Overview of Layer 2 Protocol Tunneling

Layer 2 protocol tunneling allows Layer 2 devices to use Layer 2 tunneling technology to transparently transmit Layer 2 protocol data units (PDUs) across a backbone network. Layer 2 protocol tunneling supports standard protocols, such as Spanning Tree Protocol (STP) and Link Aggregation Control Protocol (LACP), as well as user-defined protocols.

Layer 2 protocols running between user networks, such as Spanning Tree Protocol (STP) and Link Aggregation Control Protocol (LACP), must traverse a backbone network to perform Layer 2 protocol calculation.

On the network shown in Figure 1, User Network 1 and User Network 2 both run a Layer 2 protocol, Multiple Spanning Tree Protocol (MSTP). Layer 2 protocol data units (PDUs) on User Network 1 must traverse a backbone network to reach User Network 2 to build a spanning tree. Generally, the destination MAC addresses in Layer 2 PDUs of the same Layer 2 protocol are the same. For example, the MSTP PDUs are BPDUs with the destination MAC address 0180-C200-0000. Therefore, when a Layer 2 PDU reaches an edge device on a backbone network, the edge device cannot identify whether the PDU comes from a user network or the backbone network and sends the PDU to the CPU to calculate a spanning tree.

In Figure 1, CE1 on User Network 1 builds a spanning tree together with PE1 but not with CE2 on User Network 2. As a result, the Layer 2 PDUs on User Network 1 cannot traverse the backbone network to reach User Network 2.

Figure 1 Layer 2 protocol tunneling over a backbone network

To resolve the preceding problem, use Layer 2 protocol tunneling. The NetEngine 8000 F supports tunneling for the following Layer 2 protocols:

Cisco Discovery Protocol (CDP)
Ethernet Local Management Interface (E-LMI)
Ethernet in the First Mile OAM (EOAM3AH)
Device link detection protocol (DLDP)
Dynamic Trunking Protocol (DTP)
Ethernet in the First Mile (EFM)
GARP Multicast Registration Protocol (GMRP)
GARP VLAN Registration Protocol (GVRP)
Huawei Group Management Protocol (HGMP)
Link Aggregation Control Protocol (LACP)
Link Layer Discovery Protocol (LLDP)
Multiple MAC Registration Protocol (MMRP)
Multiple VLAN Registration Protocol (MVRP)
Port Aggregation Protocol (PAgP)
Secure Socket Tunneling Protocol (SSTP)
Spanning Tree Protocol (STP)
Unidirectional Link Detection (UDLD)
VLAN Trunking Protocol (VTP)

Layer 2 PDUs can be tunneled across a backbone network if all of the following conditions are met:

All sites of a user network can receive Layer 2 PDUs from one another.
Layer 2 PDUs of a user network are not processed by the CPUs of backbone network devices.
Layer 2 PDUs of different user networks must be isolated and not affect each other.

Layer 2 protocol tunneling allows Layer 2 protocol data units (PDUs) to be transparently transmitted based on the following principles:

When Layer 2 PDUs enter a backbone network:
1. The ingress device replaces the multicast destination MAC address in the Layer 2 PDUs with a specified multicast MAC address so that it does not send the Layer 2 PDUs to its CPU for processing.
  
  The specified multicast MAC address cannot be a multicast MAC address used by well-known protocols.
2. The ingress device then determines whether to add an outer VLAN tag to the Layer 2 PDUs with a specified multicast MAC address based on the configured Layer 2 protocol tunneling type.
When Layer 2 PDUs leave the backbone network:
1. The egress device restores the original multicast destination MAC address in the Layer 2 PDUs based on the configured mapping between the multicast destination MAC address and the specified multicast MAC address.
2. The egress device then determines whether to remove the outer VLAN tag from the Layer 2 PDUs with the original multicast destination MAC address based on the configured Layer 2 protocol tunneling type.