To let a router forward broadcast packet the command ip helper-address can be used. The broadcasts will be forwarded to the unicast address which is specified with the ip helper command.
ip helper-address {ip address}
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
This can be changed by:
[no] ip forward-protocol udp {protocol or port #}
Router(config)#ip forward-protocol udp ?
<0-65535> Port number
biff Biff (mail notification, comsat, 512)
bootpc Bootstrap Protocol (BOOTP) client (68)
bootps Bootstrap Protocol (BOOTP) server (67)
discard Discard (9)
dnsix DNSIX security protocol auditing (195)
domain Domain Name Service (DNS, 53)
echo Echo (7)
isakmp Internet Security Association and Key Management Protocol
(500)
mobile-ip Mobile IP registration (434)
nameserver IEN116 name service (obsolete, 42)
netbios-dgm NetBios datagram service (138)
netbios-ns NetBios name service (137)
netbios-ss NetBios session service (139)
non500-isakmp Internet Security Association and Key Management Protocol
(4500)
ntp Network Time Protocol (123)
pim-auto-rp PIM Auto-RP (496)
rip Routing Information Protocol (router, in.routed, 520)
snmp Simple Network Management Protocol (161)
snmptrap SNMP Traps (162)
sunrpc Sun Remote Procedure Call (111)
syslog System Logger (514)
tacacs TAC Access Control System (49)
talk Talk (517)
tftp Trivial File Transfer Protocol (69)
time Time (37)
who Who service (rwho, 513)
xdmcp X Display Manager Control Protocol (177)
Example configuration:
A remote site is using 10.11.4.0/24 on the LAN interface. There is a DHCP at the central location with the ip address 10.100.22.11.
The following configuration is needed to forward the DHCP broadcast from the remote LAN to the DHCP server at the central location.
Interface FastEhternet 0/0
ip address 10.11.4.254 255.255.255.0
ip helper-address 10.100.22.11
!
Interface Serial 0/0
Ip address 10.200.0.2 255.255.255.252
!
Ip route 0.0.0.0 0.0.0.0 10.200.0.1
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