H3C S3600 Operation Manual-Release 1602(V1.02)

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32-UDP Helper Operation
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UDP Helper Configuration

When configuring UDP helper, go to these sections for information you are interested in:

l          Introduction to UDP Helper

l          Configuring UDP Helper

l          Displaying and Maintaining UDP Helper

l          UDP Helper Configuration Example

Introduction to UDP Helper

Sometimes, a host needs to forward broadcasts to obtain network configuration information or request the names of other devices on the network. However, if the server or the device to be requested is located in another broadcast domain, the host cannot obtain such information through broadcast.

To solve this problem, S3600 series Ethernet switches provide the UDP Helper function to relay specified UDP packets. In other words, UDP Helper functions as a relay agent that converts UDP broadcast packets into unicast packets and forwards them to a specified destination server.

With UDP Helper enabled, the device decides whether to forward a received UDP broadcast packet according to the UDP destination port number of the packet.

l          If the destination port number of the packet matches the one pre-configured on the device, the device modifies the destination IP address in the IP header and then sends the packet to the specified destination server.

l          Otherwise, the device sends the packet to the upper layer protocol for processing.

 

Relay forwarding of BOOTP/DHCP broadcast packets is implemented by the DHCP relay function using UDP ports 67 and 68, so these two ports cannot be configured as UDP Helper relay ports.

 

By default, with UDP Helper enabled, the device forwards broadcast packets with the six UDP destination port numbers listed in Table 1-1.

Table 1-1 List of default UDP ports

Protocol

UDP port number

DNS (Domain Name System)

53

NetBIOS-DS (NetBIOS Datagram Service)

138

NetBIOS-NS (NetBIOS Name Service)

137

TACACS (Terminal Access Controller Access Control System)

49

TFTP (Trivial File Transfer Protocol)

69

Time Service

37

 

Configuring UDP Helper

Follow these steps to configure UDP Helper:

To do…

Use the command…

Remarks

Enter system view

system-view

Enable UDP Helper

udp-helper enable

Required

Disabled by default.

Specify a UDP port number

udp-helper port { port-number | dns | netbios-ds | netbios-ns | tacacs | tftp | time }

Optional

By default, the device enabled with UDP Helper forwards the broadcast packets containing any of the six port numbers 53, 138, 137, 49, 69 and 37.

Enter VLAN interface view

interface Vlan-interface vlan-id

Specify the destination server to which the UDP packets are to be forwarded

udp-helper server ip-address

Required

No destination server is specified by default.

 

l          On an S3600 Series Ethernet Switch, the reception of directed broadcast packets to a directly connected network is disabled by default. As a result, UDP Helper is available only when the ip forward-broadcast command is configured in system view. For details about the ip forward-broadcast command, refer to the IP Address and Performance part of this manual.

l          You need to enable UDP Helper before specifying any UDP port to match UDP broadcasts; otherwise, the configuration fails. When the UDP helper function is disabled, all configured UDP ports are disabled, including the default ports.

l          The dns, netbios-ds, netbios-ns, tacacs, tftp, and time keywords correspond to the six default ports. You can configure the default ports by specifying port numbers or the corresponding parameters. For example, udp-helper port 53 and udp-helper port dns specify the same port.

l          You can specify up to 20 destination server addresses on a VLAN interface.

l          If UDP Helper is enabled after a destination server is configured for a VLAN interface, the broadcasts from interfaces belonging to the VLAN and having a matching UDP port will be unicast to the destination server.

 

Displaying and Maintaining UDP Helper

To do…

Use the command…

Remarks

Display the UDP broadcast relay forwarding information of a specified VLAN interface on the switch

display udp-helper server [ interface vlan-interface vlan-id ]

Available in any view

Clear statistics about packets forwarded by UDP Helper

reset udp-helper packet

Available in user view

 

UDP Helper Configuration Example

Cross-Network Computer Search Through UDP Helper

Network requirements

PC A resides on network segment 192.168.1.0/24 and PC B on 192.168.10.0/24; they are connected through Switch A and are routable to each other. It is required to configure UDP Helper on the switch, so that PC A can find PC B through computer search. (Broadcasts with UDP port 137 are used for searching.)

Network diagram

Figure 1-1 Network diagram for UDP Helper configuration

 

Configuration procedure

# Enable Switch A to receive directed broadcasts to a directly connected network.

<Sysname> system-view

[Sysname] ip forward-broadcast

# Enable UDP Helper on Switch A.

[Sysname] udp-helper enable

# Configure the switch to forward broadcasts containing the destination UDP port number 137. (By default, the device enabled with UDP Helper forwards the broadcasts containing the destination UDP port number 137.)

[Sysname] udp-helper port 137

# Specify the destination server IP address on Vlan-interface 1.

[Sysname] interface Vlan-interface 1

[Sysname-Vlan-interface1] udp-helper server 192.168.10.2

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