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Applies to RouterOS: v3, v4



Bonding is a technology that allows aggregation of multiple ethernet-like interfaces into a single virtual link, thus getting higher data rates and providing failover.


  • Packages required: system
  • License required: Level1
  • Submenu level: /interface bonding
  • Standards and Technologies: None
  • Hardware usage: Not significant

Quick Setup Guide

Let us assume that we have 2 NICs in each router (Router1 and Router2) and want to get maximum data rate between 2 routers. To make this possible, follow these steps:

  • Make sure that you do not have IP addresses on interfaces which will be enslaved for bonding interface!
  • Add bonding interface on Router1:
[admin@Router1] interface bonding> add slaves=ether1,ether2

And on Router2:

[admin@Router2] interface bonding> add slaves=ether1,ether2

Add addresses to bonding interfaces:

[admin@Router1] ip address> add address= interface=bonding1
[admin@Router2] ip address> add address= interface=bonding1

Test the link from Router1:

[admin@Router1] interface bonding> /pi ping timeout ping timeout ping timeout 64 byte ping: ttl=64 time=2 ms 64 byte ping: ttl=64 time=2 ms

Note: bonding interface needs a couple of seconds to get connectivity with its peer.

Link monitoring

It is critical that one of the available link monitoring options is enabled. In the above example, if one of the bonded links were to fail, the bonding driver will still continue to send packets over the failed link which will lead to network degradation. Bonding in RouterOS currently supports two schemes for monitoring a link state of slave devices: MII and ARP monitoring. It is not possible to use both methods at the same time due to restrictions in the bonding driver.

ARP Monitoring

ARP monitoring sends ARP queries and uses the response as an indication that the link is operational. This also gives assurance that traffic is actually flowing over the links. If balance-rr and balance-xor modes are set, then the switch should be configured to evenly distribute packets across all links. Otherwise all replies from the ARP targets will be received on the same link which could cause other links to fail. ARP monitoring is enabled by setting three properties link-monitoring, arp-ip-targets and arp-interval. Meaning of each option is described later in this article. It is possible to specify multiple ARP targets that can be useful in High Availability setups. If only one target is set, the target itself may go down. Having additional targets increases the reliability of the ARP monitoring.

Enable ARP monitoring

[admin@Router1] interface bonding> set 0 link-monitoring=arp arp-ip-targets=
[admin@Router2] interface bonding> set 0 link-monitoring=arp arp-ip-targets=

We will not change arp-interval value in our example, RouterOS sets arp-interval to 100ms by default.

Unplug one of the cables to test if the link monitoring works correctly, you will notice some ping timeouts until arp monitoring detects link failure.

[admin@Router1] interface bonding> /pi ping timeout 64 byte ping: ttl=64 time=2 ms ping timeout 64 byte ping: ttl=64 time=2 ms ping timeout 64 byte ping: ttl=64 time=2 ms 64 byte ping: ttl=64 time=2 ms 64 byte ping: ttl=64 time=2 ms

MII monitoring

MII monitoring monitors only the state of the local interface. In RouterOS it is possible to configure MII monitoring in two ways:

  • MII Type 1 - device driver determines whether link is up or down. If device driver does not support this option then link will appear as always up.
  • MII Type 2 - deprecated calling sequences within the kernel are used to determine if link is up. This method is less efficient but can be used on all devices. This mode should be set only if MII type 1 is not supported.

Main disadvantage is that MII monitoring can't tell if the link can actually pass packets or not, even if the link is detected as being up.

MII monitoring is configured by setting the variables link-monitoring mode and mii-interval.

Enable MII Type2 monitoring:

 [admin@Router1] interface bonding> set 0 link-monitoring=mii-type-2
 [admin@Router2] interface bonding> set 0 link-monitoring=mii-type-2

We will leave mii-interval to it's default value (100ms)

When unplugging one of the cables, the failure will be detected almost instantly compared to ARP link monitoring.

Bonding modes


802.3ad mode is an IEEE standard also called LACP (Link Aggregation Control Protocol). It includes automatic configuration of the aggregates, so minimal configuration of the switch is needed. This standard also mandates that frames will be delivered in order and connections should not see mis-ordering of packets. The standard also mandates that all devices in the aggregate must operate at the same speed and duplex mode and works only with MII link monitoring.

LACP balances outgoing traffic across the active ports based on hashed protocol header information and accepts incoming traffic from any active port. The hash includes the Ethernet source and destination address and if available, the VLAN tag, and the IPv4/IPv6 source and destination address. How this is calculated depends on transmit-hash-policy parameter.


Note: layer-3-and-4 transmit hash mode is not fully compatible with LACP.

Configuration example


Example connects two ethernet interfaces on a router to the Edimax switch as a single, load balanced and fault tolerant link. More interfaces can be added to increase throughput and fault tolerance. Since frame ordering is mandatory on Ethernet links then any traffic between two devices always flows over the same physical link limiting the maximum speed to that of one interface. The transmit algorithm attempts to use as much information as it can to distinguish different traffic flows and balance across the available interfaces.

Router R1 configuration:

/inteface bonding add slaves=ether1,ether2 mode=802.3ad lacp-rate=30secs link-monitoring=mii-type1 \

Configuration on a switch:

                  Intelligent Switch : Trunk Configuration
 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 M1 M2
1 -  v  -  v  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
2 -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
3 -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
4 -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
5 -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
6 -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
7 -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -

TRK2   Disable
TRK3   Disable
TRK4   Disable
TRK5   Disable
TRK6   Disable
TRK7   Disable

Notice that LACP is enabled on first trunk group (TRK1) and switch ports on first trunk group are bound with 'v' flag. In our case port 2 and port4 will run LACP.

Verify if LACP is working: On the switch we should first verify if LACP protocol is enabled and running:

                  Intelligent Switch : LACP Port State Active Configuration

     Port         State Activity             Port         State Activity
     ---------------------------             ---------------------------
     2            Active
     4            Active

After that we can ensure that LACP negotiated with our router. If you don't see both ports on the list then something is wrong and LACP is not going to work.

                  Intelligent Switch : LACP Group Status

                [Actor]                      [Partner]

   Priority:    1                            65535

   MAC     :    000E2E2206A9                 000C42409426

   Port_No   Key     Priority   Active       Port_No   Key     Priority
   2         513     1          selected     1         9       255
   4         513     1          selected     2         9       255

After we verified that switch successfully negotiated LACP with our router, we can start traffic from Client1 and Client2 to the Server and check how traffic is evenly forwarded through both bonding slaves:

[admin@test-host] /interface> monitor-traffic ether1,ether2,bonding1 
    rx-packets-per-second: 8158     8120     16278
      rx-drops-per-second: 0        0        0
     rx-errors-per-second: 0        0        0
       rx-bits-per-second: 98.8Mbps 98.2Mbps 197.0Mbps
    tx-packets-per-second: 4833     4560     9394
      tx-drops-per-second: 0        0        0
     tx-errors-per-second: 0        0        0
       tx-bits-per-second: 2.7Mbps  3.0Mbps  5.8Mbps


Note: On some switches you need to set correct link aggregation protocol, to make balancing work in both directions


If this mode is set, packets are transmitted in sequential order from the first available slave to the last.
Balance-rr is the only mode that will send packets across multiple interfaces that belong to the same TCP/IP connection.
When utilizing multiple sending and multiple receiving links, packets are often received out of order, which result in segment retransmission, for other protocols such as UDP it is not a problem if client software can tolerate out-of-order packets.
If switch is used to aggregate links together, then appropriate switch port configuration is required, however many switches do not support balance-rr.
Quick setup guide demonstrates use of the balance-rr bonding mode. As you can see, it is quite simple to set up. Balance-rr is also useful for bonding several wireless links, however it requires equal bandwidth for all bonded links. If bandwidth of one bonded link drops, then total bandwidth of bond will be equal to the bandwidth of the slowest bonded link.


This mode uses only one active slave to transmit packets. The additional slave only becomes active if the primary slave fails. The MAC address of the bonding interface is presented onto the active port to avoid confusing the switch. Active-backup is the best choice in high availability setups with multiple switches that are interconnected.

ARP monitoring in this mode will not work correctly if both routers are directly connected. In such setups mii-type1 or mii-type2 monitoring must be used or a switch should be put between routers.


This mode balances outgoing traffic across the active ports based on the hashed protocol header information and accepts incoming traffic from any active port. Mode is very similar to LACP except that it is not standardized and works with layer-3-and-4 hash policy.


When ports are configured with broadcast mode, all slave ports transmit the same packets to the destination to provide fault tolerance. This mode does not provide load balancing.


This mode balances outgoing traffic by peer. Each link can be a different speed and duplex mode and no specific switch configuration is required as for the other modes. Downside of this mode is that only MII link monitoring is supported and incoming traffic is not balanced. Incoming traffic will use the link that is configured as "primary".

Configuration example
Lets assume than router has two links - ether1 max bandwidth is 10Mbps and ether2 max bandwidth is 5Mbps.
First link has more bandwidth so we set it as primary link

/interface bonding add mode=balance-tlb slaves=ether1,ether2 primary=ether1

No additional configuration is required for the switch.
Image above illustrates how balance-tlb mode works. As you can see router can communicate to all the clients connected to the switch with a total bandwidth of both links (15Mbps). But as you already know, balance-tlb is not balancing incoming traffic. In our example clients can communicate to router with total bandwidth of primary link which is 10Mbps in our configuration.


Mode is basically the same as balance-tlb but incoming traffic is also balanced. Only additional downside of this mode is that it requires device driver capability to change MAC address. Most of the cheap cards do not support this mode.
Image above illustrates how balance-alb mode works. Compared to balance-tlb mode, traffic from clients can also use the secondary link to communicate with the router.

Property Description

Property Description
arp (disabled | enabled | proxy-arp | reply-only; Default: enabled) Address Resolution Protocol for the interface.
  • disabled - the interface will not use ARP
  • enabled - the interface will use ARP
  • proxy-arp - the interface will use the ARP proxy feature
  • reply-only - the interface will only reply to requests originated from matching IP address/MAC address combinations which are entered as static entries in the "/ip arp" table. No dynamic entries will be automatically stored in the "/ip arp" table. Therefore for communications to be successful, a valid static entry must already exist.
arp-interval (time; Default: 00:00:00.100) time in milliseconds which defines how often to monitor ARP requests
arp-ip-targets (IP address; Default: ) IP target address which will be monitored if link-monitoring is set to arp. You can specify multiple IP addresses, separated by comma
down-delay (time; Default: 00:00:00) if a link failure has been detected, bonding interface is disabled for down-delay time. Value should be a multiple of mii-interval
lacp-rate (1sec | 30secs; Default: 30secs) Link Aggregation Control Protocol rate specifies how often to exchange with LACPDUs between bonding peer. Used to determine whether link is up or other changes have occurred in the network. LACP tries to adapt to these changes providing failover.
link-monitoring (arp | mii-type1 | mii-type2 | none; Default: none) method to use for monitoring the link (whether it is up or down)
  • arp - uses Address Resolution Protocol to determine whether the remote interface is reachable
  • mii-type1 - uses Media Independent Interface type1 to determine link status. Link status determination relies on the device driver
  • mii-type2 - similar as mii-type1, but status determination does not rely on the device driver
  • none - no method for link monitoring is used.
Note: some bonding modes require specific link monitoring to work properly.
mii-interval (time; Default: 00:00:00.100) how often to monitor the link for failures (parameter used only if link-monitoring is mii-type1 or mii-type2)
mode (802.3ad | active-backup | balance-alb | balance-rr | balance-tlb | balance-xor | broadcast; Default: balance-rr) Specifies one of the bonding policies
  • 802.3ad - IEEE 802.3ad dynamic link aggregation. In this mode, the interfaces are aggregated in a group where each slave shares the same speed. Provides fault tolerance and load balancing. Slave selection for outgoing traffic is done according to the transmit-hash-policy more>
  • active-backup - provides link backup. Only one slave can be active at a time. Another slave only becomes active, if first one fails. more>
  • balance-alb - adaptive load balancing. The same as balance-tlb but received traffic is also balanced. Device driver should have support for changing it's MAC address. more>
  • balance-rr - round-robin load balancing. Slaves in bonding interface will transmit and receive data in sequential order. Provides load balancing and fault tolerance. more>
  • balance-tlb - Outgoing traffic is distributed according to the current load on each slave. Incoming traffic is not balanced and is received by the current slave. If receiving slave fails, then another slave takes the MAC address of the failed slave. more>
  • balance-xor - Transmit based on the selected transmit-hash-policy. This mode provides load balancing and fault tolerance. more>
  • broadcast - Broadcasts the same data on all interfaces at once. This provides fault tolerance but slows down traffic throughput on some slow machines. more>
mtu (integer; Default: 1500) Maximum Transmit Unit in bytes
name (string; Default: ) descriptive name of bonding interface
primary (string; Default: ) Interface is used as primary output interface. If primary interface fails, only then are other slaves used. This value works only with active-backup mode
slaves (string; Default: none) at least two ethernet-like interfaces separated by a comma, which will be used for bonding
up-delay (time; Default: 00:00:00) if a link has been brought up, bonding interface is disabled for up-delay time and after this time it is enabled. Value should be a multiple of mii-interval
transmit-hash-policy (layer-2 | layer-2-and-3 | layer-3-and-4; Default: layer-2) Selects the transmit hash policy to use for slave selection in balance-xor and 802.3ad modes
  • layer-2 - Uses XOR of hardware MAC addresses to generate the hash. This algorithm will place all traffic to a particular network peer on the same slave. This algorithm is 802.3ad compliant.
  • layer-2-and-3 - This policy uses a combination of layer2 and layer3 protocol information to generate the hash. Uses XOR of hardware MAC addresses and IP addresses to generate the hash. This algorithm will place all traffic to a particular network peer on the same slave. For non-IP traffic, the formula is the same as for the layer2 transmit hash policy. This policy is intended to provide a more balanced distribution of traffic than layer2 alone, especially in environments where a layer3 gateway device is required to reach most destinations. This algorithm is 802.3ad compliant.
  • layer-3-and-4 - This policy uses upper layer protocol information, when available, to generate the hash. This allows for traffic to a particular network peer to span multiple slaves, although a single connection will not span multiple slaves. For fragmented TCP or UDP packets and all other IP protocol traffic, the source and destination port information is omitted. For non-IP traffic, the formula is the same as for the layer2 transmit hash policy. This algorithm is not fully 802.3ad compliant.


Link failure detection and failover is working significantly better with expensive network cards, for example, made by Intel, then with more cheap ones. On Intel cards for example, failover is taking place in less than a second after link loss, while on some other cards, it may require up to 20 seconds. Also, the Active load balancing (mode=balance-alb) does not work on some cheap cards.

L2 MTU of bonding interface is determined by taking smallest value of all slaves.

See also

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