PCs Connected to a Switch Cannot Obtain IP Addresses Through DHCP or Obtain IP Addresses After a Long Delay

Publication Date:  2015-06-08 Views:  651 Downloads:  0
Issue Description
The S5700 functions as a DHCP server to connect to an access switch. The access switch connects to PCs. PCs cannot obtain IP addresses.

Figure 1-1 Networking diagram


Handling Process
1. Check the DHCP server configuration according to product manual. If the configuration is correct, check the network cables.

2. Connect PCs to the S5700 directly to check whether the PCs can obtain IP addresses.

− If the PCs obtain IP addresses, the access switch may not transparently transmit VLAN packets. Check whether the related interfaces can transparently transmit VLAN packets.
− If the PCs cannot obtain IP addresses, go to step 3.

3. Check whether there is available IP address in the address pool.

<Switch> display ip pool interface vlanif1
  Pool-name      : Vlanif1
  Pool-No        : 6
  Lease          : 1 Days 0 Hours 0 Minutes
  Domain-name    : -
  DNS-server0    : -              
  NBNS-server0   : -              
  Netbios-type   : -              
  Position       : Interface       Status           : Unlocked
  Gateway-0      : 10.10.90.253   
  Mask           : 255.255.255.0
  VPN instance   : --
-----------------------------------------------------------------------------
         Start           End     Total  Used  Idle(Expired)  Conflict  Disable
-----------------------------------------------------------------------------
      10.10.90.1    10.10.90.254   253     1        252(0)         0        0
-----------------------------------------------------------------------------

If no available IP address exists:
   a. If all IP addresses have been allocated, run the reset ip pool interface Vlanif1 +expired/ Conflict command in the user view to reclaim conflicting IP addresses or expired IP addresses.
   b. If the address pool does not contain conflicting or expired IP address, reduce the IP address lease period. This is applicable to the scenarios where users often change.
If the address pool contains conflicting available IP addresses, enable DHCP debugging or capture DHCP packets. Check whether the DHCP server has received the DHCP DISCOVER packets from clients, and whether the DHCP server has replied with DHCP OFFER packets.

Note:
PCs can obtain IP addresses only after several DHCP packets have been exchanged between the switch and PCs.

4. Enabling STP may cause a delay to obtain IP addresses. Disable STP or set the interfaces connected to PCs as STP edge ports.
Root Cause
STP is enabled on the switch.
Solution
Follow the preceding steps.
Suggestions
On live networks, it is a frequent problem that PCs cannot obtain IP addresses. You are advised to check basic configurations and network cables first, and then check whether IP address lease expires or IP addresses are exhausted. If the problem is not caused by the preceding reasons, the client may work abnormally. Replace the client.

END