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Replace vmbr0 by recreating its actual function: a Linux bridge with the physical Ethernet interface and each guest tap interface attached. Put the host’s management address on that bridge (or on a VLAN interface connected to it), make VLAN handling consistent from the switch through the host to the guest, and verify an untagged DHCP path before introducing tags. A bridge can show UP while DHCP is still failing because the broadcast is on the wrong VLAN, no DHCP server is reachable, or the guest is connected to the wrong virtual network.
What changes when you leave Proxmox
Proxmox’s vmbrX devices are ordinary Linux bridges used as virtual switches. The bridge connects a physical NIC, such as eno1, to guest tap devices. When you move to libvirt, another hypervisor, or a hand-built Linux host, there is no automatic replacement for that switch. You must create the bridge, attach the NIC, attach guest interfaces, and decide where Layer 3 addressing and VLAN tagging belong.
The name does not matter. vmbr0, br0, and lan-bridge are equivalent names if they provide the same forwarding behavior. What matters is the topology:
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- The physical Ethernet port is normally a bridge port, not an independently addressed interface.
- The host’s management address and default route live on the bridge, or on a VLAN interface built on that bridge when management traffic is tagged.
- Guest tap or veth interfaces are attached to the bridge or to a deliberately selected VLAN-aware network.
- The switch port, host bridge, and guest network must agree about whether frames are tagged or untagged.
Proxmox documentation describes the requirement directly: a Linux bridge interface is needed to connect guests to the underlying physical network. Your replacement stack must reproduce that relationship even if it uses different configuration files or a different bridge name.
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Inventory the old path before changing it
Do not begin by deleting the old configuration. Record enough information to reconstruct the path and to recover remotely if the new bridge cuts access. Keep a console, IPMI, serial session, or another out-of-band route available.
Host and guest inventory
- Physical interface name and MAC address. Run
ip -br linkandip link show. - Current addresses and routes. Run
ip -br addressandip route. - Bridge membership. Run
bridge linkandbridge vlanwhere available. - Guest NIC model, MAC address, and the virtual network or bridge to which it is connected.
- Whether each guest expects an untagged access network or sends 802.1Q tags itself.
- Switch-port mode, native or untagged VLAN, and the complete allowed VLAN list.
- Where DHCP is supposed to run: an upstream router or server, a service on a tagged VLAN, or a libvirt-managed NAT network.
Interface names can change after a hardware replacement or operating-system migration. Match the replacement configuration by stable attributes such as the MAC address and physical port, rather than assuming the old name will return.
Build the replacement in layers
Use this order: physical carrier, bridge, host Layer 3 configuration, one untagged guest, then VLANs. Testing in another order makes a failed DHCP request ambiguous.
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Check that the intended NIC has carrier and is not being configured simultaneously by two network managers:
ip -br link
ethtool eno1
networkctl status eno1The exact status command depends on your distribution. A carrier reported by ethtool proves only that the cable and link negotiation work. It does not prove that the switch is carrying the VLANs you need.
2. Create the bridge and enslave the NIC
Choose one configuration owner. Mixing systemd-networkd, NetworkManager, and distribution-generated files for the same interface commonly produces competing addresses, routes, or bridge membership. Stop or disable the old manager only when you have console access and a rollback plan.
3. Move Layer 3 configuration
Remove the host address from the physical NIC and assign it to the bridge. A physical port carrying bridge traffic should normally have no independent address. If management is tagged, create a VLAN interface on the bridge and assign the address there instead.
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4. Prove an untagged path
Connect one disposable test guest to the plain bridge. Inside the guest, confirm the virtual NIC has carrier, then watch for a DHCP exchange. If the guest never receives an offer, do not add VLAN filtering yet; resolve the bridge, switch access VLAN, or DHCP-server path first.
systemd-networkd: a minimal bridge
systemd-networkd creates virtual devices from .netdev files and applies settings to matching links through .network files. The following example creates br0, attaches eno1, and obtains the host address by DHCP. Replace the interface name and addressing with values from your inventory.
Create the bridge device
# /etc/systemd/network/10-br0.netdev
[NetDev]
Name=br0
Kind=bridgeAttach the physical port
# /etc/systemd/network/20-eno1.network
[Match]
Name=eno1
[Network]
Bridge=br0Address the bridge, not the NIC
# /etc/systemd/network/30-br0.network
[Match]
Name=br0
[Network]
DHCP=yes
IPv6AcceptRA=yesFor a static management address, replace DHCP=yes with an address and route appropriate to your network:
[Network]
Address=192.0.2.10/24
Gateway=192.0.2.1
DNS=192.0.2.1The example address range is documentation space; use your real management subnet. After staging files, reload and inspect the result:
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networkctl reload
networkctl reconfigure br0
networkctl status br0
bridge link
ip address show dev br0
ip routeIf the host loses connectivity, use the console to restore the previous files or reboot into the known-good configuration. A reboot can be safer than restarting an uncertain active manager, but schedule it because an incorrect bridge can remove remote access.
Netplan: bridge and VLAN-backed management
On systems that use Netplan, the YAML is rendered to a backend such as systemd-networkd. Keep the physical Ethernet definition empty and put addresses on the bridge:
network:
version: 2
renderer: networkd
ethernets:
eno1: {}
bridges:
br0:
interfaces: [eno1]
dhcp4: true
parameters:
stp: true
forward-delay: 4Validate before applying. Use netplan try when available so the configuration automatically rolls back if you do not confirm it. Apply permanently with netplan apply only after the generated plan is correct.
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Put management on a tagged VLAN
If the switch sends management traffic tagged as VLAN 20, define a VLAN link whose parent is the bridge and place the address on that VLAN:
network:
version: 2
renderer: networkd
ethernets:
eno1: {}
bridges:
br0:
interfaces: [eno1]
parameters:
stp: true
vlans:
mgmt:
id: 20
link: br0
addresses: [192.0.2.10/24]
routes:
- to: default
via: 192.0.2.1
nameservers:
addresses: [192.0.2.1]Do not also assign 192.0.2.10 to eno1 or br0. Two interfaces claiming the same address can produce unstable ARP replies and an apparently random default route.
VLANs: choose one tagging model
There are two valid designs. Select one per segment and document it. Most DHCP failures during migrations come from combining both.
| Design | Where the tag is added | What the guest sees | Typical use |
|---|---|---|---|
| Guest-tagged trunk | The guest network stack or virtual switch port | A trunk capable of carrying several VLANs | Routers, firewalls, and guests that intentionally create VLAN subinterfaces |
| Host VLAN access networks | A host VLAN subinterface or dedicated bridge | An ordinary untagged Ethernet segment | General-purpose guests that should belong to one VLAN without VLAN configuration inside the guest |
Do not tag a guest interface while also attaching it to a host access bridge that already inserts the same VLAN. That creates double tagging or places the DHCP request on an unexpected segment.
Configure a tagged trunk with bridge VLAN filtering
A trunk requires agreement at three points: the physical switch port, the host bridge, and the guest connection. The switch must allow every required VLAN, the bridge must filter and permit those VLANs on both its master and the physical port, and the guest must either receive the trunk or be assigned the intended access VLAN.
Switch-side checks
- Set the port to the correct trunk or hybrid mode for your switch.
- Allow the exact VLAN IDs needed by the host and guests; an allowed-list omission silently drops DHCP broadcasts.
- Confirm the native or untagged VLAN, if one exists, matches the host’s intended untagged network.
- Check the switch MAC table after a guest transmits. Learning on a different port indicates a cabling, aggregation, or loop problem.
Bridge-side checks
Linux bridge VLAN filtering must be enabled before VLAN membership has an effect. With iproute2, inspect and adjust membership using commands such as:
bridge vlan show
bridge vlan add dev br0 vid 20 self
bridge vlan add dev eno1 vid 20
bridge vlan add dev br0 vid 30 self
bridge vlan add dev eno1 vid 30The exact PVID and untagged flags depend on your design. A bridge master and its enslaved port are separate VLAN-aware objects; permitting VLAN 20 on only one of them is insufficient. Persist the settings in your network manager rather than relying on one-time commands.
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systemd-networkd VLAN membership
For networkd, enable filtering in the bridge’s .netdev definition and add matching [BridgeVLAN] sections in the relevant .network files. A representative pattern is:
# /etc/systemd/network/10-br0.netdev
[NetDev]
Name=br0
Kind=bridge
[Bridge]
VLANFiltering=yes# In the bridge and port .network files, repeat membership as required
[BridgeVLAN]
VLAN=20
[BridgeVLAN]
VLAN=30On some systemd releases, PVID and egress-untagged behavior are expressed as additional keys in the same section. Check the systemd-networkd version installed on the host and validate the generated state with networkctl and bridge vlan show. The important rule is that VLANFiltering= must be enabled and each VLAN must be explicitly permitted on the bridge and port.
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Host VLAN subinterfaces and dedicated guest bridges
Instead of exposing a trunk to guests, create one host VLAN interface per segment and attach each to a separate bridge. For example, VLAN 20 can become br20, with the host interface br20 carrying an untagged network to its guests. The physical path is still a trunk, but the guest does not need VLAN support.
# Conceptual topology
eno1 (trunk) -> br0 (VLAN-aware) -> eno1.20 -> br20 -> guest NICThe exact persistent syntax varies by network manager. Proxmox documentation commonly shows VLAN subinterfaces such as bond0.20; the same principle applies with a physical parent or bond in another stack. Ensure that the VLAN subinterface is created on the bridge or trunk parent required by your manager, not accidentally on a port that is already enslaved and has no Layer 3 role.
Why DHCP fails even when the bridge is up
ip link reporting UP means the interface is administratively enabled and may have local carrier. It does not prove that a DHCPDISCOVER reached a server or that the reply returned on the same VLAN.
The broadcast is on the wrong VLAN
Inspect the guest’s VLAN mode, the bridge VLAN table, and the switch allowed list. A guest configured for VLAN 30 on a bridge that only permits VLAN 20 will transmit frames that are discarded locally. Conversely, a guest expecting an untagged access network may send untagged traffic into a trunk with no native VLAN configured.
No DHCP service exists on that segment
On a physically bridged network, the upstream router or DHCP server must listen on the selected VLAN. A successful bridge setup cannot manufacture a lease. Check the DHCP server’s scope, relay configuration, and logs for the guest MAC address.
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libvirt NAT is a different path
A libvirt NAT network can provide guest DNS and DHCP from its own virtual network, independent of the physical trunk. A guest attached to a libvirt NAT network should receive an address from that network’s configured range, not from the physical LAN. Inspect networks and leases with:
virsh net-list --all
virsh net-dumpxml default
virsh net-dhcp-leases defaultIf the guest must appear directly on the physical LAN, attach its virtual NIC to the Linux bridge instead of the NAT network. If isolation and outbound access are sufficient, keep NAT and troubleshoot the libvirt network rather than the switch trunk.
The host has competing addresses or routes
Remove stale addresses from the NIC, old bridge, and unused VLAN interfaces. Then inspect:
ip -br address
ip route get 192.0.2.1
ip neighMultiple default routes, duplicate addresses, or an ARP entry learned on the wrong interface can make the host appear intermittently reachable and can distract from the guest’s actual Layer 2 failure.
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Do not assume a Wi-Fi interface can replace an Ethernet bridge port. Wireless interfaces generally cannot be attached to a normal Linux host bridge because 802.11 client mode does not provide the same transparent source-MAC forwarding behavior as Ethernet. Libvirt documentation explicitly warns that wireless interfaces cannot be attached to a Linux host bridge.
Use wired Ethernet for a true bridged guest network. If only Wi-Fi is available, use routed or NAT networking, or a design specifically supported by the wireless hardware and access point. Upstream switches, cloud providers, and managed networks may also block unknown source MAC addresses, bridge protocols, or promiscuous traffic; authorization may be required even when the Linux configuration is correct.
A verification workflow that finds the failing layer
Work from the outside inward so each observation narrows the fault:
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- Physical NIC: confirm carrier and counters with
ethtool eno1andip -s link show eno1. - Bridge membership: run
bridge linkand confirm the physical NIC and the guest tap device are attached to the intended bridge. - VLAN table: run
bridge vlan showand verify the VLAN is present on the bridge master, physical port, and any guest-facing port. - Packet capture: capture simultaneously on the guest tap, bridge, and physical NIC. A DHCPDISCOVER visible on the tap but absent from the NIC indicates a host bridge or VLAN-filtering issue; a request leaving the NIC with no reply points toward the switch, relay, or DHCP server.
- DHCP service: check the server or relay logs for the guest MAC and the expected VLAN scope.
- Guest configuration: confirm the guest NIC is enabled, its MAC is the one assigned in the hypervisor, and it is not applying an unintended VLAN tag.
A useful capture filter is:
tcpdump -ni eno1 -e -vv 'port 67 or port 68'Run an equivalent capture on the bridge and guest tap while renewing the lease. Seeing a DHCPDISCOVER at one point and not the next identifies the segment where forwarding stops.
Common migration mistakes and precise fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Host reachable, guest has no lease | Guest tap is not attached, or VLAN membership excludes it | Check bridge link, then bridge vlan show; attach the tap to the intended bridge and permit its VLAN. |
| Host loses access after moving the address | Address remains on the NIC, bridge is not connected, or management VLAN is wrong | Use console access; remove the NIC address, verify bridge carrier, and place the address on the bridge or correct VLAN interface. |
| Untagged guest works; tagged guest fails | Switch trunk or bridge filtering is incomplete | Allow the VLAN on the switch and on both bridge and physical-port entries; verify the guest tag. |
| Only NAT guests obtain leases | Physical DHCP is unavailable or the bridge path is wrong | Inspect the physical VLAN and upstream DHCP scope; do not infer LAN health from libvirt’s private DHCP. |
| Intermittent ARP and duplicate-address warnings | Same host address configured on multiple interfaces or managers | Choose one Layer 3 owner and disable stale profiles. |
| Bridge works on Ethernet but not Wi-Fi | Wireless client mode cannot provide transparent bridging | Use NAT/routing or a wired uplink. |
Making the configuration durable and recoverable
- Keep one source of truth: networkd files, Netplan, NetworkManager profiles, or another selected manager.
- Save a dated copy of the working configuration before changing it.
- Use syntax validation and a temporary-apply mechanism such as
netplan trywhen supported. - Apply one layer at a time and record the observed carrier, bridge membership, VLAN table, and DHCP result.
- Prefer stable interface matching by MAC or a predictable naming policy when hardware may change.
- Document switch port, native VLAN, allowed VLANs, host bridge, guest network, and DHCP owner in one diagram.
- Keep an out-of-band path until a reboot has completed and every required guest has renewed successfully.
Choosing the replacement networking stack
The correct choice depends less on the hypervisor name than on who owns configuration and where VLAN and DHCP decisions are made.
| Decision axis | systemd-networkd | Netplan | NetworkManager | libvirt network XML |
|---|---|---|---|---|
| Configuration ownership | Direct .netdev and .network files |
YAML rendered to a backend | Connection profiles managed with nmcli or a GUI |
Virtual network definition and domain NIC attachment |
| VLAN model | Bridge filtering and [BridgeVLAN] membership, or VLAN links |
Bridge and VLAN declarations rendered to the selected backend | Bridge, VLAN, and port profiles | NAT, routed, isolated, or bridge attachment; port groups where configured |
| DHCP location | External server on a bridged segment, or a separate service | Same, depending on rendered topology | Same, depending on profile | libvirt can provide DHCP/DNS on its private networks |
| Operational checks | networkctl, bridge, packet capture |
netplan generate, netplan try, backend tools |
nmcli, bridge, packet capture |
virsh net-list, net-dumpxml, lease inspection |
For a small host, the simplest reliable design is usually one bridge for an untagged LAN, with the host address on that bridge and one test guest used to validate DHCP. Add a VLAN-aware trunk only when the switch, bridge, and guest requirements are documented. For isolated lab guests, a libvirt NAT network is often easier because its DHCP and DNS are local to that virtual network; it should not be mistaken for a physical-LAN bridge.
Quick Recap
A final pre-reboot checklist
- The intended physical NIC has carrier and is attached to exactly one bridge.
- The host address and default route are on the bridge or the correct tagged VLAN interface, never duplicated on the NIC.
- The switch port mode and allowed VLAN list match the host design.
- Bridge VLAN filtering and membership are persistent, not only entered at a shell prompt.
- A plain untagged test guest has completed DHCP successfully.
- Each tagged guest uses exactly one tagging model: guest trunk tagging or host access-network mapping.
- The DHCP owner and expected scope are known for every network.
- Console or out-of-band recovery remains available through the first reboot.
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