240
•
LLDP frame encapsulated in SNAP
Figure 70
SNAP-encapsulated LLDP frame
Table 16
Fields in a SNAP-encapsulated LLDP frame
Field
Description
Destination MAC address
MAC address to which the LLDP frame is advertised. It is the same as that
for Ethernet II-encapsulated LLDP frames.
Source MAC address
MAC address of the sending port.
Type
SNAP type for the upper layer protocol. It is 0xAAAA-0300-0000-88CC
for LLDP.
Data
LLDPDU. An LLDP frame contains only one LLDPDU.
FCS
Frame check sequence, a 32-bit CRC value used to determine the validity
of the received Ethernet frame.
LLDPDUs
LLDP uses LLDPDUs to exchange information. An LLDPDU includes multiple TLVs. Each TLV carries a type
of device information, as shown in
Figure 71
LLDPDU encapsulation format
An LLDPDU can carry up to 32 types of TLVs. Mandatory TLVs include Chassis ID TLV, Port ID TLV, Time
to Live TLV, and End of LLDPDU TLV. Other TLVs are optional.
TLVs
A TLV is an information element that contains the type, length, and value fields.
LLDPDU TLVs include the following categories:
•
Basic management TLVs
•
Organizationally (IEEE 802.1 and IEEE 802.3) specific TLVs
•
LLDP-MED (media endpoint discovery) TLVs
Basic management TLVs are essential to device management.