DELL N3000 User Manual

Page of 1460
Configuring L2 and L3 Relay Features
1089
Enabling L2 Relay on VLANs
You can enable L2 DHCP relay 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.
What Is the IP Helper Feature?
The IP Helper feature provides the ability for a router to forward configured 
UDP broadcast packets to a particular IP address. This allows applications to 
reach servers on non-local subnets. This is possible even when the application 
is designed to assume a server is always on a local subnet or when the 
application uses broadcast packets to reach the server (with the limited 
broadcast address 255.255.255.255, or a network directed broadcast address).
You can configure relay entries globally and on routing interfaces. Each relay 
entry maps an ingress interface and destination UDP port number to a single 
IPv4 address (the helper address). Multiple relay entries may be configured 
for the same interface and UDP port, in which case the relay agent relays 
matching packets to each server address. Interface configuration takes priority 
over global configuration. If the destination UDP port for a packet matches 
any entry on the ingress interface, the packet is handled according to the 
interface configuration. If the packet does not match any entry on the ingress 
interface, the packet is handled according to the global IP helper 
configuration.
You can configure discard relay entries. Discard entries are used to discard 
packets received on a specific interface when those packets would otherwise 
be relayed according to a global relay entry. Discard relay entries may be 
configured on interfaces, but are not configured globally.
Additionally, you can configure which UDP ports are forwarded. Certain UDP 
port numbers can be selected from the web interface or specified by name in 
the CLI, but you can also configure a relay entry with any UDP port number. 
You may configure relay entries that do not specify a destination UDP port. 
The relay agent assumes that these entries match packets with the UDP 
destination ports listed in Table 34-1 (the list of default ports).