[mrtg] Re: Max Speed question

Daniel J McDonald dmcdonald at digicontech.com
Wed Aug 15 18:26:38 MEST 2001

1) How is the value obtained?

	By grabbing ifSpeed from snmp, which is populated by the bandwidth
interface configuration command under IOS.

2) Why do all the values appear to be based on the number 125?  That is,
where I'd expect to see 100 MBytes/s i would see 125 MBytes/s instead.

	No, you expect to see 100Mbits/s, not 100MBytes/s, unless Cisco has built a
fiberchannel blade I don't know about.  Obviously, dividing bits by 8 to
produce bytes would give you the values that you see.

	Network guys talk in terms of bits because we always have - old serial
connections were 9, 10 or 11 bits per byte (one start bit, 7 or 8 data bits,
one or two stop bits) so the Byte per second rating (usually labeled cps
(characters per second) in those days) was not immediately obvious from the
underlying infrastructure.  Even today on T1's there is not an absolute 1:8
relationship between bps and cps.  So, we talk in terms of bps because it is
precise when building networks.

	SNMP counters, on the other hand, don't care about such trivia as whether a
byte was all zeros and thus was represented as two BPV's instead of 8 bits,
or whether it was all 1's and was represented by a particular 9 bit pattern
instead of 8 bits.  In SNMP only the bytes are counted.  MRTG by default
displays the bytes.  We can estimate bits by multiplying by 8, but it is
only an estimate.  It does make the graphs look prettier, though, when being
shown to bit-centric people.

	You can change the display to bits by using the options[_]: bits command.

Daniel J McDonald, CCIE 2495, CNX
Principal Network Specialist
Digicon Technologies

Digicon, a Cisco Partner, Silver Certified.

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