Layer 2 transparent transmission on S series switch

Layer 2 transparent transmission mechanism on S series switches, except S1700:
PEs replace the standard multicast destination MAC address of user-side Layer 2 protocol packets with a specified multicast MAC address according to the mappings between multicast destination MAC addresses and Layer 2 protocols.
Internal nodes on the backbone network forward the packets across the backbone network as common Layer 2 packets.
The egress device of the backbone network restores the original destination MAC address of the packets according to the mappings between multicast destination MAC addresses and Layer 2 protocols, and then forwards the packets to user networks.

After the destination MAC address in a user-side packet is replaced, the packet traverses the backbone network, but will not be terminated. The new MAC address in packet is configured by the l2protocol-tunnel group-mac command. S series switches can transparently transmit the following packets:
1. Spanning Tree Protocol (STP)
2. Link Aggregation Control Protocol (LACP)
3. Ethernet Operation, Administration, and Maintenance 802.3ah (EOAM3ah)
4. Link Layer Discovery Protocol (LLDP)
5. Generic VLAN Registration Protocol (GVRP)
6. Generic Multicast Registration Protocol (GMRP)
7. HUAWEI Group Management Protocol (HGMP)
8. VLAN Trunking Protocol (VTP)
9. Unidirectional Link Detection (UDLD)
10. Port Aggregation Protocol (PAGP)
11. Cisco Discovery Protocol (CDP)
12. Per VLAN Spanning Tree Plus (PVST+)
13. Shared Spanning Tree Protocol (SSTP), only supported by fixed switches
14. Dynamic Trunking Protocol (DTP)
15. Device Link Detection Protocol (DLDP)
16. User-defined protocol packets

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