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Configuration Guide - Ethernet Switching

CloudEngine 8800, 7800, 6800, and 5800 V200R003C00

This document describes the configuration of Ethernet services, including configuring MAC address table, link aggregation, VLANs, MUX VLAN, Voice VLAN, VLAN mapping, QinQ, GVRP, VCMP, STP/RSTP/MSTP, VBST, SEP, RRPP, ERPS, LBDT, and Layer 2 protocol transparent transmission.
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M-LAG Loop Prevention Mechanism

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.
Figure 4-4 Traffic forwarding when an M-LAG is connected to a Layer 2 network

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.
    NOTE:
    Unidirectional isolation in the M-LAG loop prevention mechanism takes effect only for flooding traffic such as broadcast 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.

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.
M-LAG devices use the ACL rule group to implement unidirectional isolation between peer-link interfaces and M-LAG member interfaces. Flooding traffic such as broadcast traffic from a peer-link interface to an M-LAG member interface is isolated. When an M-LAG device detects that the local M-LAG member interface is in Down state, the device sends M-LAG synchronization packets through the peer-link to instruct the remote device to revoke the automatically delivered unidirectional isolation ACL rule group of the corresponding M-LAG member interface.
Figure 4-5 M-LAG unidirectional isolation
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Updated: 2019-05-08

Document ID: EDOC1100004351

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