A UDP Helper Address is a special
router configuration used to forward
broadcast
Broadcasting is the distribution (business), distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications medium, but typically one using the electromagnetic spectrum (radio ...
network traffic from a
client
Client(s) or The Client may refer to:
* Client (business)
* Client (computing), hardware or software that accesses a remote service on another computer
* Customer or client, a recipient of goods or services in return for monetary or other valuabl ...
machine on one
subnet to a
server in another
subnet.
Usage example
The
Internet Protocol
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
...
requires every
network interface controller
A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network.
E ...
to be assigned at least one unique
IP address
An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
. Groups of machines with similar addresses are considered to be part of the same logical subnet. One method of assigning IP addresses is
DHCP
The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on Internet Protocol (IP) networks for automatically assigning IP addresses and other communication parameters to devices connected to the network using a cli ...
in which addresses typically are issued by a DHCP server running on one or more hosts. If one of these machines is on the same subnet as its clients, the DHCP server can respond to their
broadcast
Broadcasting is the distribution (business), distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications medium, but typically one using the electromagnetic spectrum (radio ...
DHCP requests and issue an address. But the DHCP servers may be hosted on a different subnet and, by default, most routers do not pass broadcast messages to nodes outside their own subnet.
To resolve this, a UDP helper address is established in the router configuration to forward broadcast network traffic outside the local subnet. If a DHCP client outside the DHCP server's subnet broadcasts an address request, it is the helper that forwards the message to the DHCP server. The server then chooses an address and sends the client a
unicast
Unicast is data transmission from a single sender (red) to a single receiver (green). Other devices on the network (yellow) do not participate in the communication.
In computer networking, unicast is a one-to-one transmission from one point in ...
message, using the helper to send the message back to the client's subnet. The address is reserved for a limited time while the DHCP server waits for a response. If the client responds with another broadcast message, the DHCP server distributes the address. Helper addresses also can be used to forward other
UDP traffic (for example,
BOOTP
The Bootstrap Protocol (BOOTP) is a computer networking protocol used in
Internet Protocol networks to automatically assign an IP address to network devices from a configuration server. The BOOTP was originally defined in RFC 951.
While some p ...
).
Implementation
Cisco's first implementation of this protocol was introduced in version 10 of their router software. It is implemented through the use of the router configuration commands
ip helper-address and
ip forward-protocol.
ip helper-address
To enable the forwarding of User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address command in interface configuration mode. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.
* Syntax Description:
ip helper-address
globaladdress
edundancy vrg-name no ip helper-address
globaladdress
edundancy vrg-name
;vrf ''name''
:(Optional) Enables
VPN
A virtual private network (VPN) extends a private network across a public network and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. The be ...
routing and forwarding (VRF) instance and VRF name.
;global
:(Optional) Configures a global routing table.
;''address''
:Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address per interface.
;redundancy ''vrg-name''
:(Optional) Defines the VRG group name.
Special consideration
The use of UDP helper addresses can cause issues with some Windows-based network configurations..
According to Microsoft these issues stem from the fact that ports 137,138 are forwarded by default on Cisco routers. Since these ports are used by
NetBIOS
NetBIOS () is an acronym for Network Basic Input/Output System. It provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network. As strictly an API, N ...
to help determine network configuration the added broadcasts can confuse the system.
See also
*
Packet forwarding
Packet forwarding is the relaying of packets from one network segment to another by nodes in a computer network. The network layer in the OSI model is responsible for packet forwarding.
Models
The simplest forwarding modelunicastinginvolve ...
*
Port forwarding
References
{{DEFAULTSORT:Udp Helper Address
Internet protocols
Internet Standards