[mrtg-developers] Re: cfgmaker reports wrong Max Speed with SNMPv2 (2.10.15)

[Larry Fahnoe]

Here's a patch that implements what I've suggested below.  It works
for me on a large number of Cisco & Nortel switches and routers.

--Larry

-----------------------snip-----------------------
--- cfgmaker-2.10.15    2004-08-17 11:13:54.000000000 -0500
+++ cfgmaker    2004-11-23 12:25:35.000000000 -0600
@@ -164,12 +164,19 @@
         debug('base',"Walking ifHighSpeed");
         my @ifHighSpeed = snmpwalk($snmphost, 'ifHighSpeed');
         for ( my $i=0; $i<=$#ifHighSpeed; $i++ ) {
-                my ($if,$value) = split /:/, $ifHighSpeed[$i], 2;
-                $value = $value * 1000000;  # highSpeed = contador * 10^6
-                if ( !$value ) {
-                        ($if, $value) = split /:/, $ifSpeed[$i], 2;
-                        #debug('base',"Speed: $if - $value");
-                }
+                my $value;
+                my ($if, $highSpeed) = split /:/, $ifHighSpeed[$i], 2;
+                my (undef, $speed) = split /:/, $ifSpeed[$i], 2;
+                if ( $speed < 2**32 - 1) {
+                    $value = $speed;
+                    #debug('base',"Speed: $if - $value (ifSpeed)");
+                } elsif ( $highSpeed > 0) {
+                    $value = $highSpeed * 1000000;
+                    #debug('base',"Speed: $if - $value (ifHighSpeed)");
+                } else {
+                    $value = 0;
+                    #debug('base',"Speed: $if - $value");
+                }
                 $routers->{$router}{$if}{'ifSpeed'} = $value;
         }
     } else {
-----------------------snip-----------------------

On Tue, Nov 23, 2004 at 11:06:38AM -0600, Larry Fahnoe wrote:
> I agree that it is important not to gratuitously break something in an
> effort to fix something else, and that fixing the problem at the
> source rather than patching downstream is a good rule of thumb to live
> by.
> 
> What I'm seeing on these cisco routers is that both ifSpeed (bits/sec)
> and ifHighSpeed (Mbits/sec) are being populated.  cfgmaker looks to
> see if there is a value in ifHighSpeed, and if so, scales it up and
> then uses it.  If ifHighSpeed doesn't contain a value, ifSpeed is
> used.  In the case of the T1 interfaces, ifSpeed contains 1544000 and
> ifHighSpeed contains 2 thus cfgmaker uses 2000000.
> 
> Upon reading the if-mib's definition of ifSpeed I see that it says:
> 
>      "If the bandwidth of the interface is greater than the maximum
>      value reportable by this object then this object should report
>      its maximum value (4,294,967,295) and ifHighSpeed must be used
>      to report the interface's speed.  For a sub-layer which has no 
>      concept of bandwidth, this object should be zero."
> 
> So, if SNMPv2 queries are being used, it seems reasonable for cfgmaker
> to consider the values of both objects and decide the speed via logic
> similar to the following:
> 
>     if ( ifSpeed < 4,294,967,295) {
>         speed = ifSpeed
>     } elsif ( ifHighSpeed > 0) {
>         speed = ifHighSpeed * 10^6
>     } else {
>         speed = 0
>     }
> 
> The primary rationale is to avoid using the rounded value from
> ifHighSpeed when there is a more precise value in ifSpeed.  Comments?
> 
> 
> On Tue, Nov 23, 2004 at 04:31:28PM +0100, Alex van den Bogaerdt wrote:
> > On Tue, Nov 23, 2004 at 07:50:57AM -0600, Larry Fahnoe wrote:
> > 
> > > I pointed out what I was observing and where I thought the problem
> > > was.  As I see it, when using SNMPv2 against a bunch of cisco routers
> > > here, some interface speeds are being reported incorrectly by mrtg,
> > > and yes, I consider this a bug.  I see that cfgmaker seeks to make a
> > > bunch of decisions about what the correct values should be, and I see
> > > that there is some attempt in the code to adjust to improper values.
> > 
> > Suppose cfgmaker would make the other decision and use what is, in your
> > case, the correct value.  Don't you agree that there's a not so slim
> > chance that others will have problems?
> > 
> > > Given the number of cisco routers out there, what is your opinion of
> > > the problem that I've reported?  Are my observations and judgment off
> > > base?
> > 
> > IMHO problems should be fixed at the root.  Patching at the wrong place
> > will introduce problems.
> > 
> > cfgmaker tries to help.  
> > 
> > ..."to try and wrestle some better information from willing
> >     livingston and cisco routers ... (may not work)"...
> > 
> > It may not work.
> > 
> > (Perhaps I'm not looking at the most recent cfgmaker.  No problem,
> > my comment still holds.)
> > 
> > Alex
> > 
> 

-- 
Larry Fahnoe, Fahnoe Technology Consulting, fahnoe at FahnoeTech.com
952/925-0744      Minneapolis, Minnesota       www.FahnoeTech.com 

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