bond0.40
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```
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### Verify LACP on switch
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```bash
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ssh admin@clab-arista-evpn-fabric-leaf1 "show port-channel 1 detailed"
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```
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## References
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- Alpine Linux ifupdown-ng documentation
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- Linux bonding documentation: `/usr/src/linux/Documentation/networking/bonding.txt`
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- Arista MLAG configuration guide
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- srl-labs/srl-evpn-mh-lab (reference implementation)
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