Link Layer Discovery Protocol
LLDP it’s a nonproprietary protocol that works at layer 2. It is an open IEEE-standard 802.1AB. LLDP is an
alternative to CDP. LLDP runs over the data
link layer and it collects information about the neighboring devices. This information
will help us in troubleshooting and documentation. LLDP and Cisco CDP work similarly
but the big difference is that LLDP is a standard while CDP is a Cisco proprietary.
LLDP has similar capabilities to CDP and there is an enhanced version to specifically address the voice application this version is called LLDP-MED (media endpoint discovery) but LLDP and LLDP-MED
are not compatible.
LLDP supports a defined set of attributes that contain a T-type, L-length, and V-value, these are called TLVs. It is used to discover neighbors’
devices. LLDP support devices use TLV to send and receive information and
store the information in a local table.
How link layer discovery protocol work?
First, you must enable it on the devices because by default LLDP is disabled, and remember not all Cisco IOS routers support LLDP. LLDP-enabled
devices send LLDP advertisements to LLDP neighbors and the information is
stored in the MIB database locally. This information can be accessed using SNMP.
What information is stored?
- Device name and description
- Port name
- IP management address
- Capabilities
- MAC address
- MDI power
- Link aggregation
- VLAN
LLDP default configuration: -
IOU1(config)#lldp ?
holdtime Specify the holdtime (in sec) to be sent in
packets
reinit Delay (in sec) for LLDP initialization on
any interface
run Enable LLDP
timer Specify the rate at which LLDP packets
are sent (in seconds)
tlv-select Selection of LLDP TLVs to send
- LLDP is disabled by default
- LLDP hold time – 120 seconds
- LLDP reinitialization delay – 2 seconds
- LLDP timer (packet update frequency) – 30 seconds
- LLDP tlv-select – to send and receive all TLVs.
Let's see the configuration: -
- enable LLDP in global configuration mode.
- change the hold time to 180, reinit to 5 sec, and the timer to 20 sec.
- clear the LLDP table.
- disable the LLDP
