SNMP polling discovering interfaces which don't exist

Unanswered Question
Feb 22nd, 2007

I noticed on our network monitoring server that it's detecting an Ethernet0/0 interface on various routers on our network. The problem is these routers don't have an ethernet interface, they only have fast ethernet. I have seen it so far on a 1760 and a 1721, running different IOS. To eliminate the monitoring software, I used a MIB browser to walk the interfaces on the router, and sure enough, it comes up with this ethernet 0/0. But if I look at the config on the router, there is no ethernet interface. Also, the monitoring station also shows the interface in an up, up status. How can this be? Is there a known issue with this happening that I have not heard about?

I have this problem too.
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Joe Clarke Fri, 02/23/2007 - 07:56

What object(s) is reporting Ethernet0/0 as existing? Try doing an snmpget on sysDescr.0 and sysName.0 followed by a walk of ifDescr. We have seen issues where certain interfaces stick in the ifTable until a reboot, but I don't believe we/ve had any issues with a phantom Ethernet0/0 interface showing up.

mjsully Fri, 02/23/2007 - 10:31

The ifDesc shows "Ethernet0/0", and the MAC address field shows up as 0000.0000.0000. The admin status shows as up, up. The sysDesc0 shows that its a 1700 series router. We have several that are doing this.

Joe Clarke Fri, 02/23/2007 - 10:39

I still don't see any bugs on this, but it does sound like a real problem given the strange MAC address. What version of IOS is this particular 1700 running? Can you reload one of these 1700s to see if the phantom interface goes away?

mjsully Fri, 02/23/2007 - 12:19

I cannot reload them, as they are production routers. I don't think it will matter, as I've counted at least two dozen that are showing this phantom interface. So far they are running 12.3(11)T4 for code. However, I have seen a few routers running this code that do not display the phantom interface.

Joe Clarke Sun, 02/25/2007 - 11:34

I still cannot find a bug that explains this behavior. I suggest you open a TAC service request and include a complete problem description along with a show tech from one of these 1700s and an snmpwalk of the ifTable. This will most likely require a recreate.

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