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Summary

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. 

Interface bonding does not create an interface with a larger link speed. Interface bonding creates a virtual interface that can load balance traffic over multiple interfaces. More details can be found in the LAG interfaces and load balancing page.

Quick Setup Guide

Let us assume that we have two Ethernet interfaces on each router (Router1 and Router2) and want to get the maximum data rate between these two routers. To make this possible, follow these steps:

  1. Make sure that you do not have IP addresses on interfaces that will be enslaved for bonding interface.
  2. Add bonding interface and IP address on the Router1:

    /interface bonding add slaves=ether1,ether2 name=bond1
    /ip address add address=172.16.0.1/24 interface=bond1
  3. Do the same thing on the Router2:

    /interface bonding add slaves=ether1,ether2 name=bond1
    /ip address add address=172.16.0.2/24 interface=bond1
  4. Test the link from Router1:

    [admin@Router1] > ping 172.16.0.2
      SEQ HOST                                 SIZE TTL TIME  STATUS                   
        0 172.16.0.2                             56  64 0ms  
        1 172.16.0.2                             56  64 0ms  
        2 172.16.0.2                             56  64 0ms  
        sent=3 received=3 packet-loss=0% min-rtt=0ms avg-rtt=0ms max-rtt=0ms

The bonding interface needs a couple of seconds to get connectivity with its peers.

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. The ARP replies are not validated, so any received packet by the slave interface will result in the slave interface considered as active. This 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. The 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.

To enable ARP monitoring on Router1:

/interface bonding set [find name=bond1] link-monitoring=arp arp-ip-targets=172.16.0.2

and Router2:

/interface bonding set [find name=bond1] link-monitoring=arp arp-ip-targets=172.16.0.1

We will not change the 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 might notice some ping timeouts until arp monitoring detects link failure.

[admin@MikroTik] > ping 172.16.0.2
  SEQ HOST                                     SIZE TTL TIME  STATUS                                 
    0 172.16.0.2                                 56  64 0ms  
    1 172.16.0.2                                 56  64 0ms  
    2 172.16.0.2                                 56  64 0ms  
    3 172.16.0.2                                 56  64 0ms  
    4 172.16.0.2                                              timeout                                
    5 172.16.0.2                                 56  64 0ms  
    6 172.16.0.2                                 56  64 0ms  
    sent=7 received=6 packet-loss=14% min-rtt=0ms avg-rtt=0ms max-rtt=0ms


For ARP monitoring to work properly it is not required to have any IP address on the device, ARP monitoring will work regardless of the IP address that is set on any interface.

When ARP monitoring is used, bonding slaves will send out ARP requests without a VLAN tag, even if an IP address is set on a VLAN interface in the same subnet as the arp-ip-targets

MII monitoring

MII monitoring monitors only the state of the local interface. MII Type 1 - a device driver determines whether a link is up or down. If the device driver does not support this option then the link will appear as always up. The 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 and mii-interval.

To enable MII Type1 monitoring on Router1 and Router2:

/interface bonding set [find name=bond1] link-monitoring=mii

We will leave mii-interval to its 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

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 misordering of packets. The standard also mandates that all devices in the aggregate must operate at the same speed and duplex mode.

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. The ARP link monitoring is not recommended, because the ARP replies might arrive only on one slave port due to transmit hash policy on the LACP peer device. This can result in unbalanced transmitted traffic, so MII link monitoring is the recommended option.

The layer-3-and-4 transmit hash mode is not fully compatible with LACP. More details can be found in https://www.kernel.org/doc/Documentation/networking/bonding.txt

balance-xor

This mode balances outgoing traffic across the active ports based on the hashed protocol header information and accepts incoming traffic from any active port. The mode is very similar to LACP except that it is not standardized and works with layer-3-and-4 hash policy. The mode can work together with static Link Aggregation Group (LAG) interfaces.

balance-rr

If this mode is set, packets are transmitted in sequential order from the first available slave to the last. The 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 results in segment retransmission, for other protocols such as UDP it is not a problem if a client software can tolerate out-of-order packets. If a 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 the usage 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 the bandwidth of one bonded link drops, then the total bandwidth of bond will be equal to the bandwidth of the slowest bonded link.

active-backup

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.

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

broadcast

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.

balance-tlb

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. The downside of this mode is that only MII link monitoring is supported (ARP link monitoring is ignored when configured) and incoming traffic is not balanced. Incoming traffic will use the link that is configured as "primary".

Configuration example
Let's assume than the router has two links - ether1 max bandwidth is 10Mbps and ether2 max bandwidth is 5Mbps. The first link has more bandwidth so we set it as a primary link:

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

No additional configuration is required for the switch. The 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 the router with a total bandwidth of primary link which is 10Mbps in our configuration.

balance-alb

The mode is basically the same as balance-tlb but incoming IPv4 traffic is also balanced. Only MII link monitoring is supported (ARP link monitoring is ignored when configured), the additional downside of this mode is that it requires device driver capability to change MAC address.


The 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.

Bonding monitoring

Since RouterOS 6.48 version, it is possible to monitor the bonding interface and bonding ports. For the 802.3ad bonding mode, more detailed monitoring options are available.

/interface bonding monitor [find] 
                      mode: 802.3ad           active-backup
              active-ports: ether4            ether6
                            ether5            
            inactive-ports:                   ether7
            lacp-system-id: CC:2D:E0:11:22:33 
      lacp-system-priority: 65535             
    lacp-partner-system-id: B8:69:F4:44:55:66
PropertyDescription
mode (802.3ad | active-backup | balance-alb | balance-rr | balance-tlb | balance-xor | broadcast)Used bonding mode
active-ports (interface)Shows the active bonding ports
inactive-ports (interface)Shows the inactive bonding ports (e.g. a disabled or backup interface)
lacp-system-id (MAC address)Shows the local LACP system ID
lacp-system-priority (integer)Shows the local LACP priority
lacp-partner-system-id (MAC address)Shows the partner LACP system ID

To monitor individual bonding ports, use a monitor-slaves command.

/interface bonding monitor-slaves bond1
Flags: A - active, P - partner 
 AP port=ether4 key=17 flags="A-GSCD--" partner-sys-id=D4:CA:6D:12:06:65 partner-sys-priority=65535 partner-key=9 partner-flags="A-GSCD--" 

 AP port=ether5 key=17 flags="A-GSCD--" partner-sys-id=D4:CA:6D:12:06:65 partner-sys-priority=65535 partner-key=9 partner-flags="A-GSCD--" 
PropertyDescription
port (interface)Used bonding port
key (integer)Shows the local LACP aggregation key, a unique key is generated and used by all bonding ports that are configured on the same LACP group
flags (string)

Shows the local LACP flags

partner-sys-id (MAC address)Shows the partner LACP system ID
partner-sys-priority (integer)Shows the partner LACP priority
partner-key (integer)Shows the partner LACP aggregation key, a unique key is generated and used by all bonding ports that are configured on the same LACP group
partner-flags (string)Shows the local LACP flags

Property Description

PropertyDescription
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 a comma
comment (string; Default: )Short description of the interface
disabled (yes | no; Default: no)Changes whether the bonding interface is disabled
down-delay (time; Default: 00:00:00)If a link failure has been detected, the bonding interface is disabled for a down-delay time. The value should be a multiple of mii-interval, otherwise, it will be rounded down to the nearest value. This property only has an effect when link-monitoring is set to mii.
forced-mac-address (MAC address; Default: none)By default, the bonding interface will use the MAC address of the first selected slave interface. This property allows to configure static MAC address for the bond interface (all zeros, broadcast or multicast addresses will not apply). RouterOS will automatically change the MAC address for slave interfaces and it will be visible in /interface ethernet configuration export
lacp-rate (1sec | 30secs; Default: 30secs)Link Aggregation Control Protocol rate specifies how often to exchange with LACPDUs between bonding peers. Used to determine whether a link is up or other changes have occurred in the network. LACP tries to adapt to these changes providing failover.
link-monitoring (arp | mii | none; Default: mii)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 - uses Media Independent Interface to determine link status. Link status determination relies on the device driver.
  • none - no method for link monitoring is used.
Note: some bonding modes require specific link monitoring to work properly.
min-links (integer: 0..4294967295; Default: 0)How many active slave links needed for bonding to become active
mii-interval (time; Default: 00:00:00.100)How often to monitor the link for failures (the parameter used only if link-monitoring is mii)
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. It 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 the first one fails. more>
  • balance-alb - adaptive load balancing. The same as balance-tlb but received traffic is also balanced. The device driver should have support for changing it's MAC address. more>
  • balance-rr - round-robin load balancing. Slaves in a bonding interface will transmit and receive data in sequential order. It 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. Must be smaller or equal to the smallest L2MTU value of a bonding slave. L2MTU of a bonding interface is determined by the lowest L2MTU value among its slave interfaces
name (string; Default: )Name of the bonding interface
primary (string; Default: none)Controls the primary interface between active slave ports, works only for active-backup, balance-tlb and balance-alb modes. For active-backup mode, it controls which running interface is supposed to send and receive the traffic. For balance-tlb mode, it controls which running interface is supposed to receive all the traffic, but for balance-alb mode, it controls which interface is supposed to receive the unbalanced traffic (the non-IPv4 traffic). When none of the interfaces are selected as primary, device will automatically select the interface that is configured as the first one.
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, the bonding interface is disabled for up-delay time and after this time it is enabled. The value should be a multiple of mii-interval, otherwise, it will be rounded down to the nearest value. This property only has an effect when link-monitoring is set to mii.
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.

See also

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