Dhcp Snooping Process - Juniper JUNOS OS 10.3 - SOFTWARE Manual

Complete Software Guide for Junos

DHCP Snooping Process

2830
®
OS for EX Series Ethernet Switches, Release 10.3
Entries in the DHCP database are updated in these events:
When a DHCP client releases an IP address (sends a DHCPRELEASE message), the
associated mapping entry is deleted from the database.
If you move a network device from one VLAN to another, typically the device has to
acquire a new IP address, so its entry in the database, including the VLAN ID, is updated.
When the lease time (timeout value) assigned by the DHCP server expires, the
associated entry is deleted from the database.
TIP: By default, the IP-MAC bindings are lost when the switch is rebooted
and DHCP clients (the network devices, or hosts) must reacquire bindings.
However, you can configure the bindings to persist by setting the
dhcp-snooping-file
remotely.
You can configure the switch to snoop DHCP server responses only from particular VLANs.
Doing this prevents spoofing of DHCP server messages.
You configure DHCP snooping for each VLAN, not for each interface (port). By default,
DHCP snooping is disabled for all VLANs.
TIP: For private VLANs (PVLANs), enable DHCP snooping on the primary
VLAN. If you enable DHCP snooping only on a community VLAN, DHCP
messages coming from PVLAN trunk ports are not snooped.
The basic process of DHCP snooping entails the following steps:
Device sends DHCPDISCOVER to request IP address.
1.
Switch forwards the packet to the DHCP server.
2.
Server sends DHCPOFFER to offer an address. If the DHCPOFFER is from a trusted
3.
interface, switch forwards the packet to the DHCP client.
Device sends DHCPREQUEST to accept the IP address. Switch snoops this packet
4.
and adds IP-MAC placeholder binding to the database. The entry is considered a
placeholder until a DHCPACK is received from the server. Until then, the IP address
could still be assigned to some other host.
Server sends DHCPACK to assign the IP address or DHCPNAK to deny the address
5.
request
Switch updates the the DHCP database in accordance with the type of packet received:
6.
Upon receipt of DHCPACK, switch updates lease information for the IP-MAC binding
in its database.
statement to store the database file either locally or
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