From: Jonathan V Hays (jhays@xxxxxxxx)
Date: Fri Jul 26 2002 - 16:10:25 GMT-3
Can anyone make sense of this table, from the CCO 12.1 command guides,
under "metric weights (IGRP)" ?
Perhaps this requires someone who has a really deep understanding of
protocols and maybe the IOS code (Howard, this is right up your alley
;-)
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/
ip_r/iprprt2/1rdigrp.htm#12369
I reproduce it below for purposes of discussion:
----------------------------------------------------------
Bandwidth is inverse minimum bandwidth of the path in bps scaled by a
factor of 2.56 * 1012. The range is from a 1200-bps line to 10 terabits
per second.
Delay is in units of 10 microseconds. The range of delay is from 10
microseconds to 168 seconds. A delay of all ones indicates that the
network is unreachable.
The delay parameter is stored in a 32-bit field, in increments of 39.1
nanoseconds. The range of delay is from 1 (39.1 nanoseconds) to
hexadecimal FFFFFFFF (decimal 4,294,967,040 nanoseconds). A delay of all
ones (that is, a delay of hexadecimal FFFFFFFF) indicates that the
network is unreachable.
Table 43 lists the default values used for several common media.
Table 43: Bandwidth Values by Media Type
Media Type Delay Bandwidth
---------- ----- ---------
Satellite 5120 (2 seconds) 5120 (500 megabits)
Ethernet 25600 (1 ms) 256000 (10 megabits)
1.544 Mbps 512000 (20,000 ms) 1,657,856 bits
64 kbps 512000 (20,000 ms) 40,000,000 bits
56 kbps 512000 (20,000 ms) 45,714,176 bits
10 kbps 512000 (20,000 ms) 256,000,000 bits
1 kbps 512000 (20,000 ms) 2,560,000,000 bits
----------------------------------------------------------
Q1. How is the description for "Delay" above related to "The delay
parameter" in the succeeding paragraph?
When I did some calculations (see below) I had to question my
assumptions.
Q2. So what are the units under the Delay column in the first table
entry?
Let's take the first entry of "5120 (2 seconds)."
By the above descriptions, a delay of 1/255 (10 microseconds) is
represented by the "delay parameter" in software in increments of 39.1
nanoseconds. Then a unit of 1/255 = (10 microseconds/39.1 nanoseconds)
and is approximately 255.75. Let's assume that Cisco rounds it off to
256. So the 2 seconds referred to in the first table entry is 200,000 (I
mean two hundred thousand, for those in other countries who use the
comma differently) of these 10 microsecond units. 200000 x 256 =
51200000. Note that this is NOT 5120 !!
Then the next entry states the delay is 25600 (1 ms). Since I don't
fully understand the first metric I will divide the 2 second Satellite
delay by the 1 ms Ethernet delay to see that the first delay in seconds
is 2000 times greater than the second. But if I multiply 5120 times 2000
the result is 10240000, not 25600 as shown in the table. If someone can
explain what these units are I'd be grateful.
Q3. Why doesn't the delay change for the last five entries in the table?
Q4. What are those numbers in the second column under Bandwidth?
I find it difficult to understand the Ethernet entry "256000 (10
megabits)", for example.
Thanks in advance for your comments.
Jonathan Hays
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