Core Issue
When configuring the ip helper-address command, the following broadcast packets will be forwarded by the router by default:
- 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
Resolution
If you do not want all the defaults to be forwarded, issue the no ip forward-protocol command to disable the port from being forwarded by the router, as shown in this example:
router#(config-if)ip helper-address x.x.x.x
router#(config)no ip forward-protocol udp tftp
With these commands, all default User Datagram Protocol (UDP) broadcasts except TFTP broadcasts are forwarded by the router.
For more information refer to ip helper-address .
Note: Configure the no ip forward-protocol command separately for each port to prevent the port from forwarding the broadcast packets by the router.
