Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.0P1 - Page 296
Configuring the DHCP secondary-subnet, Drop DHCP Packets on Snooped VLANs Only
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Debugging the IPv6 DHCP To debug the IPv6 DHCP, use the following command. • Display debug information for IPV6 DHCP. EXEC Privilege mode debug ipv6 dhcp IPv6 DHCP Snooping MAC-Address Verification Configure to enable verify source mac-address in the DHCP packet against the mac address stored in the snooping binding table. • Enable IPV6 DHCP snooping . CONFIGURATION mode ipv6 dhcp snooping verify mac-address 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. 296 Dynamic Host Configuration Protocol (DHCP)
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