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M-LAG Loop Prevention Mechanism
M-LAG has a loop prevention mechanism that helps construct a loop-free network. Figure 4-4 shows how M-LAG constructs a loop-free network. Unicast traffic from the access device or network-side to M-LAG devices is forwarded through the local device preferentially, and the peer-link does not transmit data traffic under normal circumstances. When traffic is broadcast to the remote M-LAG device through the peer-link, unidirectional traffic isolation is configured between the peer-link and M-LAG member interface. That is, traffic received through the peer-link is not forwarded through the M-LAG member interface, and therefore no loop occurs. This is the unidirectional isolation mechanism of M-LAG.
Unidirectional Isolation Mechanism
Prerequisites for the Mechanism to Take Effect
When M-LAG master and backup devices are negotiated, the system checks whether the access device is dual-homed to the M-LAG using M-LAG synchronization packets.
- If the access device is dual-homed to the M-LAG, the two M-LAG devices deliver the unidirectional isolation configuration of the corresponding M-LAG member interface to isolate traffic from peer-link interfaces to M-LAG member interfaces.
Unidirectional isolation in the M-LAG loop prevention mechanism takes effect for Layer 2 traffic (including unicast, multicast, and broadcast traffic) and Layer 3 multicast traffic, and does not take effect for Layer 3 unicast traffic.If the access device is single-homed to the M-LAG, the M-LAG does not deliver the unidirectional isolation configuration of the corresponding M-LAG member interface.
- If the access device is single-homed to the M-LAG, the M-LAG does not deliver the unidirectional isolation configuration of the corresponding M-LAG member interface.
Implementation Principles
In Figure 4-5, a device is dual-homed to an M-LAG. M-LAG devices deliver the global ACL configuration in the following sequence:
- Rule 1: Layer 3 unicast packets with a peer-link interface as the source interface and an M-LAG member interface as the destination interface are allowed to pass through.
- Rule 2: All packets with a peer-link interface as the source interface and an M-LAG member interface as the destination interface are rejected.
