A UDP Helper Address is a special router configuration used to forward

broadcast Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum ( radio waves), in a one-to-many model. Broadcasting began ...

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 valuable ...

machine on one

subnet A subnetwork or subnet is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identical ...

to a

server Server may refer to: Computing *Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients Role * Waiting staff, those who work at a restaurant or a bar attending customers and su ...

in another

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. IP h ...

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. Ear ...

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

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 part ...

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, NetB ...

to help determine network configuration the added broadcasts can confuse the system.

References

{{DEFAULTSORT:Udp Helper Address Internet protocols Internet Standards

Usage example

Implementation

Special consideration

See also

References