Dell Force10 S4810P - Page 386

386 | Dynamic Host Configuration Protocol (DHCP)
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When ip helper-address is configured, the system listens for DHCP broadcast messages on port 67. The
system rewrites packets received from the client and forwards it via unicast; the system rewrites the
destination IP address and writes its own address as the relay device. Responses from the server are unicast
back to the relay agent on port 68 and the relay agent rewrites the destination address and forwards the
packet to the client subnet via broadcast.
HCP Relay Device
To view the ip helper-address configuration for an interface, use the command show ip interface from
EXEC privilege mode, as shown in the following example.
R1_E600#show ip int gig 1/3
GigabitEthernet 1/3 is up, line protocol is down
Internet address is 10.11.0.1/24
Broadcast address is 10.11.0.255
Address determined by user input
IP MTU is 1500 bytes
Helper address is 192.168.0.1
192.168.0.2
Directed broadcast forwarding is disabled
Proxy ARP is enabled
Split Horizon is enabled
Poison Reverse is disabled
ICMP redirects are not sent
ICMP unreachables are not sent
Note: DHCP Relay is not available on Layer 2 interfaces and VLANs.
DHCP 0
01
R1(conf-if-gi-1/3)#show config
!
interface GigabitEthernet 1/3
ip address 10.11.0.3/24
ip helper-address 10.11.1.5
ip helper-address 10.11.2.5
no shutdown
1/3
Unicast
DHCP Server
10.11.1.5
DHCP Server
10.11.2.5
Broadcast
Source IP : 10.11.1.5
Destination IP: 255.255.255.255
Source Port: 67
Destination Port: 68
Unicast
Source IP : 10.11.1.3
Destination IP: 10.11.1.5
Source Port: 67
Destination Port: 67
Relay Agent Address: 10.11.0.3
1/4
Unicast
Source IP : 10.11.1.5
Destination IP: 10.11.0.3
Source Port: 67
Destination Port: 68
Broadcast
Source IP : 0.0.0.0
Destination IP: 255.255.255.255
Source Port: 68
Destination Port: 67
Relay Agent Address: 0.0.0.0