CCNP - Vlan question

Author

Vlan question

gindir

2001-03-27, 12:46 pm

Systems on one vlan can not get to DHCP Server, othersystems on other vlans on same switch works fine.

Please help

2001-03-27, 1:14 pm

Is this DHCP server located on the same VLAN or across VLAN's. If it is located across VLAN's (on a different VLAN), I would look to ensure that there is a helper address on the router or MSFC's VLAN interface that these devices use as their default gateway to forward these DHCP broadcasts.

2001-03-27, 3:29 pm

quote:
Originally posted by gindir
Systems on one vlan can not get to DHCP Server, othersystems on other vlans on same switch works fine.

Sometimes lack of DHCP configuration is related to spanning-tree. If you connect a workstation directly to a port on the switch, this port may not enter the forwarding state untill after the workstation has timed out its attempt at obtaining configuration information from the DHCP server. This is not a problem if the switchport is connected to a hub or another switch. What happens is this:
The switchport will remain in the blocked state untill the workstation is switched on and activates its NIC. Shortly after activating the NIC the workstation will broadcast DHCP discover packets. With standard DHCP configuration the switchport will take 50 seconds to enter the forwarding state. During this 50 seconds the workstation will give up getting in touch with the DHCP server.
To solve the problem do one of the following:
- Disable spanning-tree (know what you are doing or your network will die immediately)
- Set switchports that are directly connected to workstations to PORTFAST (see documentation for your switch)

Note that the problem described above is not related to VLAN. Since your problems seems to be related to one specific VLAN, make sure you have an operational DHCP server on that VLAN.

Terje

2001-03-27, 4:57 pm

Thanks a lot for your help

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