Chapter 24: Implementing IPv6 Addressing on Routers

Question 1

True or False. IPv6 routing is enabled by default on Cisco routers.

Answer 1

False. The command ‘ipv6 unicast-routing’ will need to be entered.

Question 2

What is Modified EUI-64?

Answer 2

• Modified Extended Unique Identifier
• A way of automatically assigning the second part of a prefixed IPv6 address. (e.g. if you enter FE80::/64 it will work out that the prefix is FE80:0:0:0 and then generate the interface ID using the below method)
  - Split the MAC address of the device/interface in half
  - Insert FFFE in between the two halfs
  - Invert the seventh bit of the interface ID (if it’s a 0 change it to 1 and vice versa)

Question 3

What do you add to the end of what command to configure an interface with an EUI64 generated address?

Answer 3

• EUI-64 after the address and prefix
• The full command is ‘ipv6 address <address> <prefix> eui-64</prefix>

Question 4

True or False. If an interface has a different MAC address configured against it to it’s burned-in address it will use the configured address.

Answer 4

True.

Question 5

If EUI-64 is used to dynamically assign an IPv6 address to an interface that doesn’t have a MAC (e.g. Serial) what will the device use?

Answer 5

The MAC of the lowest-numbered interface that does have a MAC.

Question 6

Which commands allow a router interface to receive an address via DHCPv6 and SLAAC?

Answer 6

• DHCPv6 - ipv6 address DHCP
• SLAAC - Ipv6 address autoconfig

Question 7

What are some examples of reserverd IPv6 Multicast addresses?

Answer 7

• FF02::1 - All hosts that use IPv6 on that link (Equivalent to IPv4 224.0.0.1)
• FF02::2 - All router interfaces that use IPv6 on that link (Equivalent to IPv4 224.0.0.2)
• FF02::5 - All OSPFv3 routers (Equivalent to IPv4 224.0.0.5)
• FF02::6 - All OSPFv3 DRs (Equivalent to IPv4 224.0.0.6)
• FF02::9 - All RIPng routers (Equivalent to IPv4 224.0.0.9)
• FF02::A - All EIGRPv6 routers (Equivalent to 224.0.0.10)
• FF02::1:2 - All routers acting as a DHCPv6 relay agent
• FF02::1:FF/104 - Solicited-node Multicast address

Question 8

Define the difference between
- Link Local (Unicast) Address
- Link Local Multicast Address
- Link Local (Multicast) Scope

Answer 8

• Link Local Address - An IPv6 address that begins FE80. Serves as an address for an interface to which devices apply a link-local scope (traffic sent here cannot be forwarded by routers)
• Link Local Multicast Address - An IPv6 address that begins with FF02. Serves as a reserved multicast address to which devices apply a link-local scope
• Link Local Scope - An IPv6 scope itsself rather than an address. The scope defines the borders within which a packet should and should not be forwarded.

Question 9

True or False. IPv6 Multicast requires less processing by hosts than IPv4 Multicast.

Answer 9

True. This means they can be discarded quicker by hosts that don’t require them.

Question 10

Why are Solicited Node Multicast Groups necessary?

Answer 10

Since ARP is replaced by NDP in IPv6, IPv4 Broadcast ARP is also replaced by IPv6 Multicast. A device attempting to discover a neighbor would know (based on the IPv6 address of the neighbor) the Solicited Node Multicast Group that the neighbor is part of (FF02::1:FF appended with the last 6 hex digits of the neighbor’s address) so it would send an ND message with this as the destination. As the scope is FF02:1 it will be recieved by all nodes on the link but only the device in the Solicited Node Multicast Group will process it.

Question 10

What is a Solicited Node Multicast address?

Answer 10

• An address associated with a Unicast address that is specifically used for NDP. These will be shown in the below format
  - FF02::1:FF…the last 6 hex digits are also the last 6 digits of the associated unicast address.

Question 11

What is the “Unkown” IPv6 address?

Answer 11

• A sequence of all 0s or simply ‘::’. This is used when a host doesn’t yet know it’s own IPv6 address. Similar to 169 addresses in IPv4.

Question 12

True or False. There is an IPv6 equivalent to 127 loopback addresses.

Answer 12

True. It is ::1 and acts in the same way.

Question 13

What is a Solicited Node Multicast address?

Answer 13

• An address associated with a Unicast address that is specifically used for NDP. These will be shown in the below format
  - FF02::1:FF…the last 6 hex digits are also the last 6 digits of the associated unicast address.

Question 13

What is a Multicast address scope? Provide examples

Answer 13

• An address scope defines a set of rules that dictate whether routers should forward a packet or not (depending on its destination address) and how far that packet is allowed to be forwarded.
• FF01- Interface Local - Defined by device - Packet remains within the device (e.g. Can be used like a loopback address)
• FF02 - Link Local - Defined by device - Packet can only be sent on the same link and cannot be forwarded by a router
• FF05 - Site Local - Configured on router - Intended to be broader than link local but narrower than organization local. Generally limits packets to not being able to cross WAN links
• FF08 - Organization Local - Configured on router - Intended to be broader than Site Local. Generally for an entire company or organization
• FF0E - Global - No boundaries

Question 13

What is an IPv6 Anycast address?

Answer 13

• When a host sends traffic to an Anycast address, the next hop router will forward the traffic to the nearest (based on routing protocol metric) router that has this address configured.
• Multiple routers can have this same address configured as it generally corresponds to a specific service that is configured on multiple routers.
• This means that the traffic would not need to be forwarded all the way to a specific router that supports this service.

Question 14

How does a router calculate the Link Local address of an interface where a Global Unicast address is present but the Link Local address has not already been assigned?

Answer 14

• EUI-64 rules are used. Add FFFE into the middle and invert the 7th bit.

Question 15

Why does EUI-64 invert the 7th bit?

Answer 15

• The 7th bit of a MAC address defines whether the MAC address is a UAA or an LAA. It is called the U/L (Universal/Local) Bit.
  - UAA - Universally Administered Address - Assigned by the manufacturer
  - LAA - Locally Administered Address - Assigned by an admin or protocol. Can be input manually on a Cisco interface using
  the mac-address command.
• If the U/L bit is 1 then the MAC is an LAA. If the U/L bit is 0 then the MAC is a UAA. However in the context of IPv6 this is reversed.

Question 16

What is the command to configure an Anycast address?

Answer 16

Just add ‘anycast’ after any IPv6 address you configure on an interface.

Question 17

What is the correct way to display the “Unspecified” IPv6 address and what is it?

Answer 17

• Displayed as ‘::’
• IPv6 equivalent of 0.0.0.0
• Can be used when a device doesn’t know its IPv6 address yet

Question 18

What is the IPv6 loopback address?

Answer 18

• Displayed as ‘::1’
• Same as the IPv4 loopback address
• IPv6 equivalent to 127.0.0.1