Networking 101

Network settings are the most common cause of failed or surprising migrations. This guide is organized by situation: find the scenario that matches yours, apply the settings, and check the expected outcome before you migrate.

Every scenario reports two separate outcomes:

• The OpenStack port — the address and MAC that OpenStack assigns to the network port.
• Inside the guest — what the migrated VM’s own network configuration looks like on first boot.

These two can differ. A preserved port address does not always mean the guest is statically configured to match it.

Note

For the underlying mechanism behind interface-name persistence and per-OS guest behavior, see Network Persistence.

The five settings

Setting	What it controls
Preserve IP	Carry the discovered source IP onto the OpenStack port.
IP address box	Leave as-is (discovered IP), type one IPv4 address, or leave empty.
Preserve MAC	Carry the source MAC, or let OpenStack generate a new one.
Fallback to DHCP	On = accept an OpenStack-assigned address rather than fail. Off = stop the migration on conflict or mismatch.
Persist source network interfaces	Restore the original interface names inside the guest. Only available when Preserve IP is on.

Quick picker

Your situation	Scenario
Same subnet, keep the exact IP and MAC, fail if the address is taken	A
Same subnet, prefer the IP but accept DHCP during a bulk move	B
Destination is a different subnet	C
Assign a specific new IP	D
Force everything onto DHCP	E
Keep the IP but re-issue the MAC	F
Source VM is powered off	G
Destination is an L2-only network	H
Attach the port with no address at all	I
Preserve IP and MAC, but Persist Network is off	J

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A — Keep the same IP and the same MAC

Use this when

• The destination network carries the same subnet as the source.
• The VM is powered on and its IP was discovered correctly.
• DNS records, firewall rules, or MAC-locked licences depend on the address surviving the move.

Settings

Setting	Value
Preserve IP	On
IP address box	Leave as-is (shows the discovered IP)
Preserve MAC	On
Fallback to DHCP	Off — you want the migration to stop rather than silently change the address
Persist source network interfaces	On — keeps the original interface names too

What you get

• OpenStack port: the same IP address, provided nothing else in the network already holds it.
• Port MAC: the same MAC address.
• Inside the guest: unchanged. A static NIC stays static on the same address; a DHCP NIC stays on DHCP. The original interface names are restored.

Caution

If the address is already in use, the migration fails with a port conflict. That is deliberate — it stops you creating a duplicate. Consider ticking Disconnect source network so the original VM releases the address first.

Persist source network interfaces must be on for the “unchanged” guarantee above. If it is off, the guest outcome depends on the OS — see Scenario J.

B — Prefer the same IP, but accept DHCP rather than fail

Use this when

• Same subnet as Scenario A, but you are migrating in bulk and would rather a few VMs come up on a different address than have the whole batch stop.

Settings

Setting	Value
Preserve IP	On
IP address box	Leave as-is
Preserve MAC	On
Fallback to DHCP	On
Persist source network interfaces	On

What you get

• OpenStack port: the same IP where possible; an OpenStack-assigned address where not.
• Port MAC: the same MAC address.
• Inside the guest: the original static configuration where the address was preserved; DHCP where it was not.

Caution

You will not be stopped when an address changes. Check the migration report afterward to see which VMs moved.

As with Scenario A, the guest outcome above assumes Persist source network interfaces is on. If it is off, see Scenario J.

C — The destination is on a different subnet

Use this when

• The destination OpenStack network does not carry the source VM’s subnet.

This is the most common cause of failed migrations.

Settings

Setting	Value
Preserve IP	Off
IP address box	Empty (or type an address valid in the new subnet — see Scenario D)
Preserve MAC	Either — your choice
Fallback to DHCP	On — required
Persist source network interfaces	Unavailable — the UI greys it out automatically

What you get

• OpenStack port: an address allocated by OpenStack from the destination subnet.
• Port MAC: the same MAC if Preserve MAC is on, otherwise a newly generated one.
• Inside the guest: DHCP.

Caution

Preserve IP on with Fallback to DHCP off is the failure case — the port cannot be created and the migration stops, but only if the source NIC actually had an address. A NIC with no discovered IP is fine.

Preserve IP on with Fallback to DHCP on is tolerable: you still land on a DHCP address and keep the MAC without editing each NIC. Turning Preserve IP off is clearer.

D — Assign a specific new IP address

Use this when

• You are re-addressing the VM as part of the move and already know the address it should get.

Settings

Setting	Value
Preserve IP	Off
IP address box	Type the new address — one IPv4 address only
Preserve MAC	Either — your choice
Fallback to DHCP	Off if a wrong address should stop the migration; on if a DHCP address is an acceptable substitute
Persist source network interfaces	Unavailable

What you get

• OpenStack port: the address you typed, provided it belongs to a subnet on the destination network.
• If it does not belong to a subnet: the migration fails (Fallback off) or the port gets an OpenStack-assigned address (Fallback on).
• Inside the guest: DHCP configuration on the assigned address.

Caution

One address per NIC. Two addresses are rejected with “Multiple IPs are not supported when Preserve IP is disabled”.

A typed address carries no prefix length, so the guest assumes /24. Verify the netmask if your subnet is not a /24.

E — Force everything onto DHCP

Use this when

• You want a clean start: OpenStack allocates all addresses and no source addressing is carried over.

Settings

Setting	Value
Preserve IP	Off on every NIC
IP address box	Empty
Preserve MAC	Either — your choice
Fallback to DHCP	On — required, see below
Persist source network interfaces	Unavailable

What you get

• OpenStack port: an address allocated by OpenStack.
• Port MAC: preserved or newly generated, per your choice.
• Inside the guest: DHCP.

Caution

Fallback to DHCP must be on. With it off, this same combination produces a port with no address at all — see Scenario I.

F — Keep the IP but let OpenStack pick a new MAC

Use this when

• The source MAC clashes with something in the destination, or you are deliberately re-issuing hardware addresses.

Settings

Setting	Value
Preserve IP	On
IP address box	Leave as-is
Preserve MAC	Off
Fallback to DHCP	Your choice — as in Scenarios A and B
Persist source network interfaces	Your choice

What you get

• OpenStack port: the same IP address.
• Port MAC: a newly generated address. The UI shows a warning triangle next to the NIC to confirm this.
• Inside the guest: DHCP. The guest cannot match its old static settings to a hardware address it has never seen.

Caution

Anything licence-locked to the MAC address will break.

Interface names inside the guest may change, because names are recovered by matching on the MAC.

G — The source VM is powered off

Use this when

• The VM is not running, so VMware Tools reported no addresses.

Settings

Setting	Value
Preserve IP	Forced off and greyed out — nothing to preserve
IP address box	Type the address you want, or leave empty for DHCP
Preserve MAC	Available, and usually worth keeping on
Fallback to DHCP	On if you left the address box empty
Persist source network interfaces	Unavailable

What you get

• OpenStack port: your typed address, an OpenStack-assigned one, or no address if the box was empty with Fallback off.
• Port MAC: preserved. The MAC is read from the virtual NIC rather than the guest, so it survives a powered-off migration.
• Inside the guest: no IP configuration is injected if no address was requested.

Tip

If you know the VM’s address, power it on briefly before migrating so vJailbreak can discover it — that unlocks Scenario A.

H — The destination is an L2-only network

Use this when

• The destination OpenStack network is tagged as an L2 network and has no subnets.

Settings

Setting	Value
Preserve IP	No effect on the port — no fixed IPs can be assigned
IP address box	Ignored
Preserve MAC	Works normally — keep it on if you need the MAC
Fallback to DHCP	Greyed out
Persist source network interfaces	Your choice

What you get

• OpenStack port: created with no fixed IP addresses.
• Port MAC: preserved or newly generated, per your choice.
• Inside the guest (Ubuntu with netplan): a wildcard configuration puts every interface on DHCP.

Caution

Nothing in vJailbreak assigns addresses on an L2 network. Make sure a DHCP server is reachable on that segment, or plan to configure the guest by hand.

I — Create the port with no address at all

Use this when

• You intend to configure addressing yourself after the migration, and want the port attached but unaddressed.

Rarely used. If you reached this by accident, you probably wanted Scenario E.

Settings

Setting	Value
Preserve IP	Off
IP address box	Empty
Preserve MAC	Either — your choice
Fallback to DHCP	Off
Persist source network interfaces	Unavailable

What you get

• OpenStack port: created and attached, with no fixed IP.
• Port MAC: preserved or newly generated, per your choice.
• Inside the guest: that interface is skipped entirely. No address, no DHCP client, nothing.

Caution

The VM will boot with an unconfigured NIC and no network reachability on it. Have console access ready.

J — Preserve IP and MAC, but Persist Network is off

Use this when

• You want the address and MAC carried over, but do not need the original interface names restored.
• Or Persist source network interfaces was simply left at its default (off) and you want to know what to expect.

This is the case Scenarios A and B point at. The port outcome is identical to them — only the guest differs.

Settings

Setting	Value
Preserve IP	On
IP address box	Leave as-is
Preserve MAC	On
Fallback to DHCP	Your choice — as in Scenarios A and B
Persist source network interfaces	Off

What you get

• OpenStack port: exactly as in Scenario A or B — the preserved IP, or a DHCP address if Fallback rescued it.
• Port MAC: the same MAC address.
• Inside the guest: this is where it differs, and it depends on the guest OS rather than on the source configuration.

Guest outcome by OS

Guest OS	Source NIC was on DHCP	Source NIC was static
Ubuntu 17.10 and newer	Converted to a static address — pinned to whatever address it happened to hold when discovered	Stays static on the same address
Ubuntu older than 17.10	Stays on DHCP. Only interface-name rules are written	Stays static, per the guest’s own configuration
RHEL family 6 and older	Legacy interface handling runs — verify after boot	Legacy interface handling runs — verify after boot
RHEL / CentOS / Rocky 7+, SUSE, other Linux	Nothing is written. The guest keeps its own configuration	Nothing is written. The guest keeps its own configuration
Windows	Nothing is written. The guest keeps its own configuration	Nothing is written. The guest keeps its own configuration

On Ubuntu 17.10 and newer, the existing /etc/netplan directory is moved aside to /etc/netplan-bkp and replaced with a single generated file. Interfaces are renamed vj0, vj1, and so on, matched by MAC address. The netmask comes from what VMware Tools reported; if it is unknown, /24 is assumed.

Caution

On Ubuntu 17.10 and newer this rewrites the guest’s networking wholesale. A NIC that was on DHCP comes back statically pinned to the address it held at discovery time. If you want it to stay on DHCP, use Scenario E.

On RHEL 7+, SUSE, other Linux, and Windows, nothing is written at all. If the conversion renamed the interface, the guest’s own configuration will point at a device that no longer exists. Have console access ready.

On a multi-homed VM, only one interface gets a default route, and which one is not predictable. Verify routing after migration.

Note

Whether the source NIC was on DHCP or static is recorded but never acted on directly by vJailbreak. It matters only because it determines what the guest’s own untouched configuration says.