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

Now, you’ll need to create a vmbridge interface as well so that VMs can use the bonded interface and that’s where the IP will go And hit apply!

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.