Dell PowerConnect 7048 - Page 909
What Is the IP Helper Feature?, Enabling L2 Relay on VLANs
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Page 909 highlights
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 33-1 (the list of default ports). Configuring L2 and L3 Relay Features 909
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1,183
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1,184
-
1,185
-
1,186
-
1,187
-
1,188
-
1,189
-
1,190
-
1,191
-
1,192
-
1,193
-
1,194
-
1,195
-
1,196
-
1,197
-
1,198
-
1,199
-
1,200
-
1,201
-
1,202
-
1,203
-
1,204
-
1,205
-
1,206
-
1,207
-
1,208
-
1,209
-
1,210
-
1,211
-
1,212
-
1,213
-
1,214
-
1,215
-
1,216
-
1,217
-
1,218
-
1,219
-
1,220
-
1,221
-
1,222
-
1,223
-
1,224
-
1,225
-
1,226
-
1,227
-
1,228
-
1,229
-
1,230
-
1,231
-
1,232
-
1,233
-
1,234