Dhcp Relay; L2 Dhcp Relay Operation Mode - Cisco SPS208G User Manual

Setup

DHCP Relay

STEP 3
DHCP Relay
SPS208G/SPS224G4/SPS2024 Service Provider Switches User Guide
•
DNS Server — Indicates the DNS server IP address.
Click Save Settings. The Network Settings are saved and the device is updated.
A DHCP Relay agent detects DHCP broadcasts from DHCP clients and relays
those broadcasts to DHCP servers possibly on different subnets. The Relay agent
information option (Option 82) in the DHCP protocol enables a DHCP relay agent to
send additional client information upon requesting an IP address.
Option 82 specifies the relaying switch's MAC address, the port identifier, and the
VLAN which forwarded the packet.
DHCP Snooping and L2 DHCP Relay implement the L2 DHCP Relay functionality in
the switch. Each feature can insert option 82 into traversing packets.
•
DHCP Snooping with option 82 insertion provides transparent L2 Relay
agent functionality (as defined in TR-101 appendix B) if the DHCP server is
on the same VLAN as the clients.
•
The L2 DHCP Relay agent provides relay service for remote servers that are
not on the same VLAN as clients.

L2 DHCP Relay Operation Mode

Transparent L2 DHCP Relay
The DHCP server must be located on the same VLAN as its clients. DHCP
broadcasts are replaced and Option 82 is added to the replacement packets
broadcast to the server.
This is implemented by the DHCP Snooping with Option 82 Insertion
configuration. Ensure that DHCP Snooping on all relevant VLANs is enabled and
that in the DHCP Snooping configuration, the DHCP server outgoing ports are set
to "trusted".
This is an implementation of transparent L2 Relay agent (as described in TR-101
appendix B).
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