3Com Router User Manual

21
CONFIGURING IP APPLICATION
This chapter contains information on the following topics:
■Configure Address Resolution Protocol (ARP)
■Configure Domain Name Resolution (DNS)
■VLAN Configuration 
■DHCP Server Configuration
■Configure DHCP Relay 
■Configure Network Address Translation (NAT)
Configure Address 
Resolution Protocol 
(ARP)ARP is mainly used for resolution from IP address to Ethernet MAC address. 
Normally, dynamic ARP is used to resolve the mapping relation from the IP address 
to the Ethernet...

328CHAPTER 21: CONFIGURING IP APPLICATION
By default, the system executes static ARP mapping.
Static ARP mapping items are valid as long as the router works normally, but 
dynamic ARP mapping items are valid for only 20 minutes.  
Display and Debug ARPTa b l e 359   Display and Debug ARP
Configure Domain 
Name Resolution 
(DNS)The TCP/IP Extranet not only provides an IP address to locate a device, but also 
designs a specific character-string host naming mechanism. This system uses a 
layered naming...

VLAN Configuration 329
By default, the system has no static domain name resolution mapping.
Pay attention that when adding a domain name mapping, if the same hostname 
has been input twice, the current configuration will overwrite the previous one. A 
static domain name resolution table can maintain a maximum of 50 mapping 
relationships between domain names and IP addresses. 
Display and Debug 
Domain Name 
Resolution Ta b l e 361   Display and Debug domain name resolution
VLAN Configuration To...

330CHAPTER 21: CONFIGURING IP APPLICATION
Figure 122   Format of VLAN tag
TPID (Tag Protocol Identifier) field has two bytes. When its value is 0X8100, it 
means the Ethernet frame header contains VLAN tag. The third and fourth byte 
are TCI (Tag Control Information) fields, with the higher three bits being user 
priority field, the fourth being the instruction of standard encapsulation format 
and the other 12 bits being VLAN IDs whose value ranges from 0 to 4094 (the 
value must begin with 1 on...

VLAN Configuration 331
In as Ethernet interface is connected with a LAN Switch port. As the Ethernet 
subinterface of every specified VLAN ID can act as an independent gateway, this 
subinterface and other Ethernet subinterface in the same VLAN ID should belong 
to the same subnet segment.
Please implement the following configuration under Ethernet subinterface view.
Ta b l e 364   Configure IP address of Ethernet subinterface
By default, no IP address is defined.
Ethernet subinterface acts as a gateway...

332CHAPTER 21: CONFIGURING IP APPLICATION
[Router] interface ethernet 0.1
bSpecify 3 as the ID number of the VLAN on which the subinterface is located.  
[Router-Ethernet0.1] vlan-type dot1q vid 3
cConfigure IP address for the subinterface.
[Router-Ethernet0.1] ip address 3.3.3.8 255.255.255.224
dCreate and enter the view of Ethernet subinterface Ethernet0.2.
[Router] interface ethernet 0.2
eSpecify 4 as the ID number of the VLAN on which the subinterface is located.
[Router-Ethernet0.2] vlan-type dot1q...

DHCP Server Configuration333
connected with PC must be set as “untagged” for the reason that PC cannot 
identify data packet marked with VLAN tag. 
Fault: Ping Two PCs, but fails to ping them through. 
Troubleshooting: The steps below can be taken.
■First, ping the IP address of Ethernet subinterface in the same VLAN from a PC. 
If the ping fails, solve the problem according to the method described in fault 
one.
■If one PC can ping through the IP address of Ethernet subinterface in the same 
VLAN, but...

334CHAPTER 21: CONFIGURING IP APPLICATION
DHCP server can provide three types of IP address allocation policies according 
to the different requirements:
■Allocate addresses manually: Administrators configure special IP addresses for a 
small number of particular hosts such as the service server WWW.
■Allocate addresses automatically: Allocate permanent IP addresses for some 
hosts connected to the network for the first time and the addresses will be 
allocated to the hosts for a long period of time....

DHCP Server Configuration335
■DHCP client logins the network for the first time
If it is the first time for a DHCP client to login to the network, it will establish a 
connection with the DHCP server through four stages:
■Discovering stage. This is the stage when the DHCP client searches the 
DHCP servers. The DHCP client broadcasts a DHCP_Discover message to 
search the DHCP servers, and every host installed with the TCP/IP suite on 
the network will receive this type of broadcast message but only the...

336CHAPTER 21: CONFIGURING IP APPLICATION
will take back the IP address. If the DHCP client wants to continue to use 
this address, it should renew the IP leasing contract such as extending the 
leasing contract of the IP address.
■In practice, the DHCP client will automatically send the message for 
renewing the IP leasing contract to the DHCP server when the DHCP client 
starts up or half of the valid period of the IP leasing contract has expired. To 
renew the IP leasing contract, the DHCP client will...