Understanding How LLDP Works With DHCP

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For years, I used to configure SIP phone VLAN membership through a DHCP

Here are the details:
– I dedicate a LAN port on a switch to voice VLAN
– somewhere else, I configure a DHCP server to serve LAN addresses within voice VLAN
– any other switch port connected to an other DHCP server is explicitely excluded from voice VLAN
– new SIP hardphones are first connected to the dedicated voice VLAN port:
after several reboots, they get an address within voice VLAN address range and save VLAN tag somewhere within their persistent memory
– SIP phones are then moved to an other switch port: as they boots, they request a LAN address using previously received VLAN tag.

Now I would like to improve this process using LLDP. I ran a couple of tests in my lab and still have some questions:

1. My lab switch sends within LLDP frames, a list of VLANs. One is named
“default” and the other is named “voice”. Do LLDP-capable phones look for a specific name to elect the VLAN tag they will later use to build DHCPDISCOVER request or do they look for something else (medPolicy) ?

2. With LLDP, do you still need your DHCP server to embed VLAN membership data within DHCPOFFER or is it a thing of the past ?

3. Have you been successfull with LLDP on a KVM guest networked to an LLDP-enabled switch through a linux bridge (see [1]) ?
Where can I find information regarding the line bellow:
echo 16384 > /sys/class/net//bridge/group_fwd_mask


Best regards

One thought on - Understanding How LLDP Works With DHCP

  • 2017-01-19 4:09 GMT-06:00 Olivier :

    Hi Oliver.

    1. Actually there are 2 protocols which must be supported on switch and ip phone, one is LLDP wich inventories both ways. IP Phone < -> Switch, to ellaborate on both devices MIB database with switching/routing/app capabilities of their partner; second is the LLDP-MED (Media Endpoint Discovery) which is capable of sending L2/L3 settings to devices. These settings are in several categories, concerning your question is “Network Policy” settings wich will be sent to the ip phone based on their app capabilities (LLDP).
    “Network Policy” settings can contain VLAN ID for voice and other ID for Data, and other for Video, etc. Once the LLDP-MED “Network Policy” settings are received on ip phone, will tag the phone traffic on the specified VLAN, On switching capable ip phones (2 or more ethernet interfaces), probably will only tag phone traffic, and leave the switched traffic on the access vlan. So ip phone sends DHCPREQUEST on the VLAN ID set by LLDP-MED.

    2. DHCP VLAN settings will probably being ignored since most of devices will prefer LLDP-MED settings.

    3. With LLDP enabled on KVM guest, you can obtain information about network devices attached, their capabilities, brand, model, etc. I have never tried but LLDP-MED supposed only to work on next switch device (Link Layer), not a propagation protocol.

    I always have deployed LLDP-MED capable ip phones on a LLDP-MED capable network is:
    1.- Enable LLDP on all access switches so they can advertise and receive LLDP information.
    2.- Configure LLDP-MED on all ports where will be connected ip phones and set the correct Network Policy, sending LLDP-MED capable voice devices to voice VLAN ID
    3.- DHCP server exists attached to voice VLAN ID, as LLDP-MED will provide same VLAN ID, phone will receive at the first boot their DHCP settings.

    Best Regards.