S9300 and S9300E V200R008(C00&C10) Configuration Guide - IP Service
This document describes the configurations of IP service, including IP address, ARP, DHCP, DNS, mDNS, UDP Helper, IP performance, IPv6, DHCPv6, IPv6 DNS, IPv6 over IPv4 tunnel and IPv4 over IPv6 tunnel.
This document describes the configurations of IP service, including IP address, ARP, DHCP, DNS, mDNS, UDP Helper, IP performance, IPv6, DHCPv6, IPv6 DNS, IPv6 over IPv4 tunnel and IPv4 over IPv6 tunnel.
How a DHCP Client Renews Its IP Address Lease
IP addresses that are dynamically allocated by a DHCP server have leases. A DHCP Discover message from a DHCP client can carry an expected lease. When allocating network parameters, the DHCP server compares the expected lease with the specified lease in the address pool and allocates an IP address with a smaller lease to the DHCP client. When the lease expires or the client goes offline, the DHCP server reclaims the IP address, which can then be allocated to other clients. This mechanism improves IP address utilization. To continue to use this IP address, the DHCP client must renew its IP address lease.
IP Address Lease Renewal Without a DHCP Relay Agent
Figure 3-5 shows how a DHCP client renews its IP address lease.
- When the lease reaches 50% (T1) of its validity period, the DHCP client sends a unicast DHCP Request message to the DHCP server to request lease renewal. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.
- If no response is received from the DHCP server when the lease reaches 87.5% (T2) of its validity period, the DHCP client sends a broadcast DHCP Request message to request lease renewal. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.
If no response is received when the lease expires, the DHCP client stops using the IP address and sends a DHCP Discover message to apply for a new IP address.
If a DHCP client does not need to use the allocated IP address before the lease expires, the DHCP client sends a DHCP Release message to the DHCP server to request IP address release. The DHCP server saves the configuration of this DHCP client and records the IP address in the allocated IP address list. The IP address can then be allocated to this DHCP client or other clients.
A DHCP client can send a DHCP Inform message to the DHCP server to request configuration update.
IP Address Lease Renewal with DHCP Relay Agents
Figure 3-6 shows how a DHCP client renews its IP address lease when a DHCP relay agent is deployed.
- When the lease reaches 50% (T1) of its validity period, the DHCP client sends a unicast DHCP Request message to the DHCP server to request lease renewal. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.
- If no response is received from the DHCP server when the lease reaches 87.5% (T2) of its validity period, the DHCP client sends a broadcast DHCP Request message to request lease renewal. The DHCP relay agent then sends a unicast DHCP Request message to the DHCP server. For details about how the DHCP relay agent processes received messages, see How a DHCP Server Allocates Network Parameters to New DHCP Clients. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.
If no response is received when the lease expires, the DHCP client stops using the IP address and sends a DHCP Discover message to apply for a new IP address.