A, DHCP Services Overview
1, DHCP (Dynamic Host Configuration Protocol ) Dynamic Host Configuration Protocol
DHCP is working by the Internet Task Force designed and developed specifically for the agreement to automatically assign TCP / IP parameters for TCP / IP computers on the network.
DHCP is working by the Internet Task Force designed and developed specifically for the agreement to automatically assign TCP / IP parameters for TCP / IP computers on the network.
2, the address information may be allocated include: IP address, subnet mask, corresponding network address, broadcast address, default gateway address, DNS server address.
3, protocol and port services
comply with udp protocol
client is a 68-port
server is a 67-port view the service ports are open: netstat -lnup | grep: 67
the DHCP server by listening to port 67 UDP protocol work, client via UDP protocol 68 port work
comply with udp protocol
client is a 68-port
server is a 67-port view the service ports are open: netstat -lnup | grep: 67
the DHCP server by listening to port 67 UDP protocol work, client via UDP protocol 68 port work
4, DHCP allocation method
automatically assigned: the IP address assigned to a permanent use of
manually assigned: specifically developed IP address by the DHCP server administrators to
dynamically allocate: after use to release the IP, for other clients to use
automatically assigned: the IP address assigned to a permanent use of
manually assigned: specifically developed IP address by the DHCP server administrators to
dynamically allocate: after use to release the IP, for other clients to use
5, DHCP lease process
client requests IP (client sends Discover broadcast packet)
server response (server sends DHCP Offer packet radio)
client selects IP (client sends Request broadcast packet)
server determines the lease (the server sends an ACK packet radio)
client requests IP (client sends Discover broadcast packet)
server response (server sends DHCP Offer packet radio)
client selects IP (client sends Request broadcast packet)
server determines the lease (the server sends an ACK packet radio)
When re-login, no need to send DHCP Discover message, DHCP Request but directly transmitted before the assigned time comprising the IP address request information. When the DHCP server receives this information packet will try to get DHCP client continues to use the original P address and answer a DHCP ACK confirmation.
If the IP address can not be allocated to the original DHCP client to use (such as IP addresses have been assigned to other clients use DHCP), DHCP server to the DHCP client to answer a DHCP Nack denied the information. When the original DHCP client receives this DHCP Nack denied the information, it must resend DHCP Discover discovery information to request a new IP address.
If the IP address can not be allocated to the original DHCP client to use (such as IP addresses have been assigned to other clients use DHCP), DHCP server to the DHCP client to answer a DHCP Nack denied the information. When the original DHCP client receives this DHCP Nack denied the information, it must resend DHCP Discover discovery information to request a new IP address.
Update leases
when the DHCP server to the client's IP address lease rent of 50%, you need to update the lease. The client sends DHCP Request packet directly to the server lease request to update the existing address lease. If the DHCP server can not respond at this time requesting DHCP client properly, the IP address of the client can continue to use the 87.5% maximum lease time.
when the DHCP server to the client's IP address lease rent of 50%, you need to update the lease. The client sends DHCP Request packet directly to the server lease request to update the existing address lease. If the DHCP server can not respond at this time requesting DHCP client properly, the IP address of the client can continue to use the 87.5% maximum lease time.
Two, DHCP relay principle
open on different network segments connected devices DHCP relay, the DHCP this particular broadcast information forwarded between VLAN, VLAN so that other clients also get there from the DHCP server IP address.
Configuration enables the configuration of the DHCP relay forwarding on Layer 3 switches, it does not contain the server's VLAN
interface vlan 2 # to enter VLAN2
ip address ip address subnet mask # Configure the IP address
ip helper-address ip add DHCP server IP address #
no shutdown #
as behind the interface vlan .... #
interface vlan 2 # to enter VLAN2
ip address ip address subnet mask # Configure the IP address
ip helper-address ip add DHCP server IP address #
no shutdown #
as behind the interface vlan .... #
DHCP relay commands
IP-address DHCPsrv-Helper the IPAddress
DHCPsrv the IPAddress-IP address for the DHCP server
IP-address DHCPsrv-Helper the IPAddress
DHCPsrv the IPAddress-IP address for the DHCP server