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Below is the output from my "show interface " command

PE1.18:01:44#sh interfaces Ethernet 3
Ethernet3 is up, line protocol is up (connected)
  Hardware is Ethernet, address is 001c.7374.3d95
  Internet address is 172.16.11.1/30
  Broadcast address is 255.255.255.255
  Address determined by manual configuration
  IP MTU 1500 bytes , BW 10000000 kbit
  Full-duplex, 10Gb/s, auto negotiation: off, uni-link: disabled
  Up 30 days, 9 minutes, 55 seconds
  Loopback Mode : None
  4 link status changes since last clear
  Last clearing of "show interface" counters 45 days, 23:02:51 ago
  5 minutes input rate 48 bps (0.0% with framing overhead), 0 packets/sec
  5 minutes output rate 48 bps (0.0% with framing overhead), 0 packets/sec
     1011859 packets input, 799292486 bytes
     Received 1 broadcasts, 1011570 multicast
     0 runts, 0 giants
     0 input errors, 0 CRC, 0 alignment, 0 symbol, 0 input discards
     0 PAUSE input
     88567 packets output, 15703989 bytes
     Sent 1 broadcasts, 88453 multicast
     0 output errors, 0 collisions
     0 late collision, 0 deferred, 0 output discards
     0 PAUSE output

I am trying to find the available capacity on this interface in terms of bytes or bits for eg. Total link capacity is 10Gb and out of that 5Gb is available. I know that the "5 minutes input rate and output" section from the output should help, but I am really confused as to what formula to be applied and how? Please help.

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  • The bandwidth is listed. it is BW 10000000 kbit. Bandwidth is how many bits per second the interface can send. What is it that you don't understand about the bandwidth?
    – Ron Maupin ♦
    Mar 27, 2018 at 1:43
  • BW 10000000 kbit is the Total bandwidth on the interface, but I want to know how much of it is currently free i.e not being utilized by traffic?
    – user258699
    Mar 27, 2018 at 1:53
  • That doesn't actually make sense. The interface is sending at 10 Gbps, or it is not sending anything. The bandwidth is how many bps it can send. If you are asking about the data usage over a certain time period, that is a completely different question.
    – Ron Maupin ♦
    Mar 27, 2018 at 1:56
  • Sorry for that. I am asking about data usage yes.
    – user258699
    Mar 27, 2018 at 2:01
  • I have made corrections to the question may be it makes more sense now.
    – user258699
    Mar 27, 2018 at 2:07

2 Answers 2

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I believe what you're really needing to know is whether the interface is being used "optimally" and how much "spare capacity" you have on that link. These are fuzzy terms, but important because you need to know whether, for example, that 10Gbps interface needs to be replaced with a 40Gbps.

From the "show" command output we know that in the last five minutes there was very little traffic on the interface. But maybe three days ago, at 2pm, there was 9 Gbps traffic for a few seconds or milliseconds. The "show" command output right now is not going to give that information. The only way for you to get that data is to monitor the interface over a period of time (like maybe a week or a month), especially when there are peak workloads.

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  • Exactly, I am looking for the "spare capacity" part. If I reduce the load-interval to say 5 seconds and subtract the input rate (48 bits in the above output) from Total capacity of link(10Gb in above output), will that give me an accurate representation of the current "spare capacity" on the link?
    – user258699
    Mar 27, 2018 at 3:55
  • No. What your method will give you is merely the "spare capacity" right now. For example, if your enterprise is a stockbroker, and this switch is used in the path for placing trades, you will get very different readings for the input/output rate depending on whether you measure it on Saturday evening or on Monday morning when the markets open.
    – mere3ortal
    Mar 27, 2018 at 4:50
  • As a matter of fact, I am looking for "spare capacity" right now. I want the spare capacity as close to the instant I do the show command. So I guess this should work right?
    – user258699
    Mar 27, 2018 at 5:22
  • Well yes, if you just want the "right now" available bandwidth, you do (10 Gbps minus 48bps (whatever is reported in the input/output rate)).
    – mere3ortal
    Mar 27, 2018 at 5:48
  • @user258699 "I want the spare capacity as close to the instant I do the show command." I have seen other people ask this type of thing. A rate is during a specific time period. In any instant, the interface is either sending a bit (1 or 0), or it isn't sending a bit. That is during 1/10,000,000,000 of a second. What you have with your Cisco stats is during a five-minute period, in the past. I'm not sure you understand what you are asking. You can't ask how busy it is right now, only how busy it was during some previous time period.
    – Ron Maupin ♦
    Mar 27, 2018 at 13:04
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OK. The 5 minute rate is how many bits were received in the last 300 seconds, divided by the number of seconds, to give you the number of bits per second over the last five minutes. It is possible that you have only a minimum frame send during the last five minutes. The frame will be sent at 10 Gbps, but your five minute rate will be much less because the entire frame will be sent in just over five hundred-millionths of a second. Even a full frame is sent in about three times that period.

The framing overhead (18 octets or 144 bits on non-tagged frames) takes into account the ethernet frame overhead on a frame (frames are sent one at a time on the link). Ethernet also has a seven-octet (56 bits) preamble, and a one-octet (eight bits) start of frame (SoF) delimiter, preceding each frame. There is also a 12-octet (48 bits) inter-frame gap between frames. All of that is layer-2 overhead before getting into the frame payload (the layer-3 packet).

Then you have the layer-3 packet overhead, the layer-4 transport protocol datagram overhead, and (possibly) the application protocol overhead before you can even start discussing the application data sent over the link.

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  • Was not asking for that at all. In Cisco if you type in the same command it gives you more or less the same output but it has two extra fields namely, txload and rxload, which essentially indicate how much of your link is utilized. The output is <some number>/255, so a link showing 1/255 is completely free 255/255 is congested. But that is cisco. I am using an Arista switch. Arista gives everything else but txload and rxload. Even if it gave, I am looking for a more detailed number and not something which is weighted out of 255. Something that will give me the answer in terms of bits or bytes.
    – user258699
    Mar 27, 2018 at 2:35
  • Actually that is pretty much exactly what you were asking: "I want to know how much of it is currently free i.e not being utilized by traffic?" There are several ways to look at this. The maximum you can transmit is 10 Gbps, and you have averaged 48 bps over the last five minutes, then 48/10,000,000,000 is the percentage of the possible bits you could have sent over the last five minutes. Really, that's all the Cisco numbers are trying to tell you.
    – Ron Maupin ♦
    Mar 27, 2018 at 2:42
  • @user258699, in reality, the link is either actually being used to send a frame at 10 Gbps, or it is not sending anything at all. What you want to know is, over a specific time period, how many bits were sent compared to how many bits could have been sent. This is all past information; either the interface is currently sending something at 10 Gbps, or it is not.
    – Ron Maupin ♦
    Mar 27, 2018 at 2:45

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