Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 117)

Multi-Service IronWare Switching Configuration Guide 87
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General operating principles
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Transmit mode
An LLDP agent sends LLDP packets to adjacent LLDP-enabled devices. The LLDP packets contain
information about the transmitting device and port.
An LLDP agent initiates the transmission of LLDP packets whenever the transmit countdown timing
counter expires, or whenever LLDP information has changed. When a transmit cycle is initiated, the
LLDP manager extracts the MIB objects and formats this information into TLVs. The TLVs are
inserted into an LLDPDU, addressing parameters are prepended to the LLDP packet, and the
information is sent out LLDP-enabled ports to adjacent LLDP-enabled devices.
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 LLDP packets 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 or 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. LLDP information exceeding 1500 bytes will be truncated. A device receiving
LLDP packets is not permitted to combine information from multiple packets.
Each LLDPDU consists of an untagged Ethernet header and a sequence of short, variable length
information elements known as TLVs.
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 and describes LLDP 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.