VLAN tags get stripped by cisco switch (SG200) ... ?!

Hi There,

trying to get this to work with my current setup.

Following this guide I was able to get everything working as advertised.

HOWEVER, when I put my cisco SG200 switch inbetween it stops working ... router A is NOT able to talk to router B. (The switch has mostly DEFAULT settings ... only PW etc. changed). It seams to mess with VLAN tags?!

Connecting router B right into the lan port of router A works just fine. So settings etc. are all good.

Putting the SWITCH in between breaks it. Why ?

This is already way above my head ... looking for pointers on how to be troubleshoot / learn about how to make this work.

Thanks in advance for any help.

UPDATE: A bit more detail about my setup ...

cable modem -> router-A -> switch -> routers-B (WAP)

I have configured 2 additional vlans on each of the routers ... connected directly. Both router know what to do with the vlan traffic.

When the switch gets in between it seems to strip the packed off the vlan ID and (can / does?) put it's own vlan id onto the traffic?!

If you have a dump switch, could you give it a try?

Or for this SG200, can you define VLANs to match what you have?

By the way, the subject line doesn't really describe the problem.

1 Like

Thanks ... changed the subject line :slight_smile:

Sorry no DUMB switch lying around ...

Added VLANS to the SG200 (same name / ids as on the routers). Still NO traffic coming through ...

I'm not familiar with managed Cisco switches, but you might have to configure a trunk link for the VLANs you want to transport over the given port.

I set the VLAN membership for the ports for RT-A & RT-B. (in Port VLAN Membership).

Still not working ... HOW DO I "configure a trunk link for the VLANs "?

At least on the SG300 series, you need to set the port to General (as I recall, surprisingly not Trunk), create the VLANs, then assign the VLANs to the ports as tagged.

Naming of VLANs is a UX convenience. Only the numeric tag is carried on the wire.


You're a bit sparse on information, remember that unless you have a "access" port the devices needs to be aware of what VLAN they're supposed to talk to otherwise the switch will translate untagged traffic to one specific VLAN.


Ok, did some more testing etc.

On the switch I have created 4 additional VLANS (same as on BOTH OpenWRT-Routers).

I made each connection port on the SWITCH a member of all VLANS and set these ports to GENERAL (tried TRUNK as well).

Again when I connect the two routers DIRECTLY ... all works as it should.

When the switch is in between ... it breaks that flow of traffic. (BTW, I could see that each VLAN traffic was hitting the other router's corresponding VLAN.)

... still NOT working ...

Thanks for all your help though !

my understanding ( although I've only worked with 3xxx series ) is the above is setting a switches internal reference to it own MIB's internal to the switch with regard to ports it will forward the frames to.

if the frames are tagged as they enter the switch then unless it's a trunk or non-access ( non-general? ) it may either strip the tags and set the frames on the native vlan or drop them?

Either way, looking at the arp / mib tables within the switch should help. As would reading the documentation regarding the native vlan etc. for that switch.