DHCP & DHCPv6

Question 1

Which DHCP option provides a TFTP server that Cisco phones can use to download a configuration?

A. DHCP Option 66
B. DHCP Option 68
C. DHCP Option 82
D. DHCP Option 57

Answer: A

DHCP options 3, 66, and 150 are used to configure Cisco IP Phones. Cisco IP Phones download their configuration from a TFTP server. When a Cisco IP Phone starts, if it does not have both the IP address and TFTP server IP address preconfigured, it sends a request with option 150 or 66 to the DHCP server to obtain this information.
+ DHCP option 150 provides the IP addresses of a list of TFTP servers.
+ DHCP option 66 gives the IP address or the hostname of a single TFTP server.

Question 2

Which three options are valid DHCPv6 functions? (Choose three)

A. Server
B. Client
C. Approver
D. Requester
E. ACK
F. Relay

Answer: A B F

Most vendor’s routers/switches have the ability to function as:
+ A DHCP client and obtain an interface IPv4 address from an upstream DHCP service
+ A DHCP relay and forward UDP DHCP messages from clients on a LAN to and from a DHCP server
+ A DHCP server whereby the router/switch services DHCP requests directly

Question 3

After testing various dynamic IPv6 address assignment methods, an engineer decides that more control is needed when distributing addresses to clients. Which two advantages does DHCPv6 have over EUI-64 (Choose two)

A. DHCPv6 requires less planning and configuration than EUI-64 requires.
B. DHCPv6 allows for additional parameters to be sent to the client, such as the domain name and DNS server.
C. DHCPv6 providers tighter control over the IPv6 addresses that are distributed to clients.
D. DHCPv6 does not require the configuration of prefix pools.
E. DHCPv6 does not require neighbor and router discovery on the network segment.

Answer: B C

Extended Unique Identifier (EUI) allows a host to assign itself a unique 64-Bit IPv6 interface identifier (EUI-64). This feature is a key benefit over IPv4 as it eliminates the need of manual configuration or DHCP as in the world of IPv4. The IPv6 EUI-64 format address is obtained through the 48-bit MAC address. The MAC address is first separated into two 24-bits, with one being OUI (Organizationally Unique Identifier) and the other being NIC specific. The 16-bit 0xFFFE is then inserted between these two 24-bits for the 64-bit EUI address. IEEE has chosen FFFE as a reserved value which can only appear in EUI-64 generated from the an EUI-48 MAC address.

Question 4

Got a diagram and asked how to configure ipv6 dhcp relay

A. DHCPv6 server facing Interface:
ipv6 address autoconfig
ipv6 enable
exit
In clients facing interface
ipv6 address
ipv6 dhcp relay destination

B. DHCPv6 server facing Interface:
ipv6 dhcp relay destination
ipv6 address
exit
In clients facing interface
ipv6 enable
ipv6 address autoconfig

Answer: A

Please notice that the “ipv6 address autoconfig” is configured on the DHCP Relay Agent (not DHCP Server). A configuration example can be found at https://community.cisco.com/t5/networking-documents/stateful-dhcpv6-relay-configuration-example/ta-p/3149338

Question 5

Which three configuration parameters can a DHCPv6 pool contain? (Choose three)

A. domain search list
B. router IP
C. default gateway
D. prefix delegation
E. DNS servers
F. subnet mask

Answer: A D E

A DHCPv6 configuration information pool is a named entity that includes information about available configuration parameters and policies that control assignment of the parameters to clients from the pool. A pool is configured independently of the DHCPv6 service and is associated with the DHCPv6 service through the command-line interface (CLI).
Each configuration pool can contain the following configuration parameters and operational information:
Prefix delegation information, which could include:
+ A prefix pool name and associated preferred and valid lifetimes
+ A list of available prefixes for a particular client and associated preferred and valid lifetimes
– A list of IPv6 addresses of DNS servers
– A domain search list, which is a string containing domain names for DNS resolution

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/configuration/15-2mt/ipv6-15-2mt-book/ip6-dhcp.html

This is how to configure a DHCPv6 pool:

ipv6 unciast-routing
ipv6 dhcp pool <pool name>
address prefix <specify address prefix> lifetime <infinite> <infinite>
dns-server <specify the dns server address>
domain-name <specify the domain name>

For example:

ipv6 dhcp pool test
address prefix 2010:AA01:10::/64 lifetime infinite infinite
dns-server AAAA:BBBB:10FE:100::15
dns-server 2010:AA01::15
domain-name example.com

Reference: https://supportforums.cisco.com/document/116221/part-1-implementing-dhcpv6-stateful-dhcpv6

So we can see DHCPv6 pool supports address prefix and domain search list, DNS servers.

Question 6

Consider this scenario. TCP traffic is blocked on port 547 between a DHCPv6 relay agent and a DHCPv6 server that is configured for prefix delegation. Which two outcomes will result when the relay agent is rebooted? (Choose two)

A. Routers will not obtain DHCPv6 prefixes.
B. DHCPv6 clients will be unreachable.
C. Hosts will not obtain DHCPv6 addresses.
D. The DHCPv6 relay agent will resume distributing addresses.
E. DHCPv6 address conflicts will occur on downstream clients.

Answer: A D

Note: A DHCPv6 relay agent is used to relay (forward) messages between the DHCPv6 client and server.

Servers and relay agents listen for DHCP messages on UDP port 547 so if a DHCPv6 relay agent cannot receive DHCP messages (because of port 547 is blocked) then the routers (clients) will not obtain DHCPv6 prefixes.

We are not sure about answer D but maybe it is related to the (absence of) “Reload Persistent Interface ID” in DHCPv6 Relay Options. This feature makes the interface ID option persistent. The interface ID is used by relay agents to decide which interface should be used to forward a RELAY-REPLY packet. A persistent interface-ID option will not change if the router acting as a relay agent goes offline during a reload or a power outage. When the router acting as a relay agent returns online, it is possible that changes to the internal interface index of the relay agent may have occurred in certain scenarios (such as, when the relay agent reboots and the number of interfaces in the interface index changes, or when the relay agents boot up and has more virtual interfaces than it did before the reboot). This feature prevents such scenarios from causing any problems.

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_dhcp/configuration/15-e/dhcp-15-e-book/dhcp-15e-book_chapter_010.html

Question 7

Refer to the exhibit. Router DHCP is configured to lease IPv4 and IPv6 addresses to clients on ALS1 and ALS2. Clients on ALS2 receive IPv4 and IPv6 addresses. Clients on ALS1 receive IPv4 addresses. Which configuration on DSW1 allows clients on ALS1 to receive IPv6 addresses?

DSW1#sh run int f0/0
Building configuration…
!
interface FastEhternet0/0
ip address 10.4.10.1 255.255.255.0
ip helper-address 4.4.4.4
duplex auto
speed auto
ipv6 address 2002:A04:A01:A04:A01/120
ipv6 enable
end

A. DSW1(config-if)#ipv6 helper address 2002:404:404::404:404
B. DSW1(config)#ipv6 route 2002:404:404::404:404/128 FastEthernet1/0
C. DSW1(dhcp-config)#default-router 2002:A04:A01::A04:A01
D. DSW1(config-if)#ipv6 dhcp relay destination 2002:404:404::404:404 GigabitEthernet1/2

Answer: D

In this topology DSW1 is the DHCPv6 Relay agent so it should relay (forward) the DHCPv6 Request packets (from the clients) out of its Gi1/2 interface to the DHCPv6 server. The command “ipv6 dhcp relay destination …” is used to complete this task.

Note: There is no “default-router” command for DHCPv6. The “ipv6 dhcp relay destination” is not required to configure on every router along the path between the client and server. It is ONLY required on the router functioning as the DHCPv6 relay agent.

Reference: http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enterprise-ipv6-solution/whitepaper_c11-689821.html

Question 8

Refer in the exhibit.

interface FastEthernet0/0
ip helper-address 192.168.145.5

A packet capture indicates that the router is not forwarding the DHCP packets that it receives on interface FastEthernet0/0. Which command needs to be entered in global configuration mode to resolve this issue?

A. ip helper-address
B. ip DHCP relay
C. service DHCP
D. ip forward-protocol

Answer: B

The “ip helper-address” command is only configured in interface mode so it is not the correct answer.

Note: The Cisco IOS software provides the global configuration command “ip forward-protocol” to allow an administrator to forward any UDP port in addition to the eight default UDP Services. For example, to forward UDP on port 517, use the global configuration command “ip forward-protocol udp 517”. But the eight default UDP Services include DHCP services so it is not the suitable answer.

Reference and good resource: http://www.ciscopress.com/articles/article.asp?p=330807&seqNum=9

A DHCP relay agent may receive a message from another DHCP relay agent that already contains relay information. By default, the relay information from the previous relay agent is replaced. If this behavior is not suitable for your network, you can use the ip dhcp relay information policy {drop | keep | replace} global configuration command to change it -> Therefore this is the correct answer.

Reference: https://www.cisco.com/en/US/docs/ios/12_4t/ip_addr/configuration/guide/htdhcpre.html

Question 9

Refer to the exhibit. The DHCP client is unable to receive a DHCP address from the DHCP server. Consider the following output:

hostname R2
!
interface fastethernet 0/0
ip address 172.31.1.1 255.255.255.0
interface serial 0/0
ip address 10.1.1.1 255.255.255.252
!
ip route 172.16.1.0 255.255.255.0 10.1.1.2

Which configuration is required on the R2 fastethernet 0/0 port in order to allow the DHCP client to successfully receive an IP address from the DHCP server?

A. R2(config-if)# ip helper-address 172.16.1.2
B. R2(config-if)# ip helper-address 172.16.1.1
C. R2(config-if)# ip helper-address 172.31.1.1
D. R2(config-if)# ip helper-address 255.255.255.255

Answer: A

If the DHCP Server is not on the same subnet with the DHCP Client, we need to configure the router on the DHCP client side to act as a DHCP Relay Agent so that it can forward DHCP messages between the DHCP Client & DHCP Server. To make a router a DHCP Relay Agent, simply put the “ip helper-address <IP-address-of-DHCP-Server>” command under the interface that receives the DHCP messages from the DHCP Client.

Question 10

DHCPv6 can obtain configuration parameters from a server through rapid two-way message exchange. Which two steps are involved in this process? (Choose two)

A. solicit
B. advertise
C. request
D. auth
E. reply

Answer: A E