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Old 11-20-2014, 06:52 AM   #1
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A cabling issue? Single Wire and two dhcp servers

A single CAT6 wire runs from front-office to back office.
In the initial setup, this wire connected the back office systems to a local network managed by a Juniper Switch. Now I want a Linux cluster in the back office to have a public IP also.
I took a 4 port switch and connected one cable from the juniper switch and another cable from the router that managed the public IP's.
Then I connected the cable to the back office to this 4 port switch.

The head node on the cluster has 4 port NIC.
eth0: : Statically assigned via the Juniper switch
eth1: gateway to the cluster and forwarded to eth0 for outgoing internet connections.
eth2: 128.224.xx.xx I assigned the available public IP
Since both eth0 and eth2 are connected to the internet, no issues there.
When I try to connect to the head node using the the public or the local IP, only one connection goes through.
a network restart ALWAYS flips the access IP ( really interesting!)
When I had two cables from two separate switches ( the public and the local)connected to eth0 and eth2, I could connect to the head node using both the IPs: and 128.224.xx.xx

I am curious to know what is the issue here.
Is,a single cable carrying two dhcp/static requests to two different servers via two managed switches, not possible in theory?
I have a limitation of running another wire from the front to the back office for the time being ( It is about 300 meters away)

This looks like a Network 101 question but I failed to find a good answer. Any tips to explore this issue is appreciated.
Old 11-20-2014, 10:03 AM   #2
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Your allocation of resources is always going to be a problem if you have your hardware split like that.
Once things know which network they are on, you can do what you want with virtual lans which isolate sets of devices using the same piece of cable. But with dhcp, boxes logging on _don't_ know what network they are on.

There is a lovely line for Fagin in the musical 'Oliver':
"I think I'd better think it out again!"

I suggest you try that. Tell your boss " To do that, I will need a & b, and c or d" (whatever you need). If he won't provide it, he's not serious about getting it done.


cable, dhcp server, networking, nic

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