UDP Helper Address
Encyclopedia
A UDP Helper Address is a special router configuration used to forward broadcast
Broadcast address
A broadcast address is a logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams...

 network traffic from a client
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

 machine on one subnet
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....

 to a server in another subnet
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....

. This is used primarily in business environments as way to save money by not having to buy a server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

 for each subnet.

Usage Example

In networking every machine is given a unique IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

. Groups of machines with similar addresses are considered to be part of the same logical subnet. One method of assigning IPs is DHCP
Dynamic Host Configuration Protocol
The Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...

 in which IPs are given out by single server machine. Normally this machine is on the same subnet as its clients and can answer to their broadcasted DHCP requests. However, in some situations the server is on a different subnet. The problem with this is that most routers in their default state do not pass broadcast
Broadcast address
A broadcast address is a logical address at which all devices connected to a multiple-access communications network are enabled to receive datagrams...

 messages to machines outside the physical subnet. Such messages are essential to the normal operation of certain network services such as DHCP.

To resolve this a UDP Helper Address is established in the router configuration to forward broadcast network traffic from the PCs to the DHCP server. The DHCP server responds with a unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...

 IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

 from the given range. It reserves the IP address for the time it can calculate that it can get a response. If the client responds again with a broadcast it distributes the IP address. A helper address can also be used to forward certain other UDP
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...

 network configuration messages. As a result a UDP helper address is sometimes created to allow two server machines to communicate across different subnets.

Implementation

Cisco's
Cisco
Cisco may refer to:Companies:*Cisco Systems, a computer networking company* Certis CISCO, corporatised entity of the former Commercial and Industrial Security Corporation in Singapore...

 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 [vrf name | global] address [redundancy' vrg-name]
no ip helper-address [vrf name | global]' address [redundancy vrg-name
]

vrf name

(Optional) Enables VPN 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.
See Microsoft's
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 knowledge base for details .

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
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, NetBIOS is not a networking protocol...

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