Cisco IP Helper-Address DHCP
a client machine on one subnet to a server in another subnet. This is used primarily in business
environments as way to save money by not having to buy a server for each subnet.
A helper address can also be used to forward certain other UDP network configuration messages.
As a result a UDP helper address is sometimes created to allow two server machines to communicate
across different subnets.
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network traffic from the PCs to the DHCP server. The DHCP server responds with a unicast IP address
from the given range. It reserves the IP address for the time it can calculate that it can get a response.
Example :
Configuration on Router A
RouterA# conf t
RouterA(config)# interface fastethernet0/0
RouterA(config-if)# ip address 192.168.1.1 255.255.255.0
RouterA(config-if)# ip helper-address 10.10.10.1
Some other important considerations for ip helper-address.
By default, the ip helper-address command forwards also some other broadcast protocols
in addition to the DHCP (BOOTP) protocol.
It forwards by default the following eight UDP broadcast protocols:
UDP 37 (Time protocol)
UDP 49 (TACACS)
UDP 53 (DNS)
UDP 67 (DHCP Server)
UDP 68 (DHCP Client)
UDP 69 (TFTP)
UDP 137 (NetBios)
UDP 138 (NetBios Datagram service)
If you want to add more broadcast protocols to be forwarded, or even remove some of the default
forwarded protocols, you can use the “ip forward-protocol” command under global config mode.
Example:
Remove the NetBios protocols (137,138) from being forwarded by default,
and add NTP protocol 123 to be forwarded by ip helper-address.
RouterA(config)# no ip forward-protocol udp 137
RouterA(config)# no ip forward-protocol udp 138
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Notes :
bootps : Bootstrap Protocol (BOOTP) server (67)