Configuring DHCP Flashcards
What is the DHCP Client Identifier?
The DHCP Client -Identifier is the client’s MAC address preceeded by the hex digits 0001
How long are DHCP leases by default?
What is the command to change lease times?
DHCP leases are 24 hours by default.
lease xxx
What is DHCPv6 Lite allow for?
It allows for stateless auto config with the use of DHCP options
What is the command for a DHCPv6 relay agent?
ipv6 dhcp relay destination x.x.x.x
Where and what do you configure to let clients running stateless auto-config know that there are DHCP options?
On the vlan interface:
ipv6 dhco server v6-users
ipv6 nd other-config-ekey
What is the command to show DHCP bindings?
What is the command to clear them (or just one)?
show ip DHCP binding
clear ip DHCP binding
Can you exclude addresses with DHCPv6?
There is currently no equivalent of ‘ip dhcp excluded-address’ for IPv6 in Cisco IOS.
What is DHCP option 150?
TFTP server for Cisco phones
What is DHCP option 43?
Location of a WLAN controller for lightweight access points
What is the command to observe DHCP process?
debug ip dhcp server
What is DHCP option 69?
SMTP server
What is DHCP option 70?
Location of POP3 mail server
What is the prefix for Link-Local Address?
What is a Link-Local Address?
FE80::/10
A link-local address is an IPv6 unicast address that can be automatically configured on any interface using the link-local prefix FE80::/10 (1111 1110 10) and the interface identifier in the modified EUI-64 format.
What is the command to configure a DNS server and domain name in a DHCP pool?
dns-server xxx
domain-name xxx
What are the 4 steps in the DHCP (client) Process?
The 4 basic steps of the DHCP Client Process are:
- Client sends DHCPDISCOVER (Broadcast)
- DHCP Server responds with DHCPOFFER (Unicast)
- Client sends DHCPREQUEST (Broadcast)
- DHCP Server responds with DHCPACK (Unicast)
A good way to remember this is the acronym DORA: D: Discover O: Offer R: Request A: Acknowledgement
Regarding Broadcast/Unicast delivery as per this RFC:
Droms Standards Track [Page 24] RFC 2131 Dynamic Host Configuration Protocol March 1997
Normally, DHCP servers and BOOTP relay agents attempt to deliver DHCPOFFER, DHCPACK and DHCPNAK messages directly to the client using uicast delivery. The IP destination address (in the IP header) is set to the DHCP ‘yiaddr’ address and the link-layer destination address is set to the DHCP ‘chaddr’ address. Unfortunately, some client implementations are unable to receive such unicast IP datagrams until the implementation has been configured with a valid IP address (leading to a deadlock in which the client’s IP address cannot be delivered until the client has been configured with an IP address).
A client that cannot receive unicast IP datagrams until its protocol software has been configured with an IP address SHOULD set the BROADCAST bit in the ‘flags’ field to 1 in any DHCPDISCOVER or DHCPREQUEST messages that client sends. The BROADCAST bit will provide a hint to the DHCP server and BOOTP relay agent to broadcast any messages to the client on the client’s subnet. A client that can receive unicast IP datagrams before its protocol software has been configured SHOULD clear the BROADCAST bit to 0. The BOOTP clarifications document discusses the ramifications of the use of the BROADCAST bit [21].
A server or relay agent sending or relaying a DHCP message directly to a DHCP client (i.e., not to a relay agent specified in the ‘giaddr’ field) SHOULD examine the BROADCAST bit in the ‘flags’ field. If this bit is set to 1, the DHCP message SHOULD be sent as an IP broadcast using an IP broadcast address (preferably 0xffffffff) as the IP destination address and the link-layer broadcast address as the link-layer destination address. If the BROADCAST bit is cleared to 0, the message SHOULD be sent as an IP unicast to the IP address specified in the ‘yiaddr’ field and the link-layer address specified in the ‘chaddr’ field. If unicasting is not possible, the message MAY be sent as an IP broadcast using an IP broadcast address (preferably 0xffffffff) as the IP destination address and the link- layer broadcast address as the link-layer destination address.
