Brocade TurboIron 24X Series Configuration Guide - Manual (Page 224)

190 Brocade TurboIron 24X Series Configuration Guide
53-1003053-01
General operating principles
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 LLDPDU, 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 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 Figure 3 , each LLDPDU has three mandatory TLVs, an End of LLDPDU TLV, plus
optional TLVs as selected by network management.
FIGURE 3 LLDPDU packet format
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 the LLDP TLV support.
Chassis ID
TLV
Port ID
TLV
Time to
Live TLV
Optional
TLV
Optional
TLV
End of
LLDPDU TLV
...
M M M M
M = mandatory TLV (required for all LLDPDUs)