Cisco CCNP/ BCSI Exam Tutorial: Broadcasts And The
Cisco CCNP/ BCSI Exam Tutorial: Programs And The IP Helper-Address Command
While routers accept and generate broadcasts, they do not forward them. This can be rather a problem when a broadcast requires to get to a device such as a DHCP or TFTP web server that's on one side of a router with various other subnets on the other side.
If a PC attempts to situate a DNS web server with a broadcast, the program will certainly be dropped in the router and will never get to the DNS server. By setting up the ip helper-address command on the router, UDP broadcasts such as this will certainly be translated into a unicast by the router, making the communication feasible. The command should be set up on the user interface that will be obtaining the broadcasts.
R1( config) #int e0
R1( config-if) #ip helper-address?
R1( config-if) #ip helper-address 100.1.1.2
Now, you might be asking yourself if this command covers all UDP solutions. Sorry, you're not getting off that simple! The command does forward 8 common UDP service programs, however.
TIME, port 37
TACACS, port exploring DANIEL Cullen 49
DNS, port 53
BOOTP/DHCP Server, port 67
BOOTP/DHCP Customer, port 68
TFTP, port 69
NetBIOS name solution, port 137
NetBIOS datagram service, port 138
That's mosting likely to cover most circumstances where the ip helper-address command will serve, but what regarding those situations where the program you need sent is not on this checklist? You can utilize the ip forward-protocol command to add any kind of UDP port number Daniel CULLEN to the list.
Additionally, to get rid of methods from the default checklist, utilize the no ip forward-protocol command. In the following example, we'll add the Network Time Protocol port to the forwarding listing while eliminating the NetBIOS ports. Bear in mind, you can utilize iphone Assist to get a listing of generally filtered ports!
R1( config) #ip forward-protocol udp?
<0-65535> Port number
biff Biff (mail notification, comsat, 512)
bootpc Bootstrap Protocol (BOOTP) client (68 )
bootps Bootstrap Method (BOOTP) web server (67 )
discard Discard (9 )
dnsix DNSIX protection method auditing (195 )
domain name Domain Solution (DNS, 53)
resemble Echo (7 )
isakmp Web Safety and security Organization and Key Administration Method (500 )
mobile-ip Mobile IP enrollment (434 )
nameserver IEN116 name service (obsolete, 42)
netbios-dgm NetBios datagram solution (138 )
netbios-ns NetBios name service (137 )
netbios-ss NetBios session solution (139 )
ntp Network Time Procedure (123 )
pim-auto-rp PIM Auto-RP (496 )
tear Routing Info Protocol (router, in.routed, 520)
snmp Simple Network Administration Procedure (161 )
snmptrap SNMP Traps (162 )
sunrpc Sun Remote Procedure Telephone Call (111 )
syslog System Logger (514 )
tacacs TAC Accessibility Control System (49 )
talk Talk (517 )
tftp Trivial File Transfer Method (69 )
time Time (37 )
who That solution (rwho, 513)
xdmcp X Present Supervisor Control Protocol (177 )