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To replace vmbr0, recreate the same network layers with a normal Linux bridge: make the physical Ethernet interface a bridge port, attach each VM tap interface to that bridge, and put the host’s management address on the bridge (or on a VLAN interface attached to it). Then make the switch port mode, bridge VLAN membership, and guest tagging agree. Test an untagged guest path before introducing VLANs, and verify DHCP packets from the guest all the way to the server.
A bridge showing UP proves only that the local link exists. A DHCP lease requires the broadcast to reach the intended VLAN and a DHCP service to answer there. The procedure below separates those layers so you can find the break without losing remote access.
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What changes when you leave Proxmox
Proxmox’s vmbrX devices are ordinary Linux bridges. They behave like software Ethernet switches: the physical NIC is one port, and each VM’s tap device is another. The replacement hypervisor does not need the same name, but it does need an equivalent bridge topology.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsIf a migration leaves guests with a bridge name that no longer exists, they may appear to have a connected virtual cable while no frames can reach the physical network. Recreating only the name is not enough. The bridge must have the correct physical port, Layer-3 address placement, VLAN policy, and guest attachment.
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Inventory the old path before changing it
Do this from a local console or an out-of-band session. A bridge mistake can remove SSH access immediately, so do not make the first change over your only remote connection.
- Record interface names and link state with
ip -br link. Save the MAC address of the physical Ethernet adapter withip link show dev eno1(substitute the actual name). - Record addresses and routes with
ip -br addrandip route. Note which address is management, which route is the default gateway, and whether any address is already on the physical NIC. - List current bridge membership with
bridge linkand VLAN membership withbridge vlan. If the old host is still available, also save its bridge and VLAN configuration before dismantling it. - Write down the switch-port mode, native or untagged VLAN, allowed tagged VLANs, and any port-security or MAC-limit settings. A trunk that allows only one VLAN cannot carry a guest network on another VLAN.
- For every guest, record the virtual NIC model, MAC address, whether the guest expects tagged frames, and the network or bridge name to which it was attached.
- Identify where DHCP is supposed to run: an upstream router or server on a physically bridged VLAN, or a libvirt-managed virtual network using NAT.
Interface names can change after a motherboard, NIC, or operating-system replacement. Match hardware by MAC address or another stable identifier rather than assuming the old eno1 name still refers to the same adapter.
Build the lowest layer first
Create a Linux bridge with systemd-networkd
systemd-networkd creates virtual devices from .netdev files and applies settings to matching links through .network files. A minimal, untagged bridge uses one file to define the bridge, one to enslave the physical NIC, and one to configure the bridge itself.
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[NetDev]
Name=br0
Kind=bridge# /etc/systemd/network/20-eno1.network
[Match]
Name=eno1
[Network]
Bridge=br0# /etc/systemd/network/30-br0.network
[Match]
Name=br0
[Network]
Address=192.0.2.10/24
Gateway=192.0.2.1
DNS=192.0.2.1Replace the example addresses with the values for your management network. Do not put the same address on both eno1 and br0. The physical interface should normally have no independent address; it is a Layer-2 bridge port.
After staging the files, check them with networkctl status br0 and networkctl status eno1. Apply changes only when you have console access or a tested rollback. If networkd is not the active manager, its files may be ignored or may conflict with another manager.
Build the equivalent bridge with Netplan
On distributions that use Netplan, define the Ethernet device as a bridge member and place the host address and default route on the bridge. This example renders to systemd-networkd:
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network:
version: 2
renderer: networkd
ethernets:
eno1: {}
bridges:
br0:
interfaces: [eno1]
addresses: [192.0.2.10/24]
routes:
- to: default
via: 192.0.2.1
nameservers:
addresses: [192.0.2.1]
parameters:
stp: true
forward-delay: 4Run netplan try where available so the configuration can automatically revert if you lose the session, then use netplan apply after confirming the result. The exact Netplan features and renderer differ by distribution release; inspect the generated configuration rather than assuming a setting was accepted.
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NetworkManager stores the same topology as connection profiles: one bridge profile, one Ethernet profile whose master is that bridge, and optional VLAN or guest profiles. The profile owning the IP address must be the bridge or VLAN profile, not the enslaved Ethernet profile. Use nmcli connection show, nmcli device status, and the manager’s active-profile details to verify ownership. Do not run networkd, NetworkManager, and a third manager against the same interfaces unless your distribution explicitly supports that arrangement.
Put Layer 3 on the right device
For an untagged management network, assign the host address, default route, and DNS to br0. The physical NIC remains a port with carrier but no competing Layer-3 configuration. An address left on the NIC can produce duplicate routes, ARP replies from the wrong interface, and traffic that works only intermittently.
When management is tagged, create a VLAN interface linked to the bridge and assign the host address there. For example, a host-side br0.20 or a manager-specific VLAN device represents VLAN 20; the address and default route belong on that VLAN interface. The bridge carries Ethernet frames, while the VLAN interface terminates the host’s Layer-3 traffic.
Use one default route unless you have deliberately configured policy routing. If the host has several VLAN interfaces, give each only the routes it needs and keep the management route on the intended VLAN. Check the result with ip route get 192.0.2.1 and ip neigh after the bridge is online.
Prove an untagged guest path before adding VLANs
Attach one test guest to the plain bridge and temporarily leave VLAN tagging disabled in the guest. This isolates physical connectivity, bridge membership, and DHCP from trunk configuration.
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- On the host, confirm carrier with
ethtool eno1orip link show eno1. A missing carrier is a cable, transceiver, switch-port, or NIC problem, not a DHCP problem. - Confirm the NIC is a bridge port with
bridge link. You should seeeno1associated withbr0, and the guest tap device should appear when the VM is running. - Inside the guest, verify its virtual NIC has carrier. The guest must send a DHCPDISCOVER on the same untagged network that the switch port presents as its native or access VLAN.
- Capture both sides while renewing the lease:
tcpdump -ni br0 -e -vv 'udp port 67 or udp port 68'and, in a second session,tcpdump -ni eno1 -e -vv 'udp port 67 or udp port 68'. - Look for a DISCOVER leaving the guest and an OFFER returning. If the DISCOVER is visible on the tap but not on the physical NIC, fix guest attachment or bridge membership. If it leaves the NIC but no OFFER returns, investigate switch VLAN selection, the DHCP server, or a filtering policy.
Do not add VLAN tags until this basic path works. Otherwise, a failed lease can have several simultaneous causes.
Make VLANs agree at every layer
A tagged network works only when three independent configurations match:
| Layer | What must match | Typical evidence |
|---|---|---|
| Switch | Port mode, native VLAN, and allowed VLAN list | Port configuration and MAC-table learning |
| Host bridge | VLAN filtering enabled and VLAN membership on the bridge and physical port | bridge vlan output and packet captures |
| Guest or host VLAN device | Either guest tags frames on a trunk, or the host presents an untagged access bridge for that VLAN | Guest NIC settings, VLAN interface state, and frame tags |
Configure the switch trunk
Set the host-facing port to the mode required by your design. An access port carries one untagged VLAN. A trunk carries tagged VLANs and may also define one native untagged VLAN. Add every VLAN needed by the host, management interface, and guests to the allowed list. If the switch uses a native VLAN, ensure the host and switch agree about which frames are untagged.
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A managed Gigabit Ethernet switch with 802.1Q VLAN support is useful when validating a migration because it exposes the allowed list and MAC-table learning. Permit only the VLANs you actually use, and check that the host MAC appears in the expected VLAN after the port comes up.
Enable filtering and membership on the Linux bridge
A bridge can forward frames locally while still dropping VLANs that are not members of a port. With systemd-networkd, enable bridge VLAN filtering and add matching [BridgeVLAN] entries for the bridge and the physical port. The exact file split depends on your distribution, but the relevant settings have this form:
[Bridge]
VLANFiltering=yes
[BridgeVLAN]
VLAN=20
[BridgeVLAN]
VLAN=30For an access-style port, mark the intended VLAN as the PVID and egress-untagged on that port. For a trunk to guests, allow the VLANs as tagged members and do not mark them untagged. Confirm the resulting state with bridge vlan show; seeing the bridge device itself in a VLAN list is not a substitute for allowing the VLAN on the physical port and the guest-facing port.
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When using another manager, look for its equivalent of bridge VLAN filtering, per-port VLAN membership, PVID, and egress-untagged flags. Configuration syntax is distribution-specific, so verify the applied kernel state with bridge vlan rather than trusting only the profile text.
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| Model | Host configuration | Guest configuration | Best use |
|---|---|---|---|
| Guest-tagged trunk | One VLAN-aware bridge carries several tagged VLANs to the VM tap devices. | Each guest NIC or guest operating system adds its VLAN tag. | Virtual routers, firewalls, and guests that need multiple VLANs. |
| Host VLAN subinterfaces | Create one VLAN interface per network and, when needed, a separate bridge for each. | Guests connect to an untagged access-style bridge. | Ordinary guests that should see one simple Ethernet segment. |
| Libvirt port groups | Define VLAN or virtual-network behavior in libvirt’s network XML and attach the VM to the selected network. | Usually no VLAN configuration inside the guest. | Centralized per-VM network selection when libvirt owns the attachment. |
Do not tag the guest and also insert a host VLAN subinterface that expects untagged traffic on the same path. That double-tagging or mismatched expectation commonly prevents DHCP and normal traffic. Decide where the tag is added and where it is removed, then keep every layer consistent with that decision.
Host VLAN example
Suppose VLAN 20 is the tagged management network and VLAN 30 is a guest network. The switch trunk allows 20 and 30. The host can terminate management on a VLAN interface linked to br0, while a separate bridge linked to VLAN 30 presents an untagged segment to guests. The host’s management address belongs on the VLAN-20 interface; guests on the VLAN-30 bridge do not need to understand 802.1Q.
Alternatively, present br0 as a filtered trunk to a virtual firewall and let that guest create VLAN 20 and VLAN 30 internally. In that model the guest NIC must receive tagged frames, and the bridge and switch must pass both VLANs.
Understand which device supplies DHCP
Physically bridged networks
On a physical bridge, the guest’s DHCP broadcast should reach the upstream router or DHCP server on the selected VLAN. The host does not need to run a DHCP service merely because it has a bridge. If the server is on VLAN 30, the switch trunk, host bridge membership, and guest path must all carry VLAN 30.
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A DHCPDISCOVER with no DHCPOFFER usually indicates a missing broadcast path, wrong VLAN, exhausted or disabled DHCP scope, or filtering. It is not fixed by changing the guest’s default gateway before the offer can arrive.
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libvirt NAT networks
A libvirt virtual network can provide its own DNS and DHCP service and NAT guest traffic through the host. This path is independent of a physical trunk. A guest on the default or another NAT network can receive a lease even when the physical bridge has no VLAN configuration, but it will not be a Layer-2 member of the upstream LAN.
Inspect available networks with virsh net-list --all, identify the active network and its address range, and check leases with virsh net-dhcp-leases <network-name>. Use NAT when isolation and outbound access are intended; use a physical bridge when guests must be directly reachable on the upstream VLAN.
Attach guests to the replacement bridge
Update each VM definition so its virtual NIC points to the new bridge or libvirt network. Preserve the guest MAC address when DHCP reservations, firewall rules, or inventory depend on it. Start one test VM first and verify that a tap device appears as a port of the expected bridge.
For a bridged libvirt interface, the XML concept is a bridge source such as <interface type='bridge'><source bridge='br0'/><model type='virtio'/></interface>. For a libvirt-managed NAT network, use a network source instead. Do not attach a guest simultaneously to an old, inactive bridge and the new one while troubleshooting; two interfaces can create competing routes and make packet captures misleading.
Verify from the outside inward
Use this order so each observation narrows the fault domain:
- Switch: confirm the host MAC is learned on the expected port and VLAN. A MAC learned on the wrong VLAN indicates switch configuration or native-VLAN mismatch.
- Physical link: check carrier, negotiated speed, and errors with
ip -s link show eno1and the switch counters. - Bridge membership: run
bridge linkand confirm the physical NIC and the guest tap are attached to the same bridge. - VLAN state: run
bridge vlan showand verify the VLAN is allowed on the bridge, the physical port, and the guest-facing port according to the chosen tagging model. - Frames: capture on the guest tap, bridge, and physical NIC. On a trunk, tagged frames should retain the expected VLAN ID; on an access path, the guest should send untagged frames.
- DHCP service: inspect the upstream or libvirt DHCP logs and lease table. Confirm the scope is active for the VLAN and that the guest MAC is not blocked.
- Layer 3: only after a lease or static address is present, test ARP with
ip neigh, the gateway withping, and routing withip route.
This sequence distinguishes a missing broadcast from a bad route. A successful ping from the host does not prove that a guest’s VLAN or DHCP path works.
Common symptoms and precise fixes
| Symptom | Likely boundary | Fix to try first |
|---|---|---|
| Bridge is UP, guest has no lease, and no DISCOVER appears on the physical NIC | Guest attachment or bridge membership | Attach the tap device to the intended bridge and confirm the VM NIC is connected. |
| DISCOVER leaves the physical NIC but no OFFER returns | Switch VLAN, DHCP scope, or upstream filtering | Check native/allowed VLANs, server scope, and switch port security. |
| Host works, guests on one VLAN fail | Bridge VLAN membership | Add that VLAN to both the bridge and the physical port; confirm filtering is enabled. |
| Only tagged guests fail | Tagging model mismatch | Choose guest-tagged trunk or host access bridge; remove the extra tag at the other layer. |
| Host loses access after moving the address | Address or route is still on the wrong device | Use console access, remove the NIC address, place it on the bridge or VLAN interface, and restore the intended default route. |
| Intermittent ARP or traffic from the host | Competing addresses or routes | Ensure only the bridge or correct VLAN interface owns the management address. |
| Guests work on NAT but not on the physical LAN | Physical trunk or DHCP path | Use NAT as a control test, then repair switch, bridge VLAN, and upstream DHCP configuration. |
Wireless uplinks and upstream restrictions
Normal Linux host bridging generally cannot attach a wireless client interface in the same way as Ethernet. Wireless station mode often cannot forward arbitrary source MAC addresses, and libvirt documentation explicitly warns that wireless interfaces cannot be attached to a Linux host bridge. Use wired Ethernet for a true Layer-2 guest bridge, or choose NAT, routed networking, or a wireless solution designed to support multiple downstream clients.
Even with wired Ethernet, an upstream switch, access point, or provider may block unknown source MAC addresses, limit learned MACs, or require explicit authorization for trunks. If the host can reach the gateway but guest MACs never appear in the switch table, check those policies before changing guest operating systems.
Quick Recap
Make the migration reversible
- Keep a local console or out-of-band path open while applying bridge changes.
- Save the current manager profiles and configuration files before editing them.
- Stage one layer at a time: physical link, plain bridge, untagged guest, VLAN filtering, then additional guests.
- Use a scheduled reboot when the active network manager is uncertain; a clean boot can reveal which configuration is actually authoritative, but schedule it because a bad bridge can still remove remote access.
- After each successful stage, record the applied state from
ip addr,ip route,bridge link, andbridge vlan. These records make rollback and later hardware replacement much faster.
Final migration checklist
- The physical NIC has carrier and is a port of the replacement bridge.
- The host management address and default route exist only on the bridge or its intended VLAN interface.
- A test guest receives DHCP on an untagged path before VLANs are introduced.
- The switch port mode, native VLAN, and allowed VLAN list match the host design.
- Bridge VLAN filtering and per-port membership are enabled for every tagged VLAN.
- Each segment uses one tagging model: guest trunk, host access bridge, or libvirt network.
- DHCP ownership is known and its logs show the guest request on the correct segment.
- Packet captures and switch MAC learning confirm the path from guest tap to physical network.
- Remote recovery and rollback access remain available before production guests are moved.
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