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Router is not forwarding DHCP requests - hosts are unable to obtain an IP address from a DHCP server

Core Issue

By default, a router will not forward any broadcasts. The problem is most likely that User Datagram Protocol (UDP) ports are not being permitted through the router.

Resolution

In order to forward UDP broadcasts, the ip helper-address command must be issued on the router interface that will be receiving the broadcasts, as shown in the following example:

router(config)#interface ethernet 0/0
router(config-if)#ip helper-address 10.1.23.5
router(config-if)#end
router#

When the ip helper-address command is configured, by default the following UDP broadcasts are forwarded:

  • TFTP - port 69
  • Domain Name System (DNS) - port 53
  • Time service - port 37
  • NetBIOS Name Server - port 137
  • NetBIOS Datagram Server - port 138
  • Bootstrap Protocol (BOOTP) - port 67
  • TACACS - port 49


Version history
Revision #:
1 of 1
Last update:
‎06-22-2009 03:36 PM
Updated by:
 
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Cisco Employee

Which one takes precedence when both are configured.

  • A dhcp pool for a particular subnet/vlan/interface.
  • An ip helper address pointing towards a dhcp server that is not the router.