What is it and how to use it?
When the DHCP Server is placed on a different subnet from the all its clients. It is important that the router at the disparate subnets be configured with an “IP helper-address” specifying the Server as its pass-through broadcast node. Here is a sample command to enable such configuration:
CORE01 (config-if)#ip helper-address 192.168.0.1
Be advised that the “IP-Helper” feature opens up a suite of ports:
|
Protocol |
UDP Port |
|
Timeserver (not same as NTP port UDP 123) |
37 |
|
TACACS |
49 |
|
DNS |
53 |
|
BOOTP/DHCP Server |
67 |
|
BOOTP/DHCP Client |
68 |
|
TFTP |
69 |
|
NetBIOS name service |
137 |
|
NetBIOS datagram service |
138 |
|
IEN-116 name service |
42 |
To enhance the network security posture, it may be necessary to close any unused port with commands such as this (Cisco):
CORE01 (config-if)#no ip forward-protocol udp 49
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