Concept Guide

Remote ID (Option
37)
This identies the host from which the message is received.
Default values: The default value of this option is the MAC address of the relay agent that adds Option 37.
DHCP Snooping
DHCP snooping is a feature that protects networks from spoong. It acts as a rewall between the DHCP server and DHCP clients.
DHCP snooping places the ports either in trusted or non-trusted mode. By default, all ports are set to the non-trusted mode. An attacker
can not connect to the DHCP server through trusted ports. While conguring DHCP snooping, manually congure ports connected to
legitimate servers and relay agents as trusted ports.
When you enable DHCP snooping, the relay agent builds a binding table — using DHCPACK messages — containing the client MAC
address, IP addresses, IP address lease time, port, VLAN ID, and binding type. Every time the relay agent receives a DHCPACK on a trusted
port, it adds an entry to the table.
The relay agent checks all subsequent DHCP client-originated IP trac (DHCPRELEASE, DHCPNACK, and DHCPDECLINE) against the
binding table to ensure that the MAC-IP address pair is legitimate and that the packet arrived on the correct port. Packets that do not pass
this check are forwarded to the server for validation. This checkpoint prevents an attacker from spoong a client and declining or releasing
the real client’s address. Server-originated packets (DHCPOFFER, DHCPACK, and DHCPNACK) that arrive on a not trusted port are also
dropped. This checkpoint prevents an attacker from acting as an imposter as a DHCP server to facilitate a man-in-the-middle attack.
Binding table entries are deleted when a lease expires, or the relay agent encounters a DHCPRELEASE, DHCPNACK, or DHCPDECLINE.
DHCP snooping is supported on Layer 2 and Layer 3 trac. DHCP snooping on Layer 2 interfaces does not require a relay agent.
NOTE
: In DHCP relay agent, congure DHCP snooping such that the packet from DHCP client must not pass through DHCP
snooping-enabled switches twice before reaching the DHCP server.
Binding table entries are deleted when a lease expires or when the relay agent encounters a DHCPRELEASE. Line cards maintain a list of
snooped VLANs. When the binding table is exhausted, DHCP packets are dropped on snooped VLANs, while these packets are forwarded
across non-snooped VLANs. Because DHCP packets are dropped, no new IP address assignments are made. However, DHCPRELEASE
and DHCPDECLINE packets are allowed so that the DHCP snooping table can decrease in size. After the table usage falls below the
maximum limit of 4000 entries, new IP address assignments are allowed.
NOTE
: DHCP server packets are dropped on all non-trusted interfaces of a system congured for DHCP snooping. To prevent
these packets from being dropped, congure ip dhcp snooping trust on the server-connected port.
DHCP Snooping for a Multi-Tenant Host
You can congure the DHCP snooping feature such that multiple IP addresses are expected for the same MAC address. You can use the
ip dhcp snooping command multiple times to map the same MAC address with dierent IP addresses. This conguration is also used
for dynamic ARP inspection (DAI) and source address validation (SAV). The DAI and SAV tables reect the same entries in the DHCP
snooping binding table.
NOTE
: If you enable DHCP Option 82 using the ip dhcp relay command, by default, the remote-ID eld contains the MAC
address of the relay agent. If you congure the remote ID as the host name in a VLT setup, congure dierent host names on
both VLT peers. If you congure the remote ID with your own string, ensure that your strings are dierent on both VLT peers.
330 Dynamic Host Conguration Protocol (DHCP)