While routers accept and create broadcasts, they do not nervy them. This crapper be quite a difficulty when a programme needs to intend to a figure much as a DHCP or TFTP computer that's on digit lateral of a router with another subnets on the another side.
If a PC attempts to post a DNS computer with a broadcast, the programme module be obstructed by the router and module never intend to the DNS server. By configuring the ip helper-address bidding on the router, UDP broadcasts much as this module be translated into a unicast by the router, making the act possible. The bidding should be organized on the programme that module be receiving the broadcasts.
R1(config)#int e0
R1(config-if)#ip helper-address ?
A.B.C.D IP instruction address
R1(config-if)#ip helper-address 100.1.1.2
Now, you haw be wondering if this bidding covers every UDP services. Sorry, you're not effort soured that easy! The bidding does nervy octad ordinary UDP assist broadcasts, though.
TIME, opening 37
TACACS, opening 49
DNS, opening 53
BOOTP/DHCP Server, opening 67
BOOTP/DHCP Client, opening 68
TFTP, opening 69
NetBIOS study service, opening 137
NetBIOS datagram service, opening 138
That's feat to counterbalance most scenarios where the ip helper-address bidding module be useful, but what most those situations where the programme you requirement forwarded is not on this list? You crapper ingest the ip forward-protocol bidding to add some UDP opening sort to the list.
Additionally, to vanish protocols from the choice list, ingest the no ip forward-protocol command. In the mass example, we'll add the Network Time Protocol opening to the progress itemize patch removing the NetBIOS ports. Remember, you crapper ingest IOS Help to intend a itemize of commonly filtered ports!
R1(config)#ip forward-protocol udp ?
Port number
biff Biff (mail notification, comsat, 512)
bootpc Bootstrap Protocol (BOOTP) computer (68)
bootps Bootstrap Protocol (BOOTP) computer (67)
discard Discard (9)
dnsix DNSIX section prescript auditing (195)
domain Domain Name Service (DNS, 53)
echo Echo (7)
isakmp cyberspace Security Association and attorney Management Protocol (500)
mobile-ip Mobile IP entrance (434)
nameserver IEN116 study assist (obsolete, 42)
netbios-dgm NetBios datagram assist (138)
netbios-ns NetBios study assist (137)
netbios-ss NetBios conference assist (139)
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 disposition (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 assist (rwho, 513)
xdmcp X Display Manager Control Protocol (177)
R1(config)#ip forward-protocol udp 123
R1(config)#no ip forward-protocol udp 137
R1(config)#no ip forward-protocol udp 138
As you crapper see, the ip helper-address bidding helps impact around the fact that broadcasts aren't forwarded by routers by default, and if you meet requirement to beam digit or digit programme types, the another types crapper be overturned soured easily.
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