Tcpdump output only shows routing messages

I have tcpdump running on a TP-Link 1043 v1 router with a link between a Linux based server and Cisco switch connected through the switch ports. The issue is the tcpdump only shows routing messages between the various network elements. Running tcpdump on the Linux server I see all the expected packets and the application (server) is working with its connection through the tp-link switch.
Is there any other settings I need to change to show all packets in the capture, I have tried setting the interface (-i option) as br-lan, eth0 and eth0.1 but it has not changed the capture results. The switch section of the tp-link is unmanaged so that I am not reporting any addresses to the connected equipment. The firewall is set to block all traffic from lan to wan. I have remote access to the wan port.
The main function for the tp link router is to capture traffic and write it directly to a connected usb memory stick for analysis.

Is the issue related to promiscuous mode?


@StuartP, welcome to the community!

LAN to LAN traffic does not pass the CPU of the router, hence it cannot be captured.

You can create another VLAN for those ports and bridge them, you will then see traffic passing, at the cost of LAN bandwidth.


Thanks for the welcome, if I create a new vlan I presume I need to include both the switch ports and the wan port in the bridge. The equipment wired through the tp link router is connected to a Cisco switch where vlan tagging is configured, would adding the bridge vlan cause issues with the traffic?

I'm not sure why you mentioned WAN. You would create e.g. a second eth0.3 Interface and bridge it to LAN. You would then add that to a LAN port of your choice as untagged. You will turn eth0.1 to off on that port. Then you should be able to capture traffic.

Some switch chips allow mirroring of ports - this may be easier for you. All these switch settings can be seen on the LuCI web GUI at Network > Switch.


I tried adding the extra VLAN and it didn’t go to plan, not long after it was added I lost connection to the Cisco switch. The LAN port status only showed one port in use. I reverted the changes and after a short period both LAN ports showed ok. My router does have the option for port mirroring but this defeats the purpose of the router, I am hoping to use it as and when required as a packet capture box which write the capture to the connected usb memory.

I assume when the VLAN was added and all the ports showed as “off” it should have had no effect on the operation of the LAN ports.

I will probably replicate the monitoring on my home router before attempting anything with the Linux server and Cisco switch, just so I can test it at my leisure with no real consequences if something is wrong.

Wrong, ports need to be "tagged" or "untagged" to be activated. If all your switch ports and VLANs were switched "off" - that's likely your issue.

The reference to the ports being off, was before I reached changing their tag/untag state, it was at that point when the VLAN had just been that shortly after I lost the visibility of one of the LAN ports. After I reverted all changes I wasn’t brave enough to try it again without further guidance.

It would really help if you show your configs (i.e. /etc/config/network), perhaps maybe even a screenshot of the Network > Switch page on the LuCI web GUI.

As requested,

/etc/config/network contents

Network page on luci interface

  • Please paste configs into a codebox next time (if I wanted to help you by editing your configs, this makes it difficult)
  • Feel free to use only one post, even for pasting multiple pics
  • I do not see a second Interface (and VLAN) in your network config
  • I asked for the Switch page (not Interfaces) - can you provide that?
  • You need the second interface created (e.g. eth0.3) before you can add it to the switch ports
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As a new forum member I am only permitted to post one image per post.

config interface 'loopback'
        option ifname 'lo'
        option proto 'static'
        option ipaddr ''
        option netmask ''

config globals 'globals'
        option ula_prefix 'fd1d:4397:a6a5::/48'

config interface 'lan'
        option type 'bridge'
        option ifname 'eth0.1'
        option proto 'none'

config interface 'wan'
        option ifname 'eth0.2'
        option proto 'static'
        option ipaddr ''
        option netmask ''

config interface 'wan6'
        option ifname 'eth0.2'
        option proto 'dhcpv6'
        option auto '0'

config switch
        option name 'switch0'
        option reset '1'
        option enable_vlan '1'
        option enable_vlan4k '1'

config switch_vlan
        option device 'switch0'
        option vlan '1'
        option ports '1 2 3 4 5t'

config switch_vlan
        option device 'switch0'
        option vlan '2'
        option ports '0 5t'

The router has not changed much from the default configuration.

OK, this means you haven't created anything I suggested.

  • Create another Interface `lan2' as eth0.3 (compare/use LAN setup as your guide)
  • Add lan2 to the LAN firewall Zone
  • Browse to Network > Switch and then create another port config for VLAN3. Here's an example to configure the fourth LAN port for another VLAN/Interface:

You can then connect the server (or switch) to this LAN4 port, you then have the ability to monitor traffic between these networks.

Thanks, I appreciate your time in trying to provide a solution.

I still get a problem when I create the new vlan port, it results in one of the lan ports showing no connection affecting the connected equipment.
You referred to creating the new interface lan2 but what interfaces is it to cover, at that point the new vlan hasn’t been created.

I thought you mentioned this previously; and I asked that you show your config so we can troubleshoot that. Do not reset to defaults, do not send the default config.

If you followed my steps above, I don't understand how that occurs - so I need to see the completed config.

  • When you create an OpenWrt Interface (lan2/VLAN3), it initially "covers" nothing
  • Please create the VLAN by going to Network > Interfaces and adding a lan2 and enumerating eth0.3 as the PHY, this is what "creates" the VLAN - I'm also happy to show more pictures
  • You then have to un configure a switch port from VLAN 1 and assign it to the new lan2 (VLAN3) - shown above in the screenshot from Network > Switch
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