HP 5500 Ei 5500 Si Switch Series Configuration Guide

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Dynamic IP address allocation process 
Figure 16 Dynamic IP address allocation process 
 
 
1. The client broadcasts a DHCP-DISCOVER message to locate a DHCP server. 
2. A DHCP server offers configuration parameters such as an IP address to the client, in a 
DHCP-OFFER message. The sending mode of the DHCP-OFFER is determined by the flag field in 
the DHCP-DISCOVER message. For related information, see  DHCP message format.
  
3. If several DHCP servers send offers to the client, the client accepts...

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DHCP message format 
Figure 17 shows the DHCP message format, which is ba sed on the BOOTP message format although 
DHCP uses some of the fields in significantly different ways. The numbers in parentheses indicate the size 
of each field in bytes. 
Figure 17  DHCP message format 
 
 
•  op—Message type defined in option field. 1 = REQUEST, 2 = REPLY 
•   htype , hlen —Hardware address type and length of a DHCP client. 
•   hops —Number of relay agents a request message traveled. 
•   xid—Transaction...

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DHCP options 
D H C P  us e s  t h e  s a m e  m e s s a g e  fo rm a t  a s  BO OT P,  b u t  D H C P  us e s  t h e  O p t io n  fi e l d  t o  c a rr y  i n fo rm a t io n  fo r  
dynamic address allocation and to provide additional configuration information to clients.  
Figure 18  DHCP option format 
 
 
Common DHCP options 
The following are common DHCP options: 
•  Option 3 —Router option. It specifies the gateway address. 
•   Option  6 —DNS server option. It specifies the DNS server’s IP...

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•  Service provider identifier, which is acquired by  the Customer Premises Equipment (CPE) from the 
DHCP server and sent to the ACS for selecting  vender-specific configurations and parameters. 
•   Preboot Execution Environment (PXE) server address, which is used to obtain the bootfile or other 
control information from the PXE server. 
1. Format of Option 43 
Network configuration parameters are carried in  different sub-options of Option 43 as shown 
in  Figure 19 . 
Figure 19  Optio

n 43...

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Figure 21 PXE server address sub-option value field 
 
 
Relay agent option (Option 82) 
Option 82 is the relay agent option in the option field of the DHCP message. It records the location 
information of the DHCP client. When a DHCP relay  agent or DHCP snooping device receives a client’s 
request, it adds Option 82 to the request message and sends it to the server. 
The administrator can locate the DHCP client to fu rther implement security control and accounting. The 
Option 82 supporting server...

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•  Verbose padding format 
{ Sub-option 1 —Contains the user-specified access node  identifier (ID of the device that adds 
Option 82 in DHCP messages), and the type,  number, and VLAN ID of the interface that 
received the client’s request. The VLAN ID field has a fixed length of 2 bytes. All the other 
padding contents of sub-option 1 are length variable. See  Figure 24. 
Figure 24  Sub-option 1 in verbose pa

dding format 
 
 { 
Sub-option 2 —Contains the MAC address of the DHCP relay agent...

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{ Sub-option 1 —Contains the VLAN ID of the interface that received the client’s request, module 
(subcard number of the receiving port) and port  (port number of the receiving port). The value 
of the sub-option type is 1, and the  value of the circuit ID type is 0. 
Figure 28   Sub-option 1 in standard padding format 
 
 { 
Sub-option 2 —Contains the MAC address of the DHCP relay agent interface or the MAC 
address of the DHCP snooping device that received the client’s request. It has the same...

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Configuring DHCP server 
The term interface in the DHCP features refers to Layer 3 interfaces, including VLAN interfaces and 
route-mode (or Layer 3) Ethernet ports. You can set an Ethernet port to operate in route mode by using the 
port  link-mode  route  command (see the  Layer 2—LAN Switching Configuration Guide ). 
Only HP 5500 EI switches support Layer 3 Ethernet port configuration. 
Overview 
The DHCP server is well suited to networks where: 
•   Manual configuration and centralized...

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 NOTE: 
The extended address pools on a DHCP server are  independent of each other and no inheritance 
relationship exists among them. 
 
Principles for selecting an address pool 
The DHCP server observes the following principles to select an address pool when assigning an IP 
address to a client: 
1.  If there is an address pool where an IP address is  statically bound to the MAC address or ID of the 
client, the DHCP server will select this address pool and assign the static ally bound IP address...

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DHCP server configuration task list 
 
Task Remarks 
Configuring an address pool for the DHCP server Required. 
Enabling DHCP Required. 
Enabling the DHCP server on an interface Required. 
Applying an extended address pool on an interface 
Required by the extended address pool configuration. 
When configuring a common address pool, ignore this 
task. 
Configuring the DHCP server security functions  Optional. 
Enabling client offline detection Optional. 
Enabling handling of Option 82 Optional....