Users Guide

Configuring the DHCP secondary-subnet
DHCP Secondary subnet feature is an extended feature of DHCP relay agent. After enabling DHCP Relay Secondary Subnet, relay agent
tries to get IP Address from secondary IP Address subnet pool, only when relay agent fails to get IP Address through primary IP Address.
1 To enable all the DHCP relay secondary — subnet interfaces.
CONFIGURATION mode
ip dhcp relay secondary-subnet
2 Sample Configuration:
Dell(conf)#ip dhcp relay secondary-subnet
Dell(conf)#interface TenGigabitEthernet 0/0
Dell(conf-if-te-0/0)#ip address 10.1.1.1/24
Dell(conf-if-te-0/0)#ip address 11.1.1.1/24 secondary
Dell(conf-if-te-0/0)#ip helper-address 2.1.1.1
Dell(conf-if-te-0/0)#no shutdown
Dell(conf-if-te-0/0)#
DHCP relay tries to get ip address for the client, through configured primary address filling giaddr (relay address) 10.1.1.1. If there is no
OFFER from DHCP Server, for three DISCOVER retries. Since secondary-subnet enabled, DHCP Relay Agent tries to get ip address
through secondary ip address filling giaddr (relay address) 11.1.1.1.
3 To disable the DHCP relay secondary-subnet:
Dell(conf)# no ip dhcp relay secondary-subnet
Drop DHCP Packets on Snooped VLANs Only
Binding table entries are deleted when a lease expires or the relay agent encounters a DHCPRELEASE.
Line cards maintain a list of snooped VLANs. When the binding table fills, DHCP packets are dropped only on snooped VLANs, while such
packets are forwarded across non-snooped VLANs. Because DHCP packets are dropped, no new IP address assignments are made.
However, DHCP release and decline packets are allowed so that the DHCP snooping table can decrease in size. After the table usage falls
below the maximum limit of 4000 entries, new IP address assignments are allowed.
To view the number of entries in the table, use the show ip dhcp snooping binding command. This output displays the snooping
binding table created using the ACK packets from the trusted port.
Dell#show ip dhcp snooping binding
Codes : S - Static D - Dynamic
IP Address MAC Address Expires(Sec) Type VLAN Interface
================================================================
10.1.1.251 00:00:4d:57:f2:50 172800 D Vl 10 Te 1/2
10.1.1.252 00:00:4d:57:e6:f6 172800 D Vl 10 Te 1/1
10.1.1.253 00:00:4d:57:f8:e8 172740 D Vl 10 Te 1/3
10.1.1.254 00:00:4d:69:e8:f2 172740 D Vl 10 Te 1/5
Total number of Entries in the table : 4
296
Dynamic Host Configuration Protocol (DHCP)