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Troubleshooting High Availability Clusters

There are typically four main problems that arise from using CARP that cause failures: Using duplicate VHIDs, use of the wrong subnet mask on a CARP VIP, attempting to use a CARP VIP outside the interface subnet, or a switch (virtual or real) improperly handling the required traffic for CARP.

Conflicting VHIDs

The VHID determines the virtual MAC address used by that CARP IP. The input validation in pfSense® software will not permit using conflicting VHIDs on a single pair of systems, however if there are multiple systems on the same broadcast domain running CARP, it’s possible to create a conflict. VRRP also uses the same virtual MAC address scheme, so a VRRP IP using the same VRID as a CARP IP VHID will also generate the same MAC address conflict.

When using CARP on the WAN interface, this also means VRRP or CARP used by the ISP can also conflict. Be sure to use VHIDs that are not in use by the ISP on that broadcast domain.

Incorrect Subnet Mask

The second sometimes needs to have the point beat home, which is to use REAL subnet mask for the IP address. Don’t use /32.

No Interface IP in that subnet

The third is that the CARP IP must already have another IP defined on a interface (vlan, lan, wan, opt) before it can be utilized. An IP Alias may be utilized to allow a CARP IP in an additional subnet on a single interface.

On 2.2 and later, this issue no longer exists. CARP addresses may be added without an existing address in the same interface.

Hypervisor users (Especially VMware ESX/ESXi)

The below settings are specifically for VMware ESX/ESXi but similar settings may be present on Hyper-V, VirtualBox, and other similar hypervisors.

  1. Enable promiscuous mode on the vSwitch

  2. Enable MAC Address changes

  3. Enable Forged transmits

  4. If multiple physical ports exist on the same vswitch, the Net.ReversePathFwdCheckPromisc option must be enabled to work around a vswitch bug where multicast traffic will loop back to the host, causing CARP to not function with “link states coalesced” messages. (See below)

ESX VDS Promisc Workaround

If a Virtual Distributed Switch is in use, a port group can be made for the firewall interfaces with promiscuous mode enabled, and a separate non-promiscuous port group may be used for other hosts. This has been reported to work by users on the forum as a way to strike a balance between the requirements for letting CARP function and for securing client ports.

ESX VDS Upgrade Issue

If a VDS (Virtual Distributed Switches) is used in ESX 4.0 or 4.1 and an upgrade from 4.0 to 4.1 or 5.0 is performed, the VDS will not properly pass CARP traffic. If a new VDS is created on 4.1 or 5.0, it will work, but the upgraded VDS will not.

It is reported that disabling promiscuous mode on the VDS and then re-enabling it will resolve the issue.

ESX VDS Port Mirroring Issue

If port mirroring is enabled on a VDS, it will break promiscuous mode. To fix it, disable promiscuous mode, then re-enable promiscuous mode.

Client Port Issues

If a physical CARP cluster is connected to a switch with an ESX box using multiple ports on the ESX box (lagg group or similar), and only certain devices/IPs are reachable by the target VM, then the port group settings in ESX may need adjusted to set the load balancing for the group to hash based on IP, not the originating interface.

Side effects of having that set incorrectly include:

  • Traffic only reaching the target VM in promisc mode on its NIC

  • Inability to reach the CARP IP from the target VM when the “real” IP of the primary firewall is reachable

  • Port forwards or other inbound connections to the target VM work from some IPs and not others.

Changing Net.ReversePathFwdCheckPromisc

Login VMware vSphere Client

For each VMware host

  • Click on host to configure and select Configuration Tab

  • Click Software Advanced Settings in left pane

  • Click on Net and scroll down to Net.ReversePathFwdCheckPromisc and set to 1

  • Click OK

Promiscuous Mode interfaces need to be set now or twiddled off and then back on. This is done per host by clicking Networking in the Hardware section

  • For each vSwitch and/or Virtual Machine Port Group.

    • NOTE: If Promiscuous is already enabled it must be disabled, saved and then re-enabled, saved.

    • Click on Properties of vSwtich

    • By Default Promiscuous Mode is Reject.

    • To Change click Edit > Security Tab

    • Select Accept from drop down

    • Click OK.

  • However, this setting is usually applied per Virtual Machine Port Group (More Secure) where the VSwitch is left at default to Reject.

    • Edit > Security > Policy Exceptions

    • Uncheck Promiscuous Mode

    • Click OK

    • Edit > Security > Policy Exceptions

    • Check Promiscuous Mode and select Accept.

More information available from VMware

VMware Workstation

If using VMware workstation on Linux for testing/modeling and CARP does not function, it is likely because VMware workstation is running non-root and cannot set the vmnet adapter in Promiscuous mode.

The permissions on /dev/vmnet* should be changed such that the user running VMware workstation is allowed to modify the /dev/vmnet* devices. See the VMware KB for details.

To make the change permanent, edit /etc/init.d/vmware, and in function vmwareStartVmnet(), add commands to chgrp and chown the vmnet devices to a group which contains user running VMware Workstation.


Be sure to use e1000 NICs (em(4)), not the ed(4) NICs or CARP VIPs will never leave init state.

VirtualBox Issues

From this thread:

  • Setting Promiscuous mode: Allow All on the relevant interfaces of the VM allows CARP to function on any interface type (Bridged, Host-Only, Internal)

Switch/Layer 2 Issues

  1. Ensure that the interfaces on both boxes (The WANs, LANs, etc, etc) are connected to the proper switch/vlan/layer 2.

  2. If the units are plugged into separate switches, ensure that the switches are properly trunking and passing broadcast/multicast traffic.

  3. If the switch on the back of a modem/CPE is being used, try a real switch instead. These built-in switches often do not properly handle CARP traffic. Often plugging the firewalls into a proper switch and then uplinking to the CPE will eliminate problems.

  4. Disable IGMP snooping or other multicast limiting and inspecting features. If they are already off, try enabling the feature and disabling it again. We have received reports of at least one Dell switch firmware mishandling multicast even with igmp snooping disabled until it is toggled, such as:

    ip igmp snooping
    no ip igmp snooping

See also

For assistance in solving problems, post on the HA/CARP/VIPs category of Netgate Forum.