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MLAG (Multi-chassis Link Aggregation) as the name suggests, allows different member ports of a LAG interface to be deployed on two different devices which appear to be on a single device. The two peer devices maintain communication by exchanging hello packets and MAC address learning of the LAG interface to ensure MAC synchronization using L2 multicast packets with the destination address 01:80:c2:00:00:0f. A downstream switch or host of the other end of the LAG link doesn’t get a feel that there are two devices linked with it on the other side of the link. The figure below shows the basic MLAG networking example.
Figure 1 MLAG Networking
MLAG is mainly applied in scenarios where a downstream switch or host has or needs dual-access to the network. In Figure 1, without deploying MLAG, SwitchB can only connect to SwitchA1 using an LACP link. If the LACP link or SwitchA1 fails, SwitchB cannot communicate with the network. By using MLAG, the downstream switch or host can have dual-access to the network, enabling link and device-level redundancy and protection.
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Figure 2. Multiple MLAGs Networking
• MLAG peer
MLAG peer devices, a pair of switches that enables the MLAG function.
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System priority will be compared first, a smaller system priority value takes precedence over a higher system priority value. The device with a lower priority becomes the Master and the other MLAG peer device becomes the Slave. If the system priorities are the same, then the system MACs are compared, device with a smaller system MAC becomes the Master, and the other MLAG peer device becomes the Slave.
NOTE: Master and Slave are defined for each MLAG domain. You can configure different MLAG on the same device of MLAG peer, so the device may be Master or Slave for different MLAGs. |
Hello Packets
Hello packets are sent periodically through the layer-3 network, for discovering and maintaining neighbor relationships for MLAG. The main parameters in a hello packet are: domain ID, system MAC, system priority, MLAG interface state, MLAG peer specified IP, MLAG peer system priority, MLAG peer system MAC, and MLAG peer LAG ID.
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Figure 3. MLAG Networking for MAC Synchronization
As shown in Figure 3, Switch1, Switch2 and the member ports connected to Switch3 form an MLAG, MLAG Domain 1; Switch1, Switch2 and the member ports connected to Switch4 form another MLAG, MLAG Domain 2. However, as Te-1/1/1 interface on Switch1 that connected to Switch5 does not belong to any MLAG domain, so the dynamic MAC address learned on Te-1/1/1 will not be synchronized to the MLAG member port ae1 on Switch2 during MLAG MAC synchronization. But as Te-1/1/1 is a single-homed port, it will be synchronized to the MLAG peer device on the MLAG peer-link port, and the address type is Peer-Sync in the MAC address table. For details about single-homed port, see Single-homed Port.
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4094 cc:37:ab:56:6e:81 Dynamic 300 ae3 xorp
NOTE:
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Anchor single port single port
Single-homed Port
| single port | |
| single port |
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MLAG member port is a single-homed port when one LAG port of the dual-homed access device is down, then the other LAG port becomes a single-homed port. We can also say that when MLAG interface state is ASY_LOCAL, then MLAG member port on local MLAG device is a single-homed port. MAC address entry learned on this port will be synchronized to the MLAG peer device on the MLAG peer-link port.
NOTE:
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Application Scenarios
As shown in Figure 5, PC 2 connects to the MLAG downlink switch (Switch 2), and communicates with PC 1 through the MLAG peer devices.
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Flood control process of traffic from uplink is similar to that of traffic from downlink, and is not mentioned here.
NOTE: All the packets received from peer-link shall be blocked to all MLAG member ports except the DHCP Offer/Ack packets. |
You can run the run show mlag internal command to view the status of flood control. For example:
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