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I have this simple topology -

[S1][p1]----[p2][S2]

Port p1 of switch S1 is connected to port p2 of S2. And port p2 on S2 is tagged.

My doubt is if an LLDP packet is sent from p1 to p2, considering p2 is tagged, will the LLDP packet be processed at CPU of S2? Or will it be dropped at hardware level?

We know untagged packets which don't have 802.1Q tag in Ethernet frames are dropped on tagged ports. I wanted to know what's the behavior if the packet is LLDP.

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  • 2
    You forgot to mention if p1 is also tagged?
    – Sebastian Wiesinger
    Jun 23, 2014 at 11:07
  • Could you also include the equipment in question as this can significantly impact the answer?
    – YLearn
    Jun 23, 2014 at 13:08
  • @Sebastian: Thanks for pointing out. p1 is untagged.
    – tcpip
    Jun 23, 2014 at 18:24
  • @YLearn: Sorry for missing that out folks. I agree this heavily relies on manufacturer. These are HP ProCurve switches, and more specifically, the packets are actually BDDP(Broadcast Domain Discovery Protocol) which is slightly different from LLDP in that it has the destination MAC address as broadcast and not multicast. This is an HP proprietary protocol used in SDN networks to recognize links between switching devices.
    – tcpip
    Jun 23, 2014 at 18:29
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 8, 2017 at 20:53

1 Answer 1

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It depends on the hardware involved. Juniper for example will drop the LLDP packets when there is no native VLAN configured (a VLAN in which all untagged traffic will be forwarded in).

Cisco for example will always send the LLDP tagged in VLAN 1. Juniper itself sends the LLDP untagged.

So it highly depends on your gear.

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  • Cisco sends them untagged in the native vlan (which is configurable)
    – Ricky
    Jun 23, 2014 at 16:44
  • Then I think HP ProCurve device is exhibiting behavior similar to Juniper's. Situation here is, S2 is connected to an OpenFlow controller to which it's supposed to send any kind of data it receives on p2. Now whilst p2 is untagged, LLDP packets are forwarded well to Controller, but that is not happening when p2 is tagged. So I guess packets are getting dropped.
    – tcpip
    Jun 23, 2014 at 18:39

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