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I'm wondering if the ARP requests should be tagged when being sent out of a VLAN tagged interface?

It seems logical to me that the ARP request should be tagged, since it needs to travel all the broadcast domain. If any switches or devices in the broadcast domain have VLAN configurations, the untagged ARP request will not pass.

I would assume that they should, but I don't have any references to confirm.

  • 3
    Yes, the switch doesn't care what's the upper layer payload is. It only looks at the L2 destination address and looks it up in the CAM table. Since ARP is destined to FF:FF:FF:FF:FF:FF it will egress all ports on the VLAN including trunk ports, which insert a VLAN tag to the frame regardless of what's inside. – sergeyrar Nov 7 '17 at 20:46
  • @sergeyrar but the insertion of tags will be only for the trunk ports. For other ports of the VLAN, no tagging should take place. – AhmedWas Dec 26 '18 at 7:28
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Whether the frame is ARP or not is irrelevent. Anything sent out a trunk port is tagged with the VLAN the frame is a member of:

pracnet.net/vlans

ARP is sent by the client, the client typically has no knowledge of what VLAN it is a member of. It is therefore unable to add a VLAN tag, and shouldn't be expected to. The ARP the client sends will arrive on the switch untagged.

If the switch sends the frame out a trunk port, the switch will add a VLAN tag.

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    But if the client has a VLAN-aware interface, technically everything going out should be tagged, including ARP requests? – Simon Nov 7 '17 at 21:07
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    If the VLANs are configure on the client, and an ARP request is needed on a VLAN tagged (virtual) interface/network, then yes, even the ARP is tagged. – mulaz Nov 7 '17 at 22:03

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