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DHCPRELEASE and DHCPDECLINE messages are dropped if the MAC addresses in the snooping
database, but the binding's interface is other than the interface where the message was
received.
On untrusted interfaces, the switch drops DHCP packets with a source MAC address that does
not match the client hardware address. This is a configurable option.
4.2.1.1.
Populating the DHCP Snooping Bindings Database
The DHCP snooping application uses DHCP messages to build and maintain the binding’s database. DHCP
snooping creates a tentative binding from DHCP DISCOVER and REQUEST messages. Tentative bindings tie a
client to a port (the port where the DHCP client message was received). Tentative bindings are completed
when DHCP snooping learns the client’s IP address from a DHCP ACK message on a trusted port. DHCP
snooping removes bindings in response to DECLINE, RELEASE, and NACK messages. The DHCP snooping
application ignores the ACK messages as a reply to the DHCP Inform messages received on trusted ports. You
can also enter static bindings into the binding database.
When a switch learns of new bindings or loses bindings, the switch immediately updates the entries in the
database. The switch also updates the entries in the binding file. The frequency at which the file is updated
is based on a configurable delay, and the updates are batched.
If the absolute lease time of the snooping database entry expires, that entry is removed. Make sure the
system time is consistent across the reboots. Otherwise, the snooping entries will not expire properly. If a
host sends a DHCP release while the switch is rebooting, when the switch receives the DHCP discovery or
request, the client’s binding goes to the tentative binding as shown in Figure 19.
Figure 4-2: DHCP Binding
The binding database includes data for clients only on untrusted ports.
4.2.1.2.
DHCP Snooping and VLANs
DHCP snooping forwards valid DHCP client messages received on non-routing VLANs. The message is
forwarded on all trusted interfaces in the VLAN.
Summary of Contents for QuantaMesh QNOS5
Page 1: ...QuantaMesh Ethernet Switch Configuration Guide QNOS5 NOS Platform ...
Page 209: ...209 Table 7 8 IPv6 Neighbor Discovery Settings ...
Page 226: ...226 Table 8 2 L3 Multicast Defaults ...
Page 254: ...254 Appendix A Term and Acronyms Table 9 5 Terms and Acronyms ...