Switch problem

Hey guys.

So, weird thing. I found another fault in this beloved network... *sigh*... To make the complicated matter very short and understandable:
I connect to a cable, get a 169.254 address (= communicating with switch, as far as I know), and no-one knows where it is. Who cares about documentation anyways, right? xD

Is there any possible way to determine the IP of the switch I'm connected to, without finding the actual switch? I haven't heard of anyone actually doing this, but it would make my life a lot easier right now, if I could do that... because this is most probably just one of many unknown switches around here.

Any help is very, very much appreciated!

Thnx, guys
Mr. Moseng
 

JasonPDK

Member
Your computer is not communicating with any DHCP server. I would guess that the switch has probably simply lost it's physical connection to the rest of the network and you need try to find it. Have you tried giving yourself a static IP on the same subnet as the rest of the network and then using nmap to scan that subnet? I'm presuming this is a managed switch?
 

beers

Moderator
Staff member
As above, 169.254/16 is an APIPA address the PC assigns itself when it doesn't receive a DHCP reply.

You should be able to see your PCs MAC on the switch if it's managed, then you can associate it with the correct VLAN.

If it's unmanaged, something else is wrong :p
 

voyagerfan99

Master of Turning Things Off and Back On Again
Staff member
Yeah that's not a real IP Address as the two above have said, so you need to assign yourself a static IP address in this case. But then you also need to know what addresses are free and what the address scheme is too.
 

Praetor

Administrator
Staff member
Oh man, I used to have an isolated lan setup where all the IPs were 169.254. Manually assigned that way ofc but that would confuse the hell out of people :p

Of course, if you set a wide enough subnet mask, it's a nonissue so long as there's physically a connection trail between two (or more) endpoints.

To get some more insight on the problem though -- can you tell us the make/model of the "switch"?
 

beers

Moderator
Staff member
Oh man, I used to have an isolated lan setup where all the IPs were 169.254. Manually assigned that way ofc but that would confuse the hell out of people :p

Haha nice. I was doing a public/private conversion on a contract once for a large theatre chain client (who at the same time was doing a windows 7 update/refresh project). Their public space was 169.224 which confused the hell out of the helpdesk staff between that and APIPA, we had a lot of issues as the public segment was not DHCP enabled whereas the private one was. :eek:
 
Well, the main problem here, as described, is that the switch is hidden in some wall somewhere... nobody knows xD But it's somewhere between my connection point and the patch rooms :p

The reason I believe it is a switch is that the 169.254 address means that I have power to my networking card. If you put a cable in there with nothing in the other end, you won't get anything at all... so, I'm connected to something, and it's not the switch back with the panels.

Ideally I would get my normal VLAN address here, but I guess that would be too easy xD lol

So just to clarify:
I know 169.254.40 is the default internal range of the networking card.
I'm connected to something, and I need to know what it is...
I have no clue what so ever if the "thing" I'm connected to is manageable
I'm looking for ways to figure out how to find the "thing"

I hope that helps you know the situation a bit better :p Tried following cables, though... did not work. They run through the cement floor, and dissapear xD
 

beers

Moderator
Staff member
Do you see your PC's MAC in the upstream-from-the-one-in-the-wall switch's MAC address table? (the one at the IDF/MDF)

You could probably open wireshark on the host and see if you capture any ARP traffic or similar that might give you an idea of what runs through that switch.

On the IDF/MDF switch do you have all of the other ports documented? You could probably find out where it ties into by ruling out the other interfaces.

Edit:
If you use a lot of Cisco gear it might even be in the CDP neighbor table.
 
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voyagerfan99

Master of Turning Things Off and Back On Again
Staff member
And if that doesn't work, just go out and buy a Fluke. They're cheap enough (And pretty handy sometimes too!)
 

beers

Moderator
Staff member
Those are always handy to have, but hard to find devices within walls and similar since it doesn't tone back into the patch panel out of the backside interface.

I worked at a theatre chain that had that problem a lot, during a remodel they would just drywall over whatever existing unmanaged switch was there lol.
 
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