Problem VLAN DHCP Server

25 pts.
Dell PowerConnect
DHCP Relay Agent
Windows Server 2008
Hi my friends! Here is my problem: I have a Switch L3 Dell Power Connect 6248 which has VLANs configured on it. Each VLAN is a LAB. Port 2: VLAN2. 172.31.2.x Port3: VLAN3. 172.31.3.x I have a DHCP Server Windows Server 2008 connected directly on port 20 of this switch and this server has the IP It is also the File Server from students put their files. This server has different scopes from each subnet (each VLAN). On the Switch is configured a DHCP Relay pointing to this server ( The PC's from each lab use DHCP. Everything always working well until the day that they asked me to "close" the vlans on the switch. So i made the following configuration of the ACLs (I made this by web console, not using command line). I made it in: Switching / Network Security / Access Control Lists / IP Access Control Lists / Rule Configuration/ Permit (with this, PCs in the same lab can watch themselves) Permit (with this, students can see their documents and PCs can get IP from DHCP) Permit (its the ISA server IP. with this, pcs can access Internet). On practice, the result of this was: PCs can see other PCs on the same VLAN and don't see PCs on other labs (its OK); PCs can access internet (its OK); students can see their docs (its OK). The problem is that it works for a few hours (maybe max 2) and after this, the PCs lost connectivity with the DCHP server (they use DHCP, no static) and they get the generic IP 169.x.x.x and everything stopped working. :( : ( When I configure Static IP on these PC's everything works fine! :) Anyone can help me to find a solution to this case? I need that everything works using DHCP an not static IP. Thanks for everything!

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Thanks. We'll let you know when a new response is added.

It sounds like your lease times are very short. If these are labs and the computers do not move around much, then lease times can be longer than if the devices are mobile and move around much. For a static lab you could use lease times of 7 days without issue. This means that computers will attempt to renew their leases 1/2 way through at about 3.5 days. If they cannot renew their leases, they will wait 50% of the remaining time and then try again – over and over until the lease time expires and then depending on the client will either keep their current address or attempt to use the locally assigned address (169.254.x.x).

I would recommend using a tool like Wireshark and running some DHCP testing using a client with ipconfig /release and ipconfig /renew. Watch the traffic at both the server (filtering by client MAC address) and on the client to see what traffic is traversing during these transactions.

Let us know if you need additional assistance.

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  • Mdurra
    I have deleted all the wrong ips's references on DNS from pcs of this lab and i have made i reservation of all the pcs. i also changed the lease time to "unlimited". so in each pc, i take off static ip, put dhcp and made ipconfig /release; ipconfig /renew. with this, all the pcs take the ip from the reservartion (active). As soon as i verify the result, i post here. thank's a lot!
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