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The IPv6 neighbor discovery process uses Internet Control Message Protocol (ICMP) messages and solicited-node multicast addresses
to determine the link-layer address of a neighbor on the same network (local link), verify the reachability of a neighbor,
and track neighboring devices.
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Information About IPv6 Neighbor Discovery
IPv6 Neighbor Discovery
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), verify the reachability of a neighbor, and track neighboring devices.
The IPv6 static cache entry for neighbor discovery feature allows static entries to be made in the IPv6 neighbor cache. Static
routing requires an administrator to manually enter IPv6 addresses, subnet masks, gateways, and corresponding Media Access
Control (MAC) addresses for each interface of each device into a table. Static routing enables more control but requires more
work to maintain the table. The table must be updated each time routes are added or changed.
IPv6 Neighbor Solicitation Message
A value of 135 in the Type field of the ICMP packet header identifies a neighbor solicitation message. Neighbor solicitation
messages are sent on the local link when a node wants to determine the link-layer address of another node on the same local
link (see the figure below). When a node wants to determine the link-layer address of another node, the source address in
a neighbor solicitation message is the IPv6 address of the node sending the neighbor solicitation message. The destination
address in the neighbor solicitation message is the solicited-node multicast address that corresponds to the IPv6 address
of the destination node. The neighbor solicitation message also includes the link-layer address of the source node.
After receiving the neighbor solicitation message, the destination node replies by sending a neighbor advertisement message,
which has a value of 136 in the Type field of the ICMP packet header, on the local link. The source address in the neighbor
advertisement message is the IPv6 address of the node (more specifically, the IPv6 address of the node interface) sending
the neighbor advertisement message. The destination address in the neighbor advertisement message is the IPv6 address of the
node that sent the neighbor solicitation message. The data portion of the neighbor advertisement message includes the link-layer
address of the node sending the neighbor advertisement message.
After the source node receives the neighbor advertisement, the source node and destination node can communicate.
Neighbor solicitation messages are also used to verify the reachability of a neighbor after the link-layer address of a neighbor
is identified. When a node wants to verify the reachability of a neighbor, the destination address in a neighbor solicitation
message is the unicast address of the neighbor.
Neighbor advertisement messages are also sent when there is a change in the link-layer address of a node on a local link.
When there is such a change, the destination address for the neighbor advertisement is the all-nodes multicast address.
Neighbor solicitation messages are also used to verify the reachability of a neighbor after the link-layer address of a neighbor
is identified. Neighbor unreachability detection identifies the failure of a neighbor or the failure of the forward path to
the neighbor, and is used for all paths between hosts and neighboring nodes (hosts or devices). Neighbor unreachability detection
is performed for neighbors to which only unicast packets are being sent and is not performed for neighbors to which multicast
packets are being sent.
A neighbor is considered reachable when a positive acknowledgment is returned from the neighbor (indicating that packets
previously sent to the neighbor have been received and processed). A positive acknowledgment from an upper-layer protocol
(such as TCP) indicates that a connection is making forward progress (reaching its destination) or the receipt of a neighbor
advertisement message in response to a neighbor solicitation message. If packets are reaching the peer, they are also reaching
the next-hop neighbor of the source. Therefore, forward progress is also a confirmation that the next-hop neighbor is reachable.
For destinations that are not on the local link, forward progress implies that the first-hop device is reachable. When acknowledgments
from an upper-layer protocol are not available, a node probes the neighbor using unicast neighbor solicitation messages to
verify that the forward path is still working.
The return of a solicited neighbor advertisement message from the neighbor is a positive acknowledgment that the forward
path is still working (neighbor advertisement messages that have the solicited flag set to a value of 1 are sent only in response
to a neighbor solicitation message). Unsolicited messages confirm only the one-way path from the source to the destination
node; solicited neighbor advertisement messages indicate that a path is working in both directions.
Note
A neighbor advertisement message that has the solicited flag set to a value of 0 must not be considered as a positive acknowledgment
that the forward path is still working.
Neighbor solicitation messages are also used in the stateless autoconfiguration process to verify the uniqueness of unicast
IPv6 addresses before the addresses are assigned to an interface. Duplicate address detection is performed first on a new,
link-local IPv6 address before the address is assigned to an interface (the new address remains in a tentative state while
duplicate address detection is performed). Specifically, a node sends a neighbor solicitation message with an unspecified
source address and a tentative link-local address in the body of the message. If another node is already using that address,
the node returns a neighbor advertisement message that contains the tentative link-local address. If another node is simultaneously
verifying the uniqueness of the same address, that node also returns a neighbor solicitation message. If no neighbor advertisement
messages are received in response to the neighbor solicitation message and no neighbor solicitation messages are received
from other nodes that are attempting to verify the same tentative address, the node that sent the original neighbor solicitation
message considers the tentative link-local address to be unique and assigns the address to the interface.
Every IPv6 unicast address (global or link-local) must be verified for uniqueness on the link; however, until the uniqueness
of the link-local address is verified, duplicate address detection is not performed on any other IPv6 addresses associated
with the link-local address. The Cisco implementation of duplicate address detection in the Cisco software does not verify
the uniqueness of anycast or global addresses that are generated from 64-bit interface identifiers.
IPv6 Router Advertisement
Message
Router advertisement
(RA) messages, which have a value of 134 in the Type field of the ICMP packet
header, are periodically sent out each configured interface of an IPv6 router.
For stateless autoconfiguration to work properly, the advertised prefix length
in RA messages must always be 64 bits.
The RA messages are
sent to the all-nodes multicast address (see the figure below).
Figure 2. IPv6 Neighbor Discovery--RA
Message
RA messages typically
include the following information:
One or more
onlink IPv6 prefixes that nodes on the local link can use to automatically
configure their IPv6 addresses
Lifetime
information for each prefix included in the advertisement
Sets of flags
that indicate the type of autoconfiguration (stateless or stateful) that can be
completed
Default router
information (whether the router sending the advertisement should be used as a
default router and, if so, the amount of time (in seconds) the router should be
used as a default router)
Additional
information for hosts, such as the hop limit and MTU a host should use in
packets that it originates
RAs are also sent in
response to router solicitation messages.
The following RA
message parameters can be configured:
The time interval
between periodic RA messages
The "router
lifetime" value, which indicates the usefulness of a router as the default
router (for use by all nodes on a given link)
The network
prefixes in use on a given link
The time interval
between neighbor solicitation message retransmissions (on a given link)
The amount of
time a node considers a neighbor reachable (for use by all nodes on a given
link)
The configured
parameters are specific to an interface. The sending of RA messages (with
default values) is automatically enabled on FDDI interfaces when the
ipv6 unicast-routing command is configured. For other
interface types, the sending of RA messages must be manually configured by
using the
no ipv6 nd ra suppress command. The sending of RA messages can be
disabled on individual interfaces by using the
ipv6 nd ra suppress command.
Default Router Preferences for Traffic Engineering
Hosts discover and select default devices by listening to Router Advertisements (RAs). Typical default device selection mechanisms
are suboptimal in certain cases, such as when traffic engineering is needed. For example, two devices on a link may provide
equivalent but not equal-cost routing, and policy may dictate that one of the devices is preferred. Some examples are as follows:
Multiple devices that route to distinct sets of prefixes—Redirects (sent by nonoptimal devices for a destination) mean that
hosts can choose any device and the system will work. However, traffic patterns may mean that choosing one of the devices
would lead to considerably fewer redirects.
Accidentally deploying a new device—Deploying a new device before it has been fully configured could lead to hosts adopting
the new device as a default device and traffic disappearing. Network managers may want to indicate that some devices are more
preferred than others.
Multihomed situations—Multihomed situations may become more common, because of multiple physical links and because of the
use of tunneling for IPv6 transport. Some of the devices may not provide full default routing because they route only to the
6-to-4 prefix or they route only to a corporate intranet. These situations cannot be resolved with redirects, which operate
only over a single link.
The default router preference (DRP) feature provides a basic preference metric (low, medium, or high) for default devices.
The DRP of a default device is signaled in unused bits in RA messages. This extension is backward compatible, both for devices
(setting the DRP bits) and hosts (interpreting the DRP bits). These bits are ignored by hosts that do not implement the DRP
extension. Similarly, the values sent by devices that do not implement the DRP extension will be interpreted by hosts that
do implement it as indicating a “medium” preference. DRPs need to be configured manually.
IPv6 Neighbor Redirect Message
A value of 137 in the type field of the ICMP packet header identifies an IPv6 neighbor redirect message. Devices send neighbor
redirect messages to inform hosts of better first-hop nodes on the path to a destination (see the figure below).
A device must be able to determine the link-local address for each of its neighboring devices in order to ensure that the
target address (the final destination) in a redirect message identifies the neighbor device by its link-local address. For
static routing, the address of the next-hop device should be specified using the link-local address of the device; for dynamic
routing, all IPv6 routing protocols must exchange the link-local addresses of neighboring devices.
After forwarding a packet, a device should send a redirect message to the source of the packet under the following circumstances:
The destination address of the packet is not a multicast address.
The packet was not addressed to the device.
The packet is about to be sent out the interface on which it was received.
The device determines that a better first-hop node for the packet resides on the same link as the source of the packet.
The source address of the packet is a global IPv6 address of a neighbor on the same link, or a link-local address.
Use theipv6 icmp error-interval command to limit the rate at which the device generates all IPv6 ICMP error messages, including neighbor redirect messages,
which ultimately reduces link-layer congestion.
Note
A device must not update its routing tables after receiving a neighbor redirect message, and hosts must not originate neighbor
redirect messages.
Configures the length of time before an IPv6 ND cache entry expires.
Step 6
ipv6 nd na glean
Example:
Device(config-if)# ipv6 nd na glean
Configures ND to glean an entry from an unsolicited NA.
Customizing IPv6 ICMP Rate Limiting
SUMMARY STEPS
enable
configure terminal
ipv6 icmp error-interval milliseconds[bucketsize]
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configure terminal
Example:
Device# configure terminal
Enters global configuration mode.
Step 3
ipv6 icmp error-interval milliseconds[bucketsize]
Example:
Device(config)# ipv6 icmp error-interval 50 20
Customizes the interval and bucket size for IPv6 ICMP error messages.
Displaying IPv6 Redirect
Messages
SUMMARY STEPS
enable
show ipv6 interface [brief ] [typenumber] [prefix ]
show ipv6 route [ipv6-address |
ipv6-prefix/prefix-length |
protocol |
interface-typeinterface-number]
show ipv6 traffic
show
hosts [vrf vrf-name |
all |
hostname |
summary ]
enable
show running-config
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device# enable
Enables
privileged EXEC mode.
Enter your
password if prompted.
Step 2
show ipv6 interface [brief ] [typenumber] [prefix ]
Example:
Device# show ipv6 interface gigabitethernet 0/0/0
Displays the
usability status of interfaces configured for IPv6.
Step 3
show ipv6 route [ipv6-address |
ipv6-prefix/prefix-length |
protocol |
interface-typeinterface-number]
Example:
Device# show ipv6 route
(Optional)
Displays the current contents of the IPv6 routing table.
Step 4
show ipv6 traffic
Example:
Device# show ipv6 traffic
(Optional)
Displays statistics about IPv6 traffic.
Step 5
show
hosts [vrf vrf-name |
all |
hostname |
summary ]
Example:
Device# show hosts
Displays the
default domain name, the style of name lookup service, a list of name server
hosts, and the cached list of hostnames and addresses.
Step 6
enable
Example:
Device> enable
Enables
privileged EXEC mode.
Enter
your password if prompted.
Step 7
show running-config
Example:
Device# show running-config
Displays the
current configuration running on the device.
Configuration Examples for IPv6 Neighbor Discovery
Example: Customizing the Parameters for IPv6 Neighbor Discovery
In the following example, IPv6 ND NA gleaning is enabled and the IPv6 ND cache expiry is set to 7200 seconds (2 hours):
interface Port-channel189
no ip address
ipv6 address FC07::789:1:0:0:3/64
ipv6 nd nud retry 1 1000 3 1000
ipv6 nd na glean
ipv6 nd cache expire 7200
no ipv6 redirects
Example: IPv6 ICMP Rate Limiting Configuration
The following example shows an interval of 50 milliseconds and a bucket size of 20 tokens being configured for IPv6 ICMP
error messages:
ipv6 icmp error-interval 50 20
Example: Displaying Information About ICMP Rate-Limited Counters
In the following example, information about ICMP rate-limited counters is displayed:
Device# show ipv6 traffic
ICMP statistics:
Rcvd: 188 input, 0 checksum errors, 0 too short
0 unknown info type, 0 unknown error type
unreach: 0 routing, 0 admin, 0 neighbor, 0 address, 0 port
parameter: 0 error, 0 header, 0 option
0 hopcount expired, 0 reassembly timeout,0 too big
0 echo request, 0 echo reply
0 group query, 0 group report, 0 group reduce
1 router solicit, 175 router advert, 0 redirects
0 neighbor solicit, 12 neighbor advert
Sent: 7376 output, 56 rate-limited
unreach: 0 routing, 15 admin, 0 neighbor, 0 address, 0 port
parameter: 0 error, 0 header, 0 option
0 hopcount expired, 0 reassembly timeout,0 too big
15 echo request, 0 echo reply
0 group query, 0 group report, 0 group reduce
0 router solicit, 7326 router advert, 0 redirects
2 neighbor solicit, 22 neighbor advert
Example: Displaying IPv6 Interface Statistics
In the following example, the
show ipv6 interface command is used to verify that IPv6 addresses are configured correctly for FastEthernet interface 1/0. Information may also
be displayed about the status of IPv6 neighbor redirect messages, IPv6 neighbor discovery messages, stateless autoconfiguration,
and MTU size.
Device# show ipv6 interface fastethernet 1/0
Ethernet0 is up, line protocol is up
IPv6 is stalled, link-local address is FE80::1
Global unicast address(es):
2001:DB8:2000::1, subnet is 2001:DB8:2000::/64
2001:DB8:3000::1, subnet is 2001:DB8:3000::/64
Joined group address(es):
FF02::1
FF02::2
FF02::1:FF00:1
MTU is 1500 bytes
ICMP error messages limited to one every 100 milliseconds
ICMP redirects are enabled
ND DAD is enabled, number of DAD attempts: 1
ND reachable time is 30000 milliseconds
ND advertised reachable time is 0 milliseconds
ND advertised retransmit interval is 0 milliseconds
ND router advertisements are sent every 200 seconds
ND router advertisements live for 1800 seconds
Hosts use stateless autoconfig for addresses.
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The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for IPv6 Neighbor Discovery
Feature Name
Releases
Feature Information
IPv6 Neighbor Discovery
12.0(22)S
12.2(2)T
12.2(14)S
12.2(17a)SX1
12.2(25)SG
12.2(28)SB
12.2(33)SRA
Cisco IOS XE Release 2.1
12.2(50)SY
15.0(1)SY
3.2.0SG
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), verify the reachability of a neighbor, and track neighboring devices.
The following commands were introduced or modified:
ipv6 nd cache expire ,
ipv6 nd na glean ,
ipv6 nd nud retry .
IPv6 neighbor discovery duplicate address detection is performed first on a new, link-local IPv6 address before the address
is assigned to an interface (the new address remains in a tentative state while duplicate address detection is performed).
No commands were introduced or modified.
IPv6 Neighbor Discovery Nonstop Forwarding
12.2(33)SRE
15.0(1)S
15.0(1)SY
15.1(1)SY
The IPv6 Neighbor Discovery Nonstop Forwarding feature provides IPv6 high availability support.
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