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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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Single-homed port is a port on the MLAG peer device which provides access device single-access to the network through either MLAG master or slave device. The single-homed port on the MLAG peer devices can connect to both hosts or servers and it can also be connected to other access switch devices. As shown in Figure 4, Switch 1 and Switch 3 are single-homed devices, the ports on the MLAG peer devices connected to Switch 1 and Switch 3 are called single-homed ports. Traffic between Switch1 and Switch3 always crosses the MLAG peer-link as Switch1 and Switch3 are active on different switches. With single-homed ports, hosts and other standalone switches are able to single-home into the network.
Figure 4. MLAG network
The MAC address entries learned on the single-homed port 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. However, the MAC synchronization on the single-homed port will be done only when there is at least one MLAG that its MLAG neighbor state is TWO-WAY or FULL. This MAC synchronization ensures that the devices connected to the single-homed port can communicate normally.
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Figure 5. Network 1 of PC 1 and PC 2 Communication in MLAG Topology
Normally, the traffic from PC 1 to PC 2 will go out of Port 1 to Switch 2. Any packet received from peer-link on MLAG slave device will be blocked to all MLAG member ports.
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Figure 6. Network 2 of PC 1 and PC 2 Communication in MLAG Topology
When considering the case of IP routing communication, as shown in Figure 7, PC1 and PC2 belong to different subnets. In this scenario, you can apply VRRP in the MLAG topology to make PC1 and PC2 can communicate with each other through IP routing. Configure two VRRP groups on the two VRRP group devices which belongs to different L3 VLAN interfaces. Configure a different virtual IP address for each VRRP group, virtual IP address 10.10.10.1 is used as the gateway for PC1 access network, and virtual IP address 20.20.20.1 is used as the gateway for PC2 access network.
Figure 7. Network 3 of PC 1 and PC 2 Communication in MLAG with VRRP Topology
Configuration Consistency
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Figure 8. MLAG Flood Control
1. All packets (Unicast, multiacst or broadcast) received from SwitchB on SwitchA1 will be flooded to any ports in the specified vlan, peer-link included.
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Figure 9. Network of IGMP Snooping Interoperation with MLAG
When the MLAG member port (ae1) on Switch A2 receives an IGMP report message from the Host, Switch A2 generates an IGMP member port multicast forwarding entry of ae1 and floods this IGMP report message to all the router ports of the same multicast group.
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Figure 10. Typical Fault Scenario of Downstream Link Down
Upstream Link to Layer-3 Device Down
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Figure 11. Typical Fault Scenario of Upstream Link Down
MLAG Peer-link Down
MLAG peer device checks the peer-link status by exchanging hello packets, if same MLAG system ID is configured on both sides of MLAG peer-link, the access switch will not be able to sense peer-link down. So the traffic will go through both links. However, if MLAG system ID is not configured, the access switch will choose the Master to transmit uplink traffic.
Figure 12. Typical Fault Scenario of Peer-link Down
MLAG Master Device Fault
When master switch reboot/shutdown, and system ID is configured, the slave device continuously uses configured system ID as the system MAC for LACP. Since system MAC for LACP has not changed, all traffic is forwarded from this functional device.
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Figure 13. Typical Fault Scene of MLAG Master Fault
