My first post – a little story about old equipment and vlans

I was called out to a customers who had an issue with surfing the web at one particular location of their network which was connected by fiber.
We were quite confused by the issue as apparently smaller website without much content, such as, would load without any issue – while bigger websites like newspapers would load just a tad or not at all.

To make things even more confusing, all the thin clients (RD) worked like a charm and the other network at this location, on VLAN 200, did not experience any problems.

RIGHT! It has to be the VLAN/IP-segment itself, as it was only used at this location. Testing the network at headquarters disproved the theory without further a due and the issue boiled down to the following statements:

  • Traffic streams containing big frames (assumption) on vlan 3123, from this particular location, will break.
  • Traffic streams on vlan 200 do not experience this problem, even though used at the samelocation.


So this was the topology: [Inet]—–[router]—-fiber—–[Switch]

I first expected the mtu to be off somewhere, and but it all checked out and I had to investigate closer as why this location experienced this issue.
Searching high and low I found out media converters were used – you know, the old type of media converters with physical switches to set speed, duplex, etc.

To my surprise, the left most switch said “1518/1522”, with the switch set in the “1518”-position.
You got to be joking? Thats why this network has trouble with big frames – The media converters bloody crops away 4 bytes of all the frames!!! Why would media converters even have a switch to “remove” the VLAN-tags?

Needless to say, setting the switch to 1522 on both media converters fixed the issue. =)

The new topology looked like this: [Inet]—–[router]—(MC)—fiber—(MC)—[Switch]

So the lesson is – If only big frames are struggling – check if there is some old equipment that has to be manually configured to support the full frame-size with VLAN tags. ^_^


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