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My point is that even if a platform lets you set all four metrics, the SPF 
computation, as defined by 10589, just adds them up and uses the sum as a 
single metric. I can only assume that the reason they are broken out 
separately is for conceptual convenience. (?) 
A human being can do this just as easily when a platform only supports a 
single metric, or when using the new-style TLVs that only define one metric.

The only real difference is that the actual values of the individual link 
properties are not reflected in the router's configuration state. 
As far as a generalized metric-selection schema, I don't think there really 
is one. How I, as a network operator, assign link metrics will depend on 
what link properties are interesting to me in my particular situation. For 
instance, if I can provision bandwidth quickly and have sufficient 
confidence in my trending and planning efforts, bandwidth isn't interesting 
to me as an IGP decision input. Delay might be, though, if I have a network 
of large geographic scope. If I run a regional or metropolitan network, my 
requirements may be quite different and I would define my metrics 
accordingly. 
-----Original Message----- 
From: Nikos Leontsinis [mailto:[email-masked]] 
Sent: Tuesday, March 12, 2002 5:48 PM 
To: Neal, Brad (Broadwing); [email-masked] 
Subject: RE: [Isis-wg] IS-IS metrics: in search for a sound way 
Although ISO 10589 defines four different metric fields, I believe the 
decision process still uses a single scalar metric because IS-IS combines 
the four fields by simple addition paragraph 7.2.2, note 13. It's up to the 
network operator to weight these fields according to his own requirements. 
No the network operator can only ammend the default cisco metric which is 
bandwith. The remaining metrics are not supported hence cannot be used what 
they rather do is to give a hint to the network administrator what to bear 
in mind when designing his/her network. I was looking for something more 
coherent than that as I think metric assinment is by far the most 
challenging task for the network designer since Cisco together with the 
remaining implemeters are fairly inefficient in this direction. I am looking

for a sound way to measure metrics. 
Point is, you can accomplish pretty much the same thing with a single 
field--just define which link properties are important to you, weight them 
according to their relative importance, add up the values and the result is 
your combined metric, just like IS-IS would have done. 
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