IPv6 Questions 3

Here you will find answers to IPv6 Questions – Part 3

Question 1

During the IPv6 autoconfiguration, what does the device append to the 64-bit prefix that it receives from the router to create its IPv6 address?

A. a pseudorandom generated number
B. its locally configured IPv4 address
C. the DHCP-supplied device ID
D. its MAC address

 

Answer: D

Explanation

The automatic configuration is a great feature of IPv6. Imagine you have to manually configure an IPv6 address with 128-bit long, what a pain! With this feature, it is no longer necessary to configure each host manually. But notice that host only autonomously configures its own Link-local address (the IP address used on a LAN). The Link-local address can be created automatically using a link-local prefix of FE80::/10 and a 64-bit interface identifier (based on 48-bit MAC address).

For example, if your MAC address is 00:12:34:56:78:9a, your 64-bit interface identifier is 0012:34FF:FE56:789a (16-bit FFFE is inserted in the middle). And notice that the notation has been changed because IPv6 addresses require 16-bit pieces to be separated by “:”.

Then, according to the RFC 3513 we need to invert the Universal/Local bit (“U/L” bit) in the 6th position of the first octet (start counting from 0). The “u” bit is set to 1 to indicate Universal, and it is set to zero (0) to indicate local scope. In this case we set this bit to 1 because the MAC address is universally unique. Thus the result is: 0212:34FF:FE56:789a (0012 = 0000 0000 0001 0010 -> turn on 6th bit we get 0000 0010 0001 0010 which is 0212).

Finally, add the link-local prefix FE80 to create the full IPv6 address: FE80:0:0:0:0212:34FF:FE56:789a (or FE80::212:34FF:FE56:789a, in short form).

Note: The reason for inverting the “U/L” bit is to allow ignoring it for short values in the manual configuration case. For example, you can manually assign the short address fc80::1 instead of the long fc80:0:0:0:0200::1.

Question 2

Which three are characteristics of IPv6? (Choose three)

A. An IPv6 address is 128 bits long.
B. An IPv6 header is 20 bits long.
C. An IPv6 header contains the next header field.
D. An IPv6 header contains the protocol field.
E. IPv6 routers send RA messages.
F. An IPv6 header contains the header checksum field.

 

Answer: A C E

Explanation

A & C are obviously correct based on the theory of IPv6.

When a client sends a Router Solicitation (RS) message, router responds with a Router Advertisement (RA) message which includes prefix, default route and lifetime (how long the host should retain information about the router) -> E is correct.

Question 3

When an IPv6 enabled host boots, it sends a router solicitation (RS) message. An IPv6 router responds with a router advertisement (RA). Which two items are contained in the RA? (Choose two)

A. IPv6 address for the host
B. lifetime of the prefix
C. prefixes for the link
D. keepalive timers
E. request for the local host IP address
F. any route advertisements it has received

 

Answer: B C

Question 4

What is IPv6 router solicitation?

A. a request made by a node to join a specified multicast group
B. a request made by a node for its IP address
C. a request made by a node for the IP address of the DHCP server
D. a request made by a node for the IP address of the local router

 

Answer: D

Question 5

Which statement is true about IPv6?

A. Only one IPv6 address is assigned per node.
B. Only one IPv6 address can be assigned to each interface.
C. Each host can autoconfigure its address without the aid of a DHCP setver.
D. IPv6 hosts use anycast addresses to assign IP addresses to interfaces.

 

Answer: C

Question 6

What does the command clear ipv6 ospf process accomplish?

A. The OSPF adjacencies are cleared and initiated again.
B. The route table is cleared. Then the OSPF neighbors are reformed.
C. The shortest path first (SPF) algorithm is performed on the LSA database.
D. The OSPF database is repopulated. Then the shortest path first (SPF) algorithm is performed.

 

Answer: D

Explanation

The command “clear ipv6 ospf” will clear the present routing table and force the OSPFv3 process to build a new one. This command is often used when something in the network was changed or for debugging purpose.

When the “process” keyword is added, which means “clear ipv6 ospf process”, the OSPF database is cleared and repopulated then the SPF algorithm is performed.

Question 7

Which statement is true about the command ipv6 ospf 1 area 0?

A. It must be issued in router global configuration mode to enable the OSPF process for IPv6.
B. It must be issued in interface configuration mode to enable the OSPF process for IPv6.
C. It must be issued before the network command to enable the OSPF process for IPv6.
D. It must be issued after the network command to enable the OSPF process for IPv6.

 

Answer: B

Question 8

Your trainee asks you, in the context of IPv6 and OSPF, what best describes a type 9 LSA? What should you tell her?

A. Link LSA
B. Interarea prefix LSA for ABRs
C. Router LSA
D. Switch LSA
E. Intra-area prefix LSA
F. None of the above

 

Answer: E

Question 9

You have been tasked with setting up OSPF on an existing router using IPv6. Which command enables OSPF for IPv6 on a router?

A. ipv6 router ospf process-id
B. router ospf process-id
C. router ospf ipv6 process-id
D. ipv6 ospf process-id area-id
E. None of the above

 

Answer: A

Explanation

The command “ipv6 router ospf process-id” is used to enable an OSPF process on the router. For example:

Router(config)#ipv6 router ospf 1

Note: This command is used in global configuration mode.

Question 10

Which statement is true about the implementation of IPv6 in an already existing IPv4 network?

A. IPv6 can be routed using the same routing protocol versions as IPv4
B. A router routing for IPv6 and IPv4 must convert IPv4 packets to IPv6 packets to route them.
C. IPv4 and IPv6 networks can be routed simultaneously.
D. Only OSPF version 3 can be utilized for routing IPv4 and IPv6.

 

Answer: C

Explanation

There are many techniques to run implement IPv6 in an already existing IPv4 network. Maybe answer C wants to mention about “dual-stacking” technique. This is the most common technique which only requires edge routers to run both IPv4 and IPv6 while the inside routers only run IPv4. At the edge network, IPv4 packets are converted to IPv6 packets before sending out.

Question 11

Refer to the exhibit. Which statement about this neighbor of R1 is true?

A. OSPFv3 adjacency has been lost, which causes the neighbor to be considered Stale.
B. Aggregate global addresses are always used between IPv6 neighbors.
C. OSPFv3 adjacency will not work between link-local addresses.
D. R1 used ICMP to learn about this neighbor.

 

Answer: D

Explanation

The “show ipv6 neighbor” command is used to display IPv6 neighbor discovery cache information. We notice that the neighbor IPv6 address is a Link Local address (in the range of FE80::/10) so this neighbor was discovered via the neighbor discovery process. The IPv6 neighbor discovery process uses ICMP messages and solicited-node multicast addresses to determine the link-layer address of a neighbor on the same network (local link), to verify the reachability of the neighbor, and to keep track of neighboring routers.

The STALE state indicates more than ReachableTime milliseconds have elapsed since the last positive confirmation was received that the forward path was functioning properly. While in stale state, the device takes no action until a packet is sent.

Note: Nodes on a local link use link-local addresses and do not require globally unique addresses to communicate. IPv6 routers do not forward packets with link-local source or destination addresses to other links.

Question 12

Which IPv6 address correctly compresses the IPv6 unicast address 2001:0:0:0:0DB8:0:0:417A?

A. 2001:0DB8:417A
B. 2001::0DB8::417A
C. 2001:::0DB8::417A
D. 2001:0DB8:0:0:417A
E. 2001::DB8:0:0:417A
F. 2001:::0DB8:0:0:417A

 

Answer: E