[mrtg] Re: CableModem Interfaces
jbarbieri at metrocastcablevision.com
Wed Jun 8 17:05:27 MEST 2005
Christopher A. Congdon wrote:
>>1) IF3 and IF4 just seem to be splitting up the data that I can also
>> IF1. Is that correct or am I missing some subtelty as to whay it
>> that way?
>I'm not sure why this is. It could have something to do with the way the
>cable modems work. I know most cable modem operators have the ability to
>limit traffic coming in or going out. In reflection of the fact that the
>cable modem we have in the office (on a commercial account) can be set
>to 2Mb up/2Mb Down, yet a residential user on the same network gets 2 Mb
>Down/128Kb Up... I think maybe they decided that bandwidth shaping was
>easier to control if inbound and outbound traffic was split onto two
I'll try and help out here, I work for a cable comapny, and it still
First off, the way the system works, is there are 2 interfaces in the
modem, one for upstream, one for downstream. This is because the
downstream runs on a different frequency then the upstream interface.
This is why when you walk it, you see two different interfaces.
Interface 3: CATV Downstream: This is the downstream interface. It will
only show you one way, so if you graph it, it will be a one way graph.
Interface 4: CATV Upstream: This is the upstream interface. It will only
show you one way, so if you graph it, it will be a one way graph.
>>2) IF1 is the same data, just in reverse, of what I am seeing on the
>> eth0. Again, is that correct or am I missing something?
>Depends on your network layout. Does all of your internet traffic go
>through the server? If so, then yes, the cable's IF1 is going to be a
>mirror image of your server's ETH0, but only if the cable modem is the
>only thing attached to your server's ETH0.
Interface 1: the ethernet interface. if your monitoring this, things are
going to be backwards (aka, if your sending data out to the internet, it
will show as inbound traffic on this interface, and not outbound). This
is because of the point of origin and where this interface is located.
Outbound from the server is inbound to the ethernet, then outbound to
the internet through the CATV upstream Interface. Inbound from the
internet is inbound on the CATV downstream interface, outbound on the
Ethernet (since its pushing the data to the server) and Inbound on the
>>3) If the above are correct, if I want to graph my internet traffic,
>> really need to grapg is my server's eth0. I don't see any
>> graphing the interfaces on the cablemode. Is this correct too?
>If all of your traffic goes through the server, then there is a
>possibility you don't need to graph the cable modem. Question however...
>Does your server have more than one NIC? If ETH0 is connected ONLY to
>your cable modem and serves no other traffic on your network, then your
>server's ETH0 graphs should match the combination of your cable modem's
What he said, if there are other devices on your network that use the
modem without going through the server, then you would have to graph the
cable modem to see the true bandwidth usage.
If I remember correctly, I believe you can put a negative sign in front
of the interface in the MRTG config file for the ethernet interface, and
this should reverse the graphs, so that outbound would be outbound to
the internet, and inbound would be inbound from the internet (not
outbound on the graph is actually inbound from the internet)
You should also be able to set up your own graph if you have the correct
OIDs for octects passed.
Lemme see if i can find those.
For an SB4100 (might be the same for others, not to sure. You can SNMP
walk the device and look for IF-MIB::ifInOctets.x and
IF-MIB::ifOutOctets.y (where x is the downstream interface, and y is the
upstream interface (exmaple, on this 4100, x=3 and y=4).
For the downstream interface: .18.104.22.168.22.214.171.124.1.10.3 (this would
be inbound on the graphs)
For the upstream interface: .126.96.36.199.188.8.131.52.1.16.4 (this
would be outbound on the graphs)
So the config could be something like:
.184.108.40.206.220.127.116.11.1.10.3&.18.104.22.168.22.214.171.124.1.16.4 at cable_modem_ip:
Title[cable_modem]: Traffic Analysis for Cable Modem
PageTop[cable_modem]: <H1>Traffic Analysis for Cable Modem</H1>
<TR><TD>System:</TD> <TD>SB4100 in sysLocation</TD></TR>
<TR><TD>ifType:</TD> <TD>ethernetCsmacd (6)</TD></TR>
<TR><TD>Max Speed:</TD> <TD>1250.0 kBytes/s</TD></TR>
<TR><TD>Ip:</TD> <TD>192.168.100.1 ()</TD></TR>
>No problem, glad to be of assistance.
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Hope this helps,
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