Basic Concepts

LAD Packet Formats

Link Automatic Discovery (LAD) packets have three different formats, depending on the link type.

When Ethernet interfaces are used on links, LAD packets are encapsulated into Ethernet frames. Figure 1 shows the LAD packet format on Ethernet interfaces.

Figure 1 LAD packet format on Ethernet interfaces

Table 1 describes the fields in an LAD packet on Ethernet interfaces.

**Table 1** Fields in an LAD packet on Ethernet interfaces
Field	Length	Description
DA	6 bytes	Destination MAC address, a broadcast MAC address fixed at 0xFF-FF-FF-FF-FF-FF
SA	6 bytes	Source MAC address, an interface's MAC address or a device's bridge MAC address
Type	2 bytes	Packet type, fixed at 0x0000
Flag	20 bytes	LAD packet identifier, fixed as Huawei Link Search
Information	20-44 bytes	LAD data unit, main part of an LAD packet
FCS	4 bytes	Frame check sequence

When Ethernet sub-interfaces are used on links, LAD packets are encapsulated into Ethernet frames. Figure 2 shows the LAD packet format on Ethernet sub-interfaces.

Figure 2 LAD packet format on Ethernet sub-interfaces

Table 2 describes the fields in an LAD packet on Ethernet sub-interfaces.

**Table 2** Fields in an LAD packet on Ethernet sub-interfaces
Field	Length	Description
DA	6 bytes	Destination MAC address, a broadcast MAC address fixed at 0xFF-FF-FF-FF-FF-FF
SA	6 bytes	Source MAC address, an interface's MAC address or a device's bridge MAC address
Tag	4 bytes	2-byte Ethernet Type field and 2-byte VLAN field included
Type	2 bytes	Packet type, fixed at 0x0806
Field	6 bytes	Four fields included: Hardware Type, fixed at 0xFF-FF Protocol Type, fixed at 0xFF-FF Hardware Length, fixed at 0x00 Protocol Length, fixed at 0x00
Flag	20 bytes	LAD packet identifier, fixed as Huawei Link Search
Information	20-44 bytes	LAD data unit, main part of an LAD packet
FCS	4 bytes	Frame check sequence

The Information field is the same in all the three LAD packet formats, meaning that the LAD data units are irrelevant to the link type. Figure 3 shows the format of the LAD data unit.

Figure 3 LAD data unit format

Table 3 describes the fields in the LAD data unit.

**Table 3** LAD data unit fields
Field	Length	Description
Type	1 byte	LAD data unit type: 1: Link Detect packet 2: Link Reply packet
Version	1 byte	LAD protocol version, fixed at 1
Length	2 bytes	LAD data unit length
Value	12-16 bytes	LAD data unit's sub TLV: Send Link Info SubTLV Recv Link Info SubTLV

LAD Packet Types

LAD packets are classified as Link Detect or Link Reply packets, depending on the LAD data unit type.

Link Detect packets: link discovery requests triggered by the NMS or command lines. Link Detect packets carry Send Link Info SubTLV in the data unit. Figure 4 shows the format of the Link Detect packet data unit.

Figure 4 Link Detect packet data unit format
Link Reply packets: link discovery replies in response to the Link Detect packets sent by remote devices. Link Reply packets carry the Send Link Info SubTLV (the same as that in the received Link Detect packets) and Recv Link Info SubTLV. Figure 5 shows the format of the Link Reply packet data unit.

Figure 5 Link Reply packet data unit format