[mrtg] Auto-Discovering IPs of Switch Ports

McDonald, Dan Dan.McDonald at austinenergy.com
Tue Oct 20 23:31:44 CEST 2009

On Wed, 2009-10-21 at 10:22 +1300, Steve Shipway wrote:
> MRTG just queries what its been told to graph, so it wont get anything
> like this.
> Cfgmaker (the utility that generates the .cfg files for MRTG to use)
> will not do this by default, but with a bit of work

More than a bit.  the BRIDGE-MIB was implemented sub-optimally, so it's
a huge endeavor to make it go right.  The problem is that you have to
determine what vlan is associated with the port, then the BRIDGE-MIB
instance associated with the vlan, then you can start to hunt down mac
addresses.  And every instance of the BRIDGE-MIB has a separate snmp
community string or snmpv3 context-id...

>  it might be possible to make an interface template for your devices
> that pulls out the last MAC address seen on a port.  Mapping this to
> an IP would be a bit more work but not impossible,

Actually, that's easier, since ipNetToMedia is a mib that is laid out in
a sane fashion.

>  though you’d probably need something running constantly creating a
> mapping database?
> The way we get around this here is that our Network people set the
> port description to be the hostname or IP of the device that is
> attached when they set up the port.  This way I can use the standard
> settings of cfgmaker to pull out the port description and use that for
> linking to the required host.

Yes, much easier...

Daniel J McDonald, CCIE # 2495, CISSP # 78281, CNX
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