Brocade FastIron Ethernet Switch Administration Guide - Manual (Page 169)

LLDP receive mode
An LLDP agent receives LLDP packets from adjacent LLDP-enabled devices. The LLDP packets
contain information about the transmitting device and port.
When an LLDP agent receives LLDP packets, it checks to ensure that the LLDPDUs contain the correct
sequence of mandatory TLVs, then validates optional TLVs. If the LLDP agent detects any errors in the
LLDPDUs and TLVs, it drops them in software. TLVs that are not recognized but do not contain basic
formatting errors, are assumed to be valid and are assigned a temporary identification index and stored
for future possible alter retrieval by network management. All validated TLVs are stored in the neighbor
database.
LLDP packets
LLDP agents transmit information about a sending device/port in packets called LLDP Data Units
(LLDPDUs). All the LLDP information to be communicated by a device is contained within a single 1500
byte packet. A device receiving LLDP packets is not permitted to combine information from multiple
packets.
As shown in the following figure, each LLDPDU has three mandatory TLVs, an End of LLDPDU TLV,
plus optional TLVs as selected by network management.
FIGURE 5 LLDPDU packet format
Each LLDPDU consists of an untagged Ethernet header and a sequence of short, variable length
information elements known as type, length, value (TLV).
TLVs have Type, Length, and Value fields, where:
• Type identifies the kind of information being sent
• Length indicates the length (in octets) of the information string
• Value is the actual information being sent (for example, a binary bit map or an alpha-numeric string
containing one or more fields).
TLV support
This section lists the LLDP and LLDP-MED TLV support.
LLDP TLVs
There are two types of LLDP TLVs, as specified in the IEEE 802.3AB standard:
• Basic management TLVs consist of both optional general system information TLVs as well as
mandatory TLVs.
Mandatory TLVs cannot be manually configured. They are always the first three TLVs in the LLDPDU,
and are part of the packet header.
LLDP receive mode
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