Link Aggregation - The switch and the server have to agree

Link aggregation gives us link redundancy and doubles the bandwidth¹.
We more or less virtualize those two physical links into a single “Bond”, “Port-Channel”, “Ether-Channel”, or “Link Aggregation/LAG/LAGG” interface using one of several protocols. I only know of LACP/802.3ad so that’s what I’ll explain from a practicioners point of view.

Layer 1 — physical. Just the cabling: two independent links, nothing aggregated yet.

flowchart LR
    subgraph Host["pve-node1"]
        nic0["Nic 0"]
        nic1["Nic 1"]
    end
    subgraph Switch["access-switch"]
        int111["Int 1/1/1"]
        int112["Int 1/1/2"]
    end
    nic0 === |"Cat6"| int111
    nic1 === |"Cat6"| int112

Layer 2 — data link. LACP (802.3ad) is what turns those two physical links into one logical one. bond0 and lag1 are the aggregate interfaces;

flowchart LR
    subgraph Host["pve-node1"]
        nic0["Nic 0"]
        nic1["Nici 1"]
        Bond["bond0 (802.3ad)"]
        nic0 --- Bond
        nic1 --- Bond
    end
    subgraph Switch["access-switch"]
        lag1["lag 1"]
        int111["int 1/1/1"]
        int112["int 1/1/2"]
        lag1 --- int111
        lag1 --- int112
    end
    Bond <-.-> |"LACP 'session'"| lag1

Layer 3 — network. The IP address lives on the aggregate interface, not on either physical NIC — nic0/nic1 never get addresses of their own.

flowchart LR
    subgraph Host["pve-node1"]
        IP1["bond0
        10.10.0.11/24"]
    end
    subgraph Switch["access-switch"]
        IP2["VLAN 10
        (no IP · bridged only)"]
    end
    subgraph Host2["pve-node2"]
        IP3["bond0
        10.10.0.12/24"]
    end
    IP1 --- IP2
    IP2 --- IP3
    IP1 <-.->|"IP reachability"| IP3

On the proxmox side, it looks about like this Datacenter/Host/Network/Create Linux Bond

On the switch, the same two physical ports need to join one port-channel with LACP active on both sides:

interface 1/1/1
    description pve-eno0
    no shutdown
    lag 1
    exit
interface 1/1/2
    description pve-eno1
    no shutdown
    lag 1
    exit
interface lag 1
    description pve-lacp
    no shutdown
    no routing
    vlan trunk native 10
    vlan trunk allowed all
    lacp mode active
    exit

Once both ends agree, cat /proc/net/bonding/bond0 will show both slaves in state: 1 (active). If it shows one link active and one stuck in an aggregator of its own, that’s the switch side — not Linux.