Chapter 1 UDP Helper Configuration
When configuring UDP helper, go to these
sections for information you are interested in:
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Introduction to
UDP Helper
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Configuring UDP Helper
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Displaying and Maintaining
UDP Helper
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UDP Helper Configuration
Example
1.1 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, S5600 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.
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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.
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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
|
1.2 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.
|
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On an S5600 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.
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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.
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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.
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You can specify up to 20 destination server
addresses on a VLAN interface.
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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.
|
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
|
1.4 UDP Helper Configuration Example
I. 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.)
Figure 1-1 Network diagram for UDP
Helper configuration
# 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
