LLDP Support
Description
LLDP (Link Layer Discovery Protocol) is a data link layer (L2) protocol used by network devices to advertise themselves to neighboring devices and to discover them.
In fastDPI, LLDP is used neither for traffic analysis nor for building a network map — fastDPI generates and sends its own LLDP packets in order to:
- Be discovered by neighboring network equipment (switches, routers).
- Integrate correctly into a cable break — fastDPI presents itself as an active device but does not forward other devices' LLDP packets. All incoming LLDP packets are unconditionally dropped.
LLDP packets are sent:
- periodically — at an interval equal to the
ttlvalue (every 120 seconds by default); - when the
fdpi_cli lldp enablecommand is executed; - on a port Link Up event.
LLDP Configuration
The lldp section in the configuration file controls LLDP behavior. All parameters within the section except device are optional.
Basic Configuration Template
lldp { enable=off trace=off pcap=off chassis= ttl=120 src_mac= dest_mac=01:80:c2:00:00:0e system_name= system_desc= device=01-00.0;enable=on;desc=port_description }
Configuration Parameters
All parameters are hot-reloadable and are applied without restarting fastDPI.
| Parameter | Description | Possible values |
|---|---|---|
enable | Enables/disables LLDP support. Default: on if the lldp section is present in the configuration, off if it is absent. | on, off |
trace | Enables LLDP tracing. When enabled, information about packet generation and sending is written to fastdpi_alert.log. | on, off |
pcap | Enables writing LLDP packets to a pcap file (named udpi_YYYYMMDDHHMMSS_XXXXX.pcap). | on, off |
chassis | Chassis-ID value sent in the LLDP packet. If not set, the MAC address from the bras_arp_mac option is used; if that is not set either, the port MAC address is used. | String |
ttl | Time To Live of the LLDP packet, in seconds. Also sets the interval for periodic packet sending. When ttl=0, periodic sending is disabled: packets are sent only on the fdpi_cli lldp enable command and on a port Link Up event. The TTL field in such packets is set to 65535, because according to the LLDP specification, TTL=0 means that LLDP is disabled on the port. | Number (default 120) |
src_mac | Source MAC address in the LLDPDU Ethernet header. If not set, the MAC from bras_arp_mac is used; otherwise, the port MAC. | MAC address (e.g., aa:bb:cc:dd:ee:ff) |
dest_mac | Destination MAC address in the LLDPDU Ethernet header. | MAC address (default 01:80:c2:00:00:0e — LLDP multicast) |
system_name | Value of the System-Name TLV in the LLDP packet. If not set, the System-Name TLV is not included. | String |
system_desc | Value of the System-Description TLV in the LLDP packet. If not set, the System-Description TLV is not included. | String |
device | Port on which LLDP packets are to be sent. The parameter can be repeated for different ports. The port name must match the in_dev/out_dev value. Subparameters are specified within the parameter, separated by semicolons:• enable=on|off — enable/disable sending on this port (on by default)• desc=string — value of the Port-Desc TLV (if not set, the Port-Desc TLV is not included) | Format: <port_name>[;enable=on|off][;desc=<string>]Example: device=01-00.0;enable=on;desc=WAN_port |
Configuration Examples
Minimum configuration required to enable LLDP support
lldp { device=01-00.0 device=02-00.0 }
Configuration with Port-Desc TLV values for each port
lldp { device=01-00.0;desc=WAN_port_1;enable=on device=02-00.0;desc=LAN_port_2;enable=on }
CLI Management
Commands for dynamic LLDP management:
- Enable generation and sending of LLDP packets (sent immediately):
fdpi_cli lldp enable [ttl=N]
where
ttl=Nis an optional parameter: the LLDP packet Time To Live and the periodic sending interval, in seconds. Whenttl=0, periodic packets are not sent; see thettlparameter description in Configuration parameters table.
Example:fdpi_cli lldp enable ttl=60
- Disable generation and sending of LLDP packets:
fdpi_cli lldp disable
When disabled, an LLDP packet with
TTL=0is sent to the ports — according to the LLDP specification, this notifies neighboring devices that LLDP has been disabled on the port.
Example output when enabling:
LLDP enabled chassis='<n/a>' TTL=120 system: name='', desc=''
src_MAC=00:00:00:00:00:00 dest_MAC=01:80:c2:00:00:0e
Devices:
'01-00.0': enabled desc=''
'02-00.0': enabled desc=''
Diagnostics and Debugging
Tracing
When the trace=on parameter is enabled in the lldp section, information about LLDP packet generation and sending is written to the fastdpi_alert.log file:
[INFO ][2026/05/25-10:56:31:700344][0x7f0a4aa34400] LLDP options:
enable : 1
trace : 1
pcap : 0
chassis : ''
ttl : 120
src_mac : -
dest_mac : 01:80:c2:00:00:0e
system_name : ''
system_desc : ''
device '01-00.0': enable=1 desc=''
device '02-00.0': enable=1 desc=''
[TRACE ][2026/05/25-10:56:31:700454][0x7f0a4aa34400] [LLDP]make_pdu: dev '01-00.0' PDU len=234
[TRACE ][2026/05/25-10:56:31:700475][0x7f0a4aa34400] [LLDP]make_pdu: dev '02-00.0' PDU len=234
[TRACE ][2026/05/25-10:56:31:850646][0x7f0a4aa34400] [LLDP]send_pdu: dev '01-00.0' via slave #0
[TRACE ][2026/05/25-10:56:31:850725][0x7f0a4aa34400] [LLDP]send_pdu: dev '02-00.0' via slave #0
PCAP Recording
When the pcap=on parameter is enabled, all generated LLDP packets are saved to a pcap file (named udpi_YYYYMMDDHHMMSS_XXXXX.pcap).
LLDPDU Example (Result)
Example of a frame generated by fastDPI (minimum configuration, bras_arp_mac=00:01:02:33:44:f1):
00:01:02:33:44:f1 > 01:80:c2:00:00:0e, ethertype LLDP (0x88cc), length 78
Chassis ID TLV (1), length 7
Subtype MAC address (4): 00:01:02:33:44:f1
Port ID TLV (2), length 8
Subtype Local (7): 01-00.0
Time to Live TLV (3), length 2: TTL 120s
End TLV (0), length 0
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