IP Services
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Instruction Manual - NXA-ENET8-POE+
Option 82 information contains information which can identify both the relay agent and the interface through which the DHCP
request was received:
The DHCP Relay Information Option Remote ID (RID) is the access node identifier - a string used to identify the switch to
the DHCP server.
The DHCP Relay Information Option Fields are the Option 82 circuit identification fields (CID - including VLAN ID, stack unit,
and port). These fields identify the requesting device by indicating the interface through which the relay agent received the
request.
Command Usage
You must specify the IP address for at least one active DHCP server. Otherwise, the switch's DHCP relay agent will not be
able to forward client requests to a DHCP server. Up to five DHCP servers can be specified in order of preference.
If any of the specified DHCP server addresses are not located in the same network segment with this switch, specify the
default router through which this switch can reach other IP subnetworks (see the
section on page 265 or the
Configuring the IPv6 Default Gateway
DHCP relay configuration will be disabled if an active DHCP server is detected on the same network segment.
DHCP Snooping Information Option 82 (see page 177) and DHCP Relay Information Option 82 cannot both be enabled at
the same time.
DHCP request packets received by the switch are handled as follows:
If a DHCP relay server has been set on the switch, when the switch receives a DHCP request packet without option 82
information from the management VLAN or a non-management VLAN, it will add option 82 relay information and the
relay agent's address to the DHCP request packet, and then unicast it to the DHCP server.
If a DHCP relay server has been set on the switch, when the switch receives a DHCP request packet with option 82
information from the management VLAN or a non-management VLAN, it will process it according to the configured
relay information option policy:
If the policy is replace, the DHCP request packet's option 82 content (the RID and CID sub-option) is replaced with
information provided by the switch. The relay agent address is inserted into the DHCP request packet, and the switch
then unicasts this packet to the DHCP server.
If the policy is keep, the DHCP request packet's option 82 content will be retained. The relay agent address is inserted
into the DHCP request packet, and the switch then unicasts this packet to the DHCP server.
If the policy is drop, the original DHCP request packet is flooded onto the VLAN which received the packet but is not
relayed.
DHCP reply packets received by the relay agent are handled as follows:
When the relay agent receives a DHCP reply packet with Option 82 information over the management VLAN, it first ensures
that the packet is destined for it.
If the RID in the DHCP reply packet is not identical with that configured on the switch, the option 82 information is retained,
and the packet is flooded onto the VLAN through which it was received.
If the RID in the DHCP reply packet matches that configured on the switch, it then removes the Option 82 information from
the packet, and sends it on as follows:
If the DHCP packet's broadcast flag is on, the switch uses the circuit-id information contained in the option 82
information fields to identify the VLAN connected to the requesting client and then broadcasts the DHCP reply packet to
this VLAN.
If the DHCP packet's broadcast flag is off, the switch uses the circuit-id information in option 82 fields to identify the
interface connected to the requesting client and unicasts the reply packet to the client.
DHCP packets are flooded onto the VLAN which received them if DHCP relay service is enabled on the switch and any of the
following situations apply:
There is no DHCP relay server set on the switch, when the switch receives a DHCP packet.
A DHCP relay server has been set on the switch, when the switch receives a DHCP request packet with a non-zero relay
agent address field (that is not the address of this switch).
A DHCP relay server has been set on the switch, when the switch receives DHCP reply packet without option 82
information from the management VLAN.
The reply packet contains a valid relay agent address field (that is not the address of this switch), or receives a reply
packet with a zero relay agent address through the management VLAN.
A DHCP relay server has been set on the switch, and the switch receives a reply packet on a non-management VLAN.
FIG. 316
Layer 3 DHCP Relay Service
DHCP
Server
Provides IP address
compatible with switch
segment to which client
is attached