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MLAG (Multi-chassis Link Aggregation Group) implementation in RouterOS allows configuring LACP bonds on two separate devices, while the client device believes to be connected on the same machine. This provides a physical redundancy in case of switch failure. All CRS3xx series devices can be configured with MLAG using the RouterOS version 7.

Both peers establish the MLAG interface and update the bridge host table over peer ports using ICCP (Inter Chassis Control Protocol). RouterOS ICCP does not require an IP configuration, but it should be isolated from the rest of the network using a dedicated untagged VLAN. This untagged VLAN can be configured with vlan-filtering and pvid. Peer ports can also be configured as LACP bonding interfaces.

The MLAG requires enabled STP or RSTP protocol, the MSTP is not supported.

Quick setup

in this example, CRS317 and CRS309 devices are used as MLAG peers and any device with two SFP+ interfaces can be used as an LACP client, see a network scheme below.

Below are configuration commands to create a regular LACP bonding in RouterOS for the Client device: 

/interface bonding
add mode=802.3ad name=bond1 slaves=sfp-sfpplus1,sfp-sfpplus2

Next, configure bonding interfaces for MLAG on Peer1 and Peer2 devices, use a matching mlag-id setting on both peer devices:

/interface bonding
add mlag-id=10 mode=802.3ad name=client-bond slaves=sfp-sfpplus2

Configure bridge with enabled vlan-filtering, and add needed interfaces as bridge ports. A dedicated untagged VLAN should be applied for the inter-chassis communication on a peer port, thus a different pvid setting is used. Below are configuration commands for Peer1 and Peer2 devices:

/interface bridge
add name=bridge1 vlan-filtering=yes
/interface bridge port
add bridge=bridge1 interface=sfp-sfpplus1 pvid=99
add bridge=bridge1 interface=client-bond

All VLANs used for bridge slaves must be also configured as tagged VLANs for peer port, so this port is now added as a tagged VLAN 1 member (the default pvid for client-bond). Other VLANs can be added if needed. Below are configuration commands for both peer devices:

/interface bridge vlan
add bridge=bridge1 tagged=sfp-sfpplus1 vlan-ids=1

Last, specify bridge and peer-port to enable MLAG. Below are configuration commands for both peer devices:

/interface bridge mlag
set bridge=bridge1 peer-port=sfp-sfpplus1

Additionally, check MLAG status on peer devices and make sure that Client LACP has both interfaces active.

# Peer1
[admin@Peer1] > /interface/bridge/mlag/monitor    
       status: connected
    system-id: 74:4D:28:11:70:6B
  active-role: primary

# Peer2
[admin@Peer2] > /interface/bridge/mlag/monitor           
       status: connected
    system-id: 74:4D:28:11:70:6B
  active-role: secondary

# Client
[admin@Client] > /interface bonding monitor bond1 
                    mode: 802.3ad
            active-ports: sfp-sfpplus1,sfp-sfpplus2
          lacp-system-id: 74:4D:28:7B:7F:96
    lacp-system-priority: 65535
  lacp-partner-system-id: 74:4D:28:11:70:6C

MLAG settings and monitoring

This section describes the available MLAG settings and monitoring options.

Sub-menu: /interface bridge mlag



bridge (interface; Default: none)The bridge interface where MLAG is being created.

peer-port  (interface; Default: none)

An interface that will be used as a peer port. Both peer devices are using inter-chassis communication over these peer ports to establish MLAG and update the host table. Peer port should be isolated on a different untagged VLAN using a pvid setting. Peer port can be configured as a bonding interface.

Use the monitor commands to see the current MLAG status.

[admin@Peer1] > /interface/bridge/mlag/monitor    
       status: connected
    system-id: 74:4D:28:11:70:6B
  active-role: primary



status (connected | connecting | disabled)The MLAG status.

system-id (MAC address)

The lowest MAC address between both peer bridges will be used as the system-id. This system-id is used for (R)STP bridge identifier.

active-role (primary | secondary)

The peer with the lowest bridge MAC address will be acting as a primary device. The primary device is responsible for sending the correct LACP system ID on all MLAG ports.

Sub-menu: /interface bonding



mlag-id (integer: 0..4294967295; Default:)Changes MLAG ID for bonding interface. The same MLAG ID should be used on both peer devices to successfully create a single MLAG.

LACP bonding interface and bonding slave ports can be monitored with monitor and monitor-slaves commands. See more details on Bonding monitoring.

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