Users Guide

1192 Layer-2 and Layer-3 Relay Features
relay agent can be used to add the information that the DHCP server needs
to perform its role in address and configuration and assignment. The
information added by the L2 relay agent can include location and
identification information that can assist the DHCP server in applying
policies such as service offerings or address assignment.
Before it relays DHCP requests from clients, the switch can add a Circuit ID
and a Remote ID. These IDs provide information about the circuit and port
number connected to the client. This information is added as suboptions in
the DHCP Option 82 packets as defined in sections 3.1 and 3.2 of RFC3046.
The switch removes this option from packets that it relays from L3 Relay
agents/DHCP servers to clients.
These sub-options may be used by the DHCP server to affect how it treats the
client and also may be used by the relay agent to limit broadcast replies to the
specific circuit or attachment point of the client.
Enabling L2 Relay on VLANs
L2 DHCP relay can be enabled on a particular VLAN. The VLAN is identified
by a service VLAN ID (S-VID), which a service provider uses to identify a
customer’s traffic while traversing the provider network to multiple remote
sites. The switch uses the VLAN membership of the switch port client (the
customer VLAN ID, or C-VID) to perform a lookup a corresponding S-VID.
If the S-VID is enabled for DHCP Relay, then the packet can be forwarded. If
the C-VID does not correspond to an S-VID that is enabled for DHCP Relay,
then the switch will not relay the DHCP request packet.
How is DHCP Option 82 Used?
The Dell EMC Networking Operating System supports insertion of DHCP
Option 82 information into DHCP messages relayed to DHCP servers. The
Dell EMC Networking N-Series switch can be configured to insert either the
Circuit ID or the Remote ID or both. When enabled, the Circuit ID contains
the port identifier over which the DHCP request was received. The Remote
ID is configurable by the administrator on a per-switch basis.
Consider a network with multiple DHCP servers, where the administrator
wishes to serve addresses from a specific server based on the switch and port
to which the user station is connected. User traffic is served on VLAN 10 or
20.