Recently, I was on a Juniper device, trying to figure out why I was getting a lower bit rate even though I knew I was sending a lot more.

Then it hit me that Juniper devices show the Layer 3 transmit rate, whereas I was sending at a Layer 2 bit rate. It has been a while since I have encountered this, and I used to be able to do a calculation to estimate the Layer 3 bit rate based on the Layer 2 bit rate using the frame-size (Ethernet, VLAN).

However, I cant seem to remember how I did that calculation, nor can I find any examples or documentation on the conversion.

Can anyone highlight how this calculation is done (preferably with an example)? Lets say a L2 bit rate of 100 Mbps, and a frame size of 70 MB.

  • 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 can provide your own answer and accept it. – Ron Maupin Aug 6 '17 at 23:56

The frame header is 14 octets without a VLAN tag, and 18 octets with a VLAN tag. Inside the frame is your payload (IP packet). There is also a 4-octet FCS at the end of the frame.

If you add that all together, you get the frame size (maximum 1518 or 1522 octets, depending on whether or not it is a trunk link).

Ethernet has more than just the frame. There is a 7-octet preamble, 1-octet start of frame delimiter, and a 12-octet inter-packet gap between frames. So you add 20 octets to the size of your frame to account for ethernet overhead.

| improve this answer | |
  • Yes, but what is the conversion. factor. So for example, if I am sending 100 Mbps L2 rate, what would i see as the L3 rate? – user2883071 Jul 18 '16 at 19:34
  • That depends on the layer-3 packet size. An ethernet frame can be as small as 64 octets (have a payload as small as 42 octets with VLAN tag, or 46 octets without), and if the layer-3 packet is less than that, ethernet will pad the payload. The maximum ethernet payload is 1500 octets. This doesn't count the ethernet overhead of 20 octets. Without knowing the exact size of the layer-3 packets, it will not really be possible to have a conversion factor. If you know the layer-3 packet size, then you can figure it out with what is in my answer. – Ron Maupin Jul 18 '16 at 19:45
  • @user2883071, what I have given you is only for ethernet. Wi-Fi or other layer-2 protocols will be different. – Ron Maupin Jul 18 '16 at 19:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.