Bonding allows two or more interfaces, its “members”, to share network traffic. From a high-level point of view, bonded interfaces act like a single port, but they have the bandwidth of multiple network devices, e.g. two 1 GB physical interfaces act like a single 2 GB interface. Bonds also increase robustness: the bonded port does not go down as long as at least one of its members is up.

In vswitchd, a bond always has at least two members (and may have more). If a configuration error, etc. would cause a bond to have only one member, the port becomes an ordinary port, not a bonded port, and none of the special features of bonded ports described in this section apply.

There are many forms of bonding of which ovs-vswitchd implements only a few. The most complex bond ovs-vswitchd implements is called “source load balancing” or SLB bonding. SLB bonding divides traffic among the members based on the Ethernet source address. This is useful only if the traffic over the bond has multiple Ethernet source addresses, for example if network traffic from multiple VMs are multiplexed over the bond.


Most of the ovs-vswitchd implementation is in vswitchd/bridge.c, so code references below should be assumed to refer to that file except as otherwise specified.

Enabling and Disabling Members

When a bond is created, a member is initially enabled or disabled based on whether carrier is detected on the NIC (see iface_create()). After that, a member is disabled if its carrier goes down for a period of time longer than the downdelay, and it is enabled if carrier comes up for longer than the updelay (see bond_link_status_update()). There is one exception where the updelay is skipped: if no members at all are currently enabled, then the first member on which carrier comes up is enabled immediately.

The updelay should be set to a time longer than the STP forwarding delay of the physical switch to which the bond port is connected (if STP is enabled on that switch). Otherwise, the member will be enabled, and load may be shifted to it, before the physical switch starts forwarding packets on that port, which can cause some data to be dropped for a time. The exception for a single enabled member does not cause any problem in this regard because when no members are enabled all output packets are dropped anyway.

When a member becomes disabled, the vswitch immediately chooses a new output port for traffic that was destined for that member (see bond_enable_member()). It also sends a “gratuitous learning packet”, specifically a RARP, on the bond port (on the newly chosen member) for each MAC address that the vswitch has learned on a port other than the bond (see bundle_send_learning_packets()), to teach the physical switch that the new member should be used in place of the one that is now disabled. (This behavior probably makes sense only for a vswitch that has only one port (the bond) connected to a physical switch; vswitchd should probably provide a way to disable or configure it in other scenarios.)

Bond Packet Input

Bonding accepts unicast packets on any member. This can occasionally cause packet duplication for the first few packets sent to a given MAC, if the physical switch attached to the bond is flooding packets to that MAC because it has not yet learned the correct member for that MAC.

Bonding only accepts multicast (and broadcast) packets on a single bond member (the “active member”) at any given time. Multicast packets received on other members are dropped. Otherwise, every multicast packet would be duplicated, once for every bond member, because the physical switch attached to the bond will flood those packets.

Bonding also drops received packets when the vswitch has learned that the packet’s MAC is on a port other than the bond port itself. This is because it is likely that the vswitch itself sent the packet out the bond port on a different member and is now receiving the packet back. This occurs when the packet is multicast or the physical switch has not yet learned the MAC and is flooding it. However, the vswitch makes an exception to this rule for broadcast ARP replies, which indicate that the MAC has moved to another switch, probably due to VM migration. (ARP replies are normally unicast, so this exception does not match normal ARP replies. It will match the learning packets sent on bond fail-over.)

The active member is simply the first member to be enabled after the bond is created (see bond_choose_active_member()). If the active member is disabled, then a new active member is chosen among the members that remain active. Currently due to the way that configuration works, this tends to be the remaining member whose interface name is first alphabetically, but this is by no means guaranteed.

Bond Packet Output

When a packet is sent out a bond port, the bond member actually used is selected based on the packet’s source MAC and VLAN tag (see bond_choose_output_member()). In particular, the source MAC and VLAN tag are hashed into one of 256 values, and that value is looked up in a hash table (the “bond hash”) kept in the bond_hash member of struct port. The hash table entry identifies a bond member. If no bond member has yet been chosen for that hash table entry, vswitchd chooses one arbitrarily.

Every 10 seconds, vswitchd rebalances the bond members (see bond_rebalance()). To rebalance, vswitchd examines the statistics for the number of bytes transmitted by each member over approximately the past minute, with data sent more recently weighted more heavily than data sent less recently. It considers each of the members in order from most-loaded to least-loaded. If highly loaded member H is significantly more heavily loaded than the least-loaded member L, and member H carries at least two hashes, then vswitchd shifts one of H’s hashes to L. However, vswitchd will only shift a hash from H to L if it will decrease the ratio of the load between H and L by at least 0.1.

Currently, “significantly more loaded” means that H must carry at least 1 Mbps more traffic, and that traffic must be at least 3% greater than L’s.

Bond Balance Modes

Each bond balancing mode has different considerations, described below.

LACP Bonding

LACP bonding requires the remote switch to implement LACP, but it is otherwise very simple in that, after LACP negotiation is complete, there is no need for special handling of received packets.

Several of the physical switches that support LACP block all traffic for ports that are configured to use LACP, until LACP is negotiated with the host. When configuring a LACP bond on a OVS host, this means that there will be an interruption of the network connectivity between the time the ports on the physical switch and the bond on the OVS host are configured. The interruption may be relatively long, if different people are responsible for managing the switches and the OVS host.

Such network connectivity failure can be avoided if LACP can be configured on the OVS host before configuring the physical switch, and having the OVS host fall back to a bond mode (active-backup) till the physical switch LACP configuration is complete. An option “lacp-fallback-ab” exists to provide such behavior on Open vSwitch.

Active Backup Bonding

Active Backup bonds send all traffic out one “active” member until that member becomes unavailable. Since they are significantly less complicated than SLB bonds, they are preferred when LACP is not an option. Additionally, they are the only bond mode which supports attaching each member to a different upstream switch.

SLB Bonding

SLB bonding allows a limited form of load balancing without the remote switch’s knowledge or cooperation. The basics of SLB are simple. SLB assigns each source MAC+VLAN pair to a link and transmits all packets from that MAC+VLAN through that link. Learning in the remote switch causes it to send packets to that MAC+VLAN through the same link.

SLB bonding has the following complications:

  1. When the remote switch has not learned the MAC for the destination of a unicast packet and hence floods the packet to all of the links on the SLB bond, Open vSwitch will forward duplicate packets, one per link, to each other switch port.

    Open vSwitch does not solve this problem.

  2. When the remote switch receives a multicast or broadcast packet from a port not on the SLB bond, it will forward it to all of the links in the SLB bond. This would cause packet duplication if not handled specially.

    Open vSwitch avoids packet duplication by accepting multicast and broadcast packets on only the active member, and dropping multicast and broadcast packets on all other members.

  3. When Open vSwitch forwards a multicast or broadcast packet to a link in the SLB bond other than the active member, the remote switch will forward it to all of the other links in the SLB bond, including the active member. Without special handling, this would mean that Open vSwitch would forward a second copy of the packet to each switch port (other than the bond), including the port that originated the packet.

    Open vSwitch deals with this case by dropping packets received on any SLB bonded link that have a source MAC+VLAN that has been learned on any other port. (This means that SLB as implemented in Open vSwitch relies critically on MAC learning. Notably, SLB is incompatible with the “flood_vlans” feature.)

  4. Suppose that a MAC+VLAN moves to an SLB bond from another port (e.g. when a VM is migrated from this hypervisor to a different one). Without additional special handling, Open vSwitch will not notice until the MAC learning entry expires, up to 60 seconds later as a consequence of rule #2.

    Open vSwitch avoids a 60-second delay by listening for gratuitous ARPs, which VMs commonly emit upon migration. As an exception to rule #2, a gratuitous ARP received on an SLB bond is not dropped and updates the MAC learning table in the usual way. (If a move does not trigger a gratuitous ARP, or if the gratuitous ARP is lost in the network, then a 60-second delay still occurs.)

  5. Suppose that a MAC+VLAN moves from an SLB bond to another port (e.g. when a VM is migrated from a different hypervisor to this one), that the MAC+VLAN emits a gratuitous ARP, and that Open vSwitch forwards that gratuitous ARP to a link in the SLB bond other than the active member. The remote switch will forward the gratuitous ARP to all of the other links in the SLB bond, including the active member. Without additional special handling, this would mean that Open vSwitch would learn that the MAC+VLAN was located on the SLB bond, as a consequence of rule #3.

    Open vSwitch avoids this problem by “locking” the MAC learning table entry for a MAC+VLAN from which a gratuitous ARP was received from a non-SLB bond port. For 5 seconds, a locked MAC learning table entry will not be updated based on a gratuitous ARP received on a SLB bond.