3Com Router User Manual

							DHCP Server Configuration337
■Configure the range of a DHCP address pool
■Configure the IP addresses that do not participate in auto-allocation in the 
DHCP address pool
■Configure the lease valid period of the IP addresses in a DHCP address pool
■Configure the IP address of the outgoing gateway router at the DHCP client
■Configure the domain name of the DHCP client
■Configure the IP address of the DNS server used by the DHCP client
■Configure the IP address of the NetBIOS server used by the DHCP client
■Set the type of NetBIOS node for DHCP client
■Configure the maximum number of ping packets sent by the DHCP server and 
the maximum time waiting for response
■Configure the user-defined options
1Enable/disable the DHCP Service
Before performing the DHCP configurations, DHCP service should be enabled first. 
Only after the DHCP service is enabled, other related DHCP configurations can 
take effect.
Perform the following configurations in system view.
Ta b l e 366   Enable/disable the DHCP service
By default, the DHCP service is disabled.
2Create a DHCP address pool
To allocate the IP addresses, the user needs to create an address pool on the DHCP 
server. When the client requests an IP address, the DHCP server will choose an 
appropriate address pool according to a certain algorithm, it will select an idle IP 
address from this address pool, and transmit it together with other parameters 
(e.g. DNS server address, the lease period of the address and so on) to the client. 
Each DHCP server can be configured with 1 and more address pools. Up to 50 
address pools are supported.
An address pool in the DHCP server is organized in the form of a“tree” structure. 
The root is the address of the natural segment, branches are the subnet addresses 
of the segment, and the leaf nodes are the manually binding client addresses. In 
this tree structure, the inheritance of configurations is fulfilled. In other words, 
subnets (son nodes) inherit the configuration of the natural segment (father 
node), and like that, the clients (grandson nodes) inherit the configuration 
parameters of the subnets (son nodes). Therefore, as for some general parameters, 
such as domain name, the user just needs to perform the configuration on the 
father node or the son nodes.
Perform the following configurations in system view.
Ta b l e 367   Create a DHCP address pool or entering the DHCP address pool view
OperationCommand
Enable the DHCP servicedhcp enable
Disable the DHCP serviceundo dhcp enable
OperationCommand
Create a DHCP address pool or enter the 
DHCP address pool viewdhcp server ip-pool pool-name 
						

							338CHAPTER 21: CONFIGURING IP APPLICATION
By default, no DHCP address pool is created.
3Configure the range of a DHCP address pool
aConfigure the static binding address allocated to the client
Some special clients (e.g., WWW server) need to be bound with fixed IP addresses, 
that is, to bind a certain client MAC address with a certain IP address. When the 
client with this MAC address applies for a DHCP address, the server will find the 
corresponding fixed IP address according to the client MAC address, and allocate it 
for the client. You can assume that a statically binding address is a special DHCP 
address pool that contains only one address.
Perform the following configurations in DHCP address pool view.
Ta b l e 368   Configure the statically binding IP address and MAC address
By default, no binding of DHCP client IP address and MAC address is configured. 
For the binding client MAC address, the default type is ethernet.
The command static-bind ip-address must be used together with the 
static-bind mac-address command. None of them have the superposition 
function, that is, the latest configuration will replace the previous one.
The command network and the commands static-bind ip-address and 
static-bind mac-address are conflicting. In other words, a DHCP address pool 
can be used either to configure statically binding addresses or the dynamic 
addresses, but not both.
bConfigure the dynamic IP addresses allocated to clients
For the addresses dynamically allocated to the clients (including the permanent 
dynamic addresses and those dynamic addresses with a limited lease period), it is 
necessary to configure the range of the address pool. Only one address segment 
can be configured in one address pool, and the mask can be used to set the 
address range.
Perform the following configurations in DHCP address pool view.
Ta b l e 369   Configure the range of the dynamically allocated IP addresses
By default, no DHCP address pool is configured, that is, there are no allocable 
addresses.
Disable the DHCP serviceundo dhcp server ip-pool pool-name
OperationCommand
Configure a statically binding IP addressstatic-bind 
ip-addressip-address [ mask 
netmask ]
Delete the statically binding IP addressundo static-bind ip-address
Configure a statically binding client MAC address static-bind mac-address 
mac-address
Delete a statically binding client MAC addressundo static-bind mac-address
OperationCommand
Configure the range of the IP addresses allocated 
dynamicallynetwork ip-address [ 
mask netmask ]
Delete the range of the IP addresses allocated dynamicallyundo network 
						

							DHCP Server Configuration339
The command network cannot be superimposed, that is, the latest configuration 
will overwrite the previous one.
The command network and the commands static-bind ip-address and 
static-bind mac-address are conflicting. In other words, for the same DHCP 
address pool, configure either dynamically binding address or the dynamic 
address, but not both.
4Configure the IP Addresses in the DHCP Address Pool not participating in 
Auto-allocation
As for a network or subnetwork, some IP addresses may have been used by some 
servers or particular hosts, like WWW server, gateway and FTP server. The DHCP 
server should exclude these addresses to ensure the normal operation of the 
network when allocating addresses. Otherwise, there will be the possibility that 
one address is allocated to two hosts, and hence causes the IP address allocation 
conflict.
By default, no IP address that does not participate in the auto-allocation is 
configured. As for some IP addresses that do not participate in the allocation, use 
the 
dhcp server forbidden-ip command to avoid their allocations.
Perform the following configurations in system view.
Ta b l e 370   Configure the IP addresses in an address pool that do not participate in 
auto-allocation
By default, IP addresses that do not participate in auto-allocation are not 
configured. That is, all the addresses are assumed to participate in auto-allocation.
This command can be superimposed. That is, the latest and the original 
configurations will take effect simultaneously. When using the undo dhcp server 
forbidden-ip
 command to delete the address-excluding setting, make sure that 
the parameters are totally consistent with those originally configured. That is, do 
not delete only some addresses originally configured.
5Configure IP Address Leasing Valid Period for DHCP Address Pool
According to various purposes of client hosts, a DHCP server can specify different 
valid periods of address leasing for different address pools and thus enhance the 
application flexibility. All the addresses in the same DHCP address pool own the 
same valid period. Address leasing valid period can not be inherited.
Perform the following configurations in DHCP address pool view.
Ta b l e 371   Configure IP address leasing valid period for DHCP address pool
OperationCommand
Configure the IP addresses in a DHCP address 
pool that will not participate in 
auto-allocationdhcp server forbidden-ip 
low-ipaddress [ high-ipaddress ]
Delete the IP addresses in a DHCP address 
pool that do not participate in auto-allocationundo dhcp server forbidden-ip 
low-ipaddress [ high -ipaddress ]
OperationCommand
Configure the leasing valid period of the dynamically 
allocated IP address for a DHCP address poolexpired { day day [ hour 
hour [ minute minute ] ] | 
unlimited }
Restore the leasing valid period of the dynamically 
allocated IP address for the DHCP address pool to the 
default valueundo expired 
						

							340CHAPTER 21: CONFIGURING IP APPLICATION
By default, the valid leasing period of IP address is 1 day.
6Configure the IP Address of Egress Gateway Router for DHCP Clients
When a DHCP client accesses a server (or host) beyond the local subnetwork, all 
the data must be sent and received via the egress gateway for the local network. 
Only a maximum of 8 egress gateway addresses can be configured in each DHCP 
address pool.
Perform the following configurations in DHCP address pool view.
Ta b l e 372   Configure the gateway router address of client
By default, the egress gateway address of DHCP client is not configured.
When specifying multiple egress gateway addresses, you need to continuously 
configure up to two addresses in the gateway-list command, instead of using 
this command repeatedly. That is because the new egress gateway address will 
replace the previous one other than superimposing it.
7Configure Domain Name of DHCP Client
In DHCP servers, the domain names used by the corresponding clients can be 
specified respectively for each address pool.
Perform the following configurations in DHCP address pool view.
Ta b l e 373   Configure the domain names of DHCP clients
By default, the domain names allocated to DHCP clients are not configured.
8Configure IP Address of DNS Used by DHCP Clients
When a computer accesses the Internet through the domain name, the domain 
name should be resolved to IP addresses. To access the DHCP client to the Internet, 
a DHCP server specifies the DNS address for the client when allocating the IP 
address to it. Each DHCP address pool can be configured with up to a maximum of 
8 DNS addresses.
Perform the following configurations in DHCP address pool view.
Ta b l e 374   Configure the DNS addresses in a DHCP address pool
By default, the IP address of DNS is not configured.
When specifying multiple DNS’s, you need to continuously configure up to two 
addresses in the dns-list command, instead of using this command repeatedly. 
OperationCommand
Configure the egress gateway address of DHCP 
clientgateway-list ipaddress1 [ 
ipaddress2 … ipaddress8 ]
Delete the egress gateway address of DHCP clientundo gateway-list { 
ip-address | all }
OperationCommand
Configure the domain name allocated to a DHCP clientdomain-name domainame
Delete the domain name allocated to a DHCP clientundo domain-name
OperationCommand
Configure the addresses of DNSes for the DHCP 
clientsdns-list ipaddress1 [ 
ipaddress2 … ipaddress8 ]
Delete the IP addresses of DNSes for the  DHCP 
clientsundo dns-list { ip-address | 
all } 
						

							DHCP Server Configuration341
That is because the new DNS address will replace the previous one rather than 
superimpose it.
9Configure IP Address of NetBIOS Server Used by DHCP Clients
Clients can communicate through the NetBIOS protocol. As for the clients installed 
with the Microsoft operating system, WINS (Windows Internet Naming Service) 
Server will provide the hostname-to-IP-address resolution mechanism for the hosts 
adopting the NetBIOS protocol. Therefore, WINS setting is necessary for most 
Windows network clients. Each DHCP address pool can be configured with up to a 
maximum of 8 NetBIOS addresses.
Perform the following configurations in DHCP address pool view.
Ta b l e 375   Configure the address of NetBIOS server used by DHCP clients
By default, the IP address of NetBIOS server is not configured.
When specifying multiple NetBIOS servers, you need to continuously configure up 
to two addresses in the nbns-list command, instead of using this command 
repeatedly. That is because the new NetBIOS server address will replace the 
previous one other than superimpose it.
10Set the type of NetBIOS node for DHCP client
When DHCP clients use the NetBIOS protocol to communicate on WANs, the 
mapping relations should be established between host names and IP addresses. 
There are four types of NetBIOS nodes for obtaining mapping relations:
■b-node: Obtain the mapping between them by means of broadcast.
■p-node: Obtain the mapping relation by means of communicating with a 
NetBIOS server.
■m-node: p-node owning part of the broadcasting features.
■h-node: b-node owning the “peer-to-peer” communicating mechanism.
Perform the following configurations in DHCP address pool view.
Ta b l e 376   Set the type of NetBIOS node for DHCP client
By default, clients adopt h-node.
11Configure Maximum Number of ping Packets Sent by the DHCP Server and the 
Longest Time Waiting for Response
Before allocating an IP address to a client, the DHCP server should detect this IP 
address. That is, checking whether there is response through pinging the host with 
this address. If no response is received after the longest time waiting for a 
response, re-send ping packets to this address until reaching the maximum 
number of ping packets allowed to be sent. If still no response is received, you can 
OperationCommand
Configure the address of NetBIOS server used 
by DHCP clientsnbns-list ip-address1 [ 
ip-address2 ... ip-address8 ]
Delete the address of NetBIOS server used by 
DHCP clientsundo nbns-list { ip-address | all 
}
OperationCommand
Set the type of NetBIOS node for DHCP clientnetbios-type { b-node | 
h-node | m-node | p-node }
Restore the type of NetBIOS node for DHCP client to 
the default valueundo netbios-type 
						

							342CHAPTER 21: CONFIGURING IP APPLICATION
assume that the IP address in this segment is not in use. Only when it is not in use 
can the IP address be allocated to the specified client.
Perform the following configurations in system view.
Ta b l e 377   Configure maximum number of ping packets sent by DHCP server & time for 
response
By default, the number of ping packets being sent is 2 and the time waiting for 
ping response packets is 500ms.
12Configure self-defined options
As DHCP continuously develops, you can support these new options/development 
by adding add them to the attribute tables of the DHCP servers by means of the 
self-defined options.
Perform the following configurations in DHCP address pool view.
Ta b l e 378   Configure DHCP self-defined options
Display and Debug 
DHCP ServerUse reset, debugging and display command in All views.
Ta b l e 379   Display and Debug DHCP servers
OperationCommand
Configure the maximum number of ping packets sent 
by the DHCP serverdhcp server ping { packets 
number }
Restore the maximum number of ping packets sent 
by the DHCP server to the default valueundo dhcp server ping 
packets
Configure the longest time waiting for response after 
ping packets are sent by the DHCP serverdhcp server ping { timeout 
milliseconds }
Restore the longest time waiting for response after 
ping packets are sent by the DHCP server to the 
default valueundo dhcp server ping 
timeout
OperationCommand
Configure DHCP self-defined optionsoption code { ascii ascii-string 
| hex hex-string [ hex-string… ] 
| ip-address ip-address }
Delete the DHCP self-defined optionsundo option code
OperationCommand
Display the address binding information of 
DHCPdisplay dhcp server ip-in-use [ 
ip-address ]
Reset all the address binding information of 
DHCPreset dhcp server ip-in-use { 
all | ip-address }
Display the statistic information of address 
conflicts of DHCPdisplay dhcp server conflict [ 
ip-address ]
Reset all the statistics of address conflicts of 
DHCPreset dhcp server conflict { all 
| ip-address }
Display the statistics of DHCP serverdisplay dhcp server statistics
Reset all the statistics of DHCP serverreset dhcp server statistics
Display the information of the available 
addresses in the DHCP address pooldisplay dhcp server expired
Display the information of the tree structure in 
the DHCP address pooldisplay dhcp server tree
Enable the DHCP server debuggingdebugging dhcp server { events | 
packet | ip-relation } 
						

							DHCP Server Configuration343
Typical DHCP Server 
Configuration ExampleThe common DHCP networking methods can be classified into two categories: 
One is that the DHCP server and the clients reside on the same subnetwork and 
they directly carry out the interaction of DHCP. Another one is that the DHCP 
server and the clients reside on different subnetworks and they must implement 
the allocation of IP addresses through the DHCP relay proxy. In both these cases, 
the DHCP configurations are the same.
I. Networking Requirements
DHCP server allocates IP addresses dynamically for the clients in the same 
segment, the address pool segment 10.1.1.0 is divided into two segments: 
10.1.1.0 (the mask is 255.255.255.128) and 10.1.1.128 (the mask is 
255.255.255.128). The two Ethernet interfaces of the DHCP server are 10.1.1.1 
(the mask is 255.255.255.128) and 10.1.1.129 (the mask is 255.255.255.128) 
respectively.
In the segment 10.1.1.0, the address lease period is 10 days and 12 hours, the 
domain name is 3com.com. The DNS address is 10.1.1.2, without NetBIOS 
address, and the outgoing router address is 10.1.1.126. In the segment 
10.1.1.128, the address lease period is 5 days, the DNS address is 10.1.1.2, the 
NetBIOS address is 10.1.1.4, and the outgoing router address is 10.1.1.254.
II. Networking Diagram
Figure 126   DHCP server and clients reside in the same network
III. Configuration Procedures
1Enable the DHCP service.
[Router] dhcp enable
2Configure the IP addresses (DNS address, NetBIOS address and outgoing gateway 
address) that do not participate in auto-allocation.
[Router] dhcp server forbidden-ip 10.1.1.2
[Router] dhcp server forbidden-ip 10.1.1.4
[Router] dhcp server forbidden-ip 10.1.1.254
3Configure the common attributes (pool address range, domain name, DNS 
address) of DHCP address 0.
[Router] dhcp server ip-pool 0
[Router-dhcp0] network 10.1.1.0 mask 255.255.255.0
[Router-dhcp0] domain-name 3com.com
[Router-dhcp0] dns-list 10.1.1.2
LANLAN NetBIOS
Serv erClientR outer
DN S Serv erDHC P S er v e r
R outer Outgoing router  B Client
Client Client
Client
Client Router Outgoing router  A 
						

							344CHAPTER 21: CONFIGURING IP APPLICATION
4Configure the attributes (address pool range, outgoing gateway and address lease 
period) in DHCP pool 1.
[Router] dhcp server ip-pool 1
[Router-dhcp1] network 10.1.1.0 mask 255.255.255.128
[Router-dhcp1] gateway-list 10.1.1.126
[Router-dhcp1] expired day 10 hour 12
5Configure the attributes (address pool range, outgoing gateway, NetBIOS address, 
and address lease period) in DHCP pool 2.
[Router] dhcp server ip-pool 2
[Router-dhcp2] network 10.10.1.128 mask 255.255.255.128
[Router-dhcp2] expired day 5
[Router-dhcp2] nbns-list 10.1.1.4
[Router-dhcp2] gateway-list 10.1.1.254
TroubleshootingFault: Dynamic IP address allocation conflict occurs at the client.
Solution: Following these steps to solve this problem.
1First of all, determine whether there is a host with this IP address on the network. 
You can perform the ping operation with relative long timeout to check the 
connectivity of this IP address.
2If a host with this IP address exists, you need to use the command dhcp server 
forbidden-ip
 to configure this IP address does not participate in dynamic address 
allocation.
3At the client, use ipconfig /release_all command to release the IP address 
dynamically, and use 
ipconfig /renew_all to re-apply dynamic addresses.
Configure DHCP Relay As the scale of networks grows and their complexities increase, network 
configurations become more and more complex. The original BOOTP protocol for 
static host configuration cannot satisfy the demands of users, especially on the 
occasions when computers are always on the move (e.g., using laptops or wireless 
network) and the number of actual computers exceeds that of the available IP 
addresses. To facilitate users to improve utilization ratio of resources and to 
support diskless networking mechanisms, the DHCP (Dynamic Host Configuration 
Protocol) based on BOOTP was developed. Similar to the BOOTP protocol, DHCP 
works in client-server mode. With this protocol, a DHCP client can dynamically 
request configuration information from a DHCP server, including important 
parameters such as assigned IP addresses, subnet masks and default gateways, 
etc. DHCP server can also conveniently configure this information dynamically for 
DHCP clients.
However, original DHCP can only take effect in a sub-net, and cannot work across 
different network segments, which is obviously not economic. So, it is necessary to 
set a DHCP server on all network segments for dynamic host configuration. This 
problem is solved by the introduction of DHCP relay, which relays relative 
messages to a destination DHCP server, so that multiple networks can share a 
DHCP server, which is  more cost-effective and convenient for centralized 
management. 
						

							Configure DHCP Relay 345
Figure 127   Schematic diagram of DHCP relay 
The above figure is the schematic diagram of DHCP relay. Its working principle is as 
follows:
After starting DHCP client, a configuration request message is broadcast and the 
DHCP relay router will send the message to the designated DHCP server on the 
other network after processing it properly. According to the information provided 
by the client, the server sends configuration information to the client via DHCP 
relay and completes the dynamic configuration of host. 
DHCP relay actually realizes transparent transmission of broadcast messages, i.e. 
transmitting broadcast messages of DHCP clients to DHCP servers on other 
network segments. Besides, the user can also configure transparent transmission 
for broadcast messages of designated protocols as required, to enable specific 
protocol broadcast messages of this network segment to arrive at other network 
segments. Similarly, the destinations are specified by IP auxiliary addresses. For 
example, transmit TFTP and DNS protocol messages transparently to 
corresponding servers.
To implement the DHCP relay, users have to configure IP auxiliary addresses to 
specify the DHCP server addresses.
Configure DHCP Relay DHCP configuration includes:
■Configure interface relay address
■Configure transparent transmission forwarding protocol.
1Configure interface relay address
To implement DHCP relay function, you need to configure IP relay address to 
specify DHCP server address. For DHCP relay, IP relay address specifies DHCP 
server. After configuration the broadcast messages received by this interface will 
be sent to the relay address. The interface configured with IP helper address 
should support broadcast mode. An interface can be configured with up to 20 
relay addresses. 
Perform the following task in Ethernet interface view.
Ta b l e 380   Configure interface relay address
By default, an interface has no IP address.
2Configure transparent transmission forwarding protocol
Network DHCP client
DHCP clientDHCP trunk router
DHCP server
OperationCommand
Configure interface relay addressip relay-address  ip-address
Delete interface relay addressundo ip relay-address [ 
ip-address ] 
						

							346CHAPTER 21: CONFIGURING IP APPLICATION
UDP needs to be forwarded Broadcast messages of common protocols usually 
adopt UDP. The destination port number of UDP is configured to set the 
transparent transmission protocol. For example, transparent transmission of TFTP 
broadcast messages (port number 69) and DNS protocol broadcast messages (port 
number 53) can be configured. At most 20 transparent transmission forwarding 
protocols can be configured. 
Perform the following task in system view.
Ta b l e 381   Configure transparent transmission forwarding protocol
By default, no transparent transmission forwarding protocol is configured.
Display and Debug 
DHCP RelayTa b l e 382   Display and debug of DHCP relay
DHCP Relay 
Configuration ExampleI. Configuration Requirement
DHCP client host is in the network segment 10.110.0.0, while DHCP server is in 
the network segment 202.38.0.0. DHCP relay router needs to relay DHCP 
messages, so that DHCP client hosts can obtain configuration information such as 
IP address from DHCP server through application.
DHCP server should be assigned with an address pool in network segment 
10.110.0.0, so that it can assign proper address information to the DHCP client 
host on the network segment. Meanwhile, the route to 10.110.0.0 should be 
available on DHCP server.
OperationCommand
Add transparent transmission forwarding 
protocolip relay protocol udp port
Delete transparent transmission forwarding 
protocolundo ip relay protocol udp [ 
port ]
OperationCommand
Display the current transparent transmission protocoldisplay ip relay protocol
Display helper addresses of respective interfacesdisplay ip relay-address
Turn on DHCP relay and transparent transmission 
debugging information switchdebugging ip relay protocol