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Reviewing IPv6
With the worldwide depletion of IP version 4 (IPv4) addresses, many organizations have
migrated, are in the process of migrating, or are considering migrating their IPv4 addresses
to IPv6 addresses. IPv6 dramatically increases the number of available IP addresses. In fact,
IPv6 offers approximately 5 * 1028 IP addresses for each person on the planet.
Beyond the increased address space, IPv6 offers many other features:
■ Simplified header.
■ IPv4 header: Uses 12 fields.
■ IPv6 header: Uses 5 fields.
■ No broadcasts.
■ No fragmentation (performs MTU discovery for each session).
■ Can coexist with IPv4 during a transition.
■ Dual stack (running IPv4 and IPv6 simultaneously).
■ IPv6 over IPv4 (tunneling IPv6 over an IPv4 tunnel).
This section reviews IPv6 address types, the IPv6 address structure, and routing options
for IPv6. Subsequent sections in this chapter address troubleshooting IPv6 routing protocols,
specifically OSPFv3 and RIPng.
IPv6 Address Types
IPv6 has three types of addresses:
■ Unicast
■ Multicast
■ Anycast
Unicast
With unicast, a single IPv6 address is applied to a single interface, as illustrated in Figure
12-1. The communication flow can be thought of as a one-to-one communication flow.
In Figure 12-1, a server (that is, AAAA::1) is sending traffic to a single client (that is,
AAAA::2).
Multicast
With multicast, a single IPv6 address (that is, a multicast group) represents multiple devices
on a network, as seen in Figure 12-2. The communication flow is one-to-many.
In Figure 12-2, a server (that is, AAAA::1) is sending traffic to a multicast group (that is,
FF00::A). Two clients (that is, AAAA::2 and AAAA::3) have joined this group. Those
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Chapter 12: IPv6 Troubleshooting 377
AAAA::2
Destination
Address: AAAA::2
AAAA::4
AAAA::1 AAAA::3
Figure 12-1 IPv6 Unicast Example
Multicast Group: FF00::A
AAAA::2
Destination
Address: FF00::A
AAAA::4
AAAA::1
AAAA::3
Figure 12-2 IPv6 Multicast Example
Anycast
With anycast, a single IPv6 address is assigned to multiple devices, as depicted in Figure
12-3. The communication flow is one-to-nearest (from the perspective of a router’s routing
table).
In Figure 12-3, a client with an IPv6 address of AAAA::1 wants to send traffic to a destination
IPv6 address of AAAA::2. Notice that two servers (that is, Server A and Server B)
have an IPv6 address of AAAA::2. In the figure, the traffic destined for AAAA::2 is sent
to Server A, via router R2, because the network on which Server A resides appears to be
closer than the network on which Server B resides, from the perspective of router R1’s
IPv6 routing table.
clients receive the traffic from the server, whereas any client that did not join the group
(for example, AAAA::4) does not receive the traffic.
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IPv6 Address Format
An IPv6 address has the following address format, where X equals a hexadecimal digit in
the range of 0–F:
XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX
A hexadecimal digit is four bits in size (that is, four binary bits can represent sixteen values).
Notice that an IPv6 address has eight fields, and each field contains four hexadecimal
digits. The following formula reveals why an IPv6 address is a 128-bit address:
4 bits per digit * 4 digits per field * 8 fields = 128 bits in an IPv6 address
Because IPv6 addresses can be difficult to work with, due to their size, the following rules
exist for abbreviating these addresses:
■ Leading zeros in a field can be omitted.
■ Contiguous fields containing all zeros can be represented with a double colon.
(NOTE: This can only be done once for a single IPv6 address.)
As an example, consider the following IPv6 address:
ABCD:0123:4040:0000:0000:0000:000A:000B
378 CCNP TSHOOT 642-832 Official Certification Guide
Destination
Address: AAAA::2
AAAA::2
AAAA::2
Server A
Server B
AAAA::1
R2
R3
R1
Figure 12-3 IPv6 Anycast Example
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Table 12-2 IPv6 Configuration Commands
Command Description
ipv6 cef Global configuration mode command that configures
Cisco Express Forwarding for IPv6.
ipv6 unicast-routing Global configuration mode command that instructs a
router to forward IPv6 traffic.
ipv6 address ipv6-
address/prefix-length [eui-64]
Interface configuration mode command that assigns an
IPv6 address to an interface. (NOTE: The eui-64 option
allows a router to complete the low-order 64 bits of an address,
based on an interface’s MAC address.)
Using the rules for abbreviation, the IPv6 address can be rewritten as
ABCD:123:4040::A:B
Also, the Extended Unique Identifier (EUI-64) format can be used to cause the router to
automatically populate the low-order 64 bits of an IPv6 address based on an interface’s
MAC address.
IPv6 Routing Options
IPv6 maintains a separate routing table from IPv4. Following are methods of populating
this IPv6 routing table:
■ Static routes: Configured similar to IPv4 static routes.
■ RIP next generation (RIPng): Has many of the same characteristics as RIPv2 (for
example, a distance vector routing protocol with a 15 hop-count maximum).
■ OSPFv3: Builds on OSPFv2 to add support for IPv6 network characteristics (for example,
128-bit network addresses and link-local addresses).
■ IS-IS for IPv6: Similar to IS-IS for IPv4, with a few IPv6 extensions added (for example,
new Type, Length, Value [TLV] attributes, and a new protocol ID).
■ Multiprotocol BGP: Allows BGP to route protocols other than IPv4 (for example,
IPv6).
■ EIGRP: Configured on the interfaces with IPv6 addressing, similar to OSPFv3.
Configuring IPv6 Support
As a reference, Table 12-2 presents basic IPv6 configuration commands.
Chapter 12: IPv6 Troubleshooting 379
Example 12-1 demonstrates how to enable a router to support IPv6 routing and how to assign
IPv6 addresses to router interfaces.
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Table 12-3 Commands Used to Tunnel IPv6 via IPv4
Command Description
interface tunnel interface-id Global configuration mode command that creates a
virtual IPv4 tunnel interface over which encapsulated
IPv6 packets can flow.
tunnel source ipv4-address Interface configuration mode command that identifies
the IPv4 address of the local end of a tunnel.
tunnel destination ipv4-address Interface configuration mode command that identifies
the IPv4 address of the remote end of a tunnel.
tunnel mode ipv6ip Interface configuration mode command that configures
an interface to act as a manual IPv6 tunnel.
ipv6 address ipv6-address/prefixlength
Interface configuration mode command that specifies
the IPv6 address assigned to a tunnel interface.
Example 12-1 IPv6 Configuration Example
R1# show run
...OUTPUT OMITTED...
ipv6 unicast-routing
ipv6 cef
!
interface FastEthernet0/0
ipv6 address B:B:B:B::2/64
!
interface FastEthernet0/1
ipv6 address A:A:A:A::2/64
...OUTPUT OMITTED...
As Example 12-1 demonstrates, IPv6 unicast routing is enabled with the ipv6 unicastrouting
command issued in global configuration mode. Cisco Express Forwarding support
for IPv6 is then globally enabled using the ipv6 cef command. Finally, under interface
configuration mode for both the Fast Ethernet 0/0 and 0/1 interfaces, IPv6 addresses are
assigned with the ipv6 address ipv6-address/prefix-length command.
Tunneling IPv6 Through an IPv4 Tunnel
When an enterprise is migrating from an IPv4 to IPv6 environment, there might be times
when IPv6 networks might be separated by one or more IPv4 networks. One option that
allows these different address formats to peacefully coexist is to tunnel IPv6 traffic over
an IPv4 tunnel.
Table 12-3 provides a command reference for setting up such a tunnel.
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Chapter 12: IPv6 Troubleshooting 381
Figure 12-4 depicts a topology tunneling IPv6 traffic over an IPv4 tunnel.
Examples 12-2 and 12-3 show the configuration of the depicted IPv6 over IPv4 tunnel.
Notice that each router specifies the source and destination IPv4 addresses of the tunnel,
from the perspective of each router. The tunnel interface is then assigned an IPv6 address,
told to participate in OSPF Area 1, and instructed to act as a manual IPv6 tunnel. The configuration
and troubleshooting of OSPFv3, which is the version of OSPF that supports
IPv6, is discussed in the next section.
Example 12-2 Configuring IPv6 Parameters on Router R1’s Tunnel Interface
Key
R1# show run Topic
...OUTPUT OMITTED...
interface tunnel 1
tunnel source 192.168.1.1
tunnel destination 192.168.1.2
ipv6 address b:b:b:b::1/64
ipv6 ospf 1 area 1
tunnel mode ipv6ip
Example 12-3 Configuring IPv6 Parameters on Router R2’s Tunnel Interface
R2# show run
...OUTPUT OMITTED...
interface tunnel 1
tunnel source 192.168.1.2
Table 12-3 Commands Used to Tunnel IPv6 via IPv4
Command Description
ipv6 ospf process-id area area-id Interface configuration mode command that allows
the IPv6 address configured on a tunnel interface to
participate in an OSPFv3 routing process.
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