Brocade Communications Systems 6650 manual Lldp packets, Lldp receive mode

General LLDP operating principles 8

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.

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 Figure 13, each LLDPDU has three mandatory TLVs, an End of LLDPDU TLV, plus optional TLVs as selected by network management.

FIGURE 13 LLDPDU packet format

Chassis ID

TLV

Port ID

TLV

Time to

Live TLV

Optional

TLV

...

Optional

TLV

End of

LLDPDU TLV

MMMM

M = mandatory TLV (required for all LLDPDUs)

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).

Brocade ICX 6650 Administration Guide	187
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