102 Routing Configuration
The relay agent only relays packets that meet the following conditions:
• The destination MAC address must be the all-ones broadcast address (FF:FF:FF:FF:FF:FF).
• The destination IP address must be the limited broadcast address (255.255.255.255) or a directed
broadcast address for the receive interface.
• The IP time-to-live (TTL) must be greater than 1.
• The protocol field in the IP header must be UDP (17).
• The destination UDP port must match a configured relay entry.
CLI Examples
Example 1: Enable/Disable IP Helper
To globally enable/disable IP Helper (relay of UDP packets) use the following command:
console (config)#ip helper enable
Example 2: Configure IP Helper Globally (DHCP)
To relay DHCP packets received on any interface to two DHCP servers (10.1.1.1 and 10.1.2.1), use the
following commands:
console (config)#ip helper-address 10.1.1.1 dhcp
console (config)#ip helper-address 10.1.2.1 dhcp
Example 3: Enable IP Helper Globally (UDP)
To relay UDP packets received on any interface for all default ports (Table 2) to the server at 20.1.1.1, use
the following commands:
console (config)#ip helper-address 20.1.1.1
Example 4: Enable IP Helper on a VLAN Routing Interface to a Server (DHCP)
To relay DHCP packets received on VLAN 100 to two DHCP servers, 192.168.10.1 and 192.168.20.1, use
the following commands:
console(config)#interface vlan 100
console(config-if-vlan100)#ip helper-address 192.168.10.1 dhcp
console(config-if-vlan100)#ip helper-address 192.168.20.1 dhcp
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