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The IPv6 neighbor discovery process uses ICMPv6 messages and solicited-node multicast addresses to determine the link-layer address of a neighbor on the same network (local link), verify the readability of a neighbor, and keep track of neighboring routers.
Nodes (hosts) use neighbor discovery to determine the link-layer addresses for neighbors known to reside on attached links and to quickly purge cashed values that become invalid. Hosts also use neighbor discovery to find neighboring routers that are willing to forward packets on their behalf. In addition, nodes use the protocol to actively keep track of which neighbors are reachable and which are not, and to detect changed link-layer addresses. When a router or the path to a router fails, a host actively searches for functioning alternates.
This section includes the following topics:
Neighbor solicitation messages (ICMPv6 Type 135) are sent on the local link by nodes attempting to discover the link-layer addresses of other nodes on the local link. The neighbor solicitation message is sent to the solicited-node multicast address. The source address in the neighbor solicitation message is the IPv6 address of the node sending the neighbor solicitation message. The neighbor solicitation message also includes the link-layer address of the source node.
After receiving a neighbor solicitation message, the destination node replies by sending a neighbor advertisement message (ICPMv6 Type 136) on the local link. The source address in the neighbor advertisement message is the IPv6 address of the node sending the neighbor advertisement message; the destination address 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 verifying 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.
The neighbor reachable time enables detecting unavailable neighbors. Shorter configured times enable detecting unavailable neighbors more quickly, however, shorter times consume more IPv6 network bandwidth and processing resources in all IPv6 network devices. Very short configured times are not recommended in normal IPv6 operation.
During the stateless autoconfiguration process, Duplicate Address Detection verifies the uniqueness of new unicast IPv6 addresses before the addresses are assigned to interfaces (the new addresses remain in a tentative state while Duplicate Address Detection is performed). Duplicate Address Detection is performed first on the new link-local address. When the link-local address is verified as unique, then Duplicate Address Detection is performed all the other IPv6 unicast addresses on the interface.
Duplicate Address Detection is suspended on interfaces that are administratively down. While an interface is administratively down, the unicast IPv6 addresses assigned to the interface are set to a pending state. An interface returning to an administratively up state restarts Duplicate Address Detection for all of the unicast IPv6 addresses on the interface.
When a duplicate address is identified, the state of the address is set to DUPLICATE, the address is not used, and the following error message is generated:
If the duplicate address is the link-local address of the interface, the processing of IPv6 packets is disabled on the interface. If the duplicate address is a global address, the address is not used. However, all configuration commands associated with the duplicate address remain as configured while the state of the address is set to DUPLICATE.
If the link-local address for an interface changes, Duplicate Address Detection is performed on the new link-local address and all of the other IPv6 address associated with the interface are regenerated (Duplicate Address Detection is performed only on the new link-local address).
The ASA uses neighbor solicitation messages to perform Duplicate Address Detection. By default, the number of times an interface performs Duplicate Address Detection is 1.
An ASA can participate in router advertisements so that neighboring devices can dynamically learn a default router address. Router advertisement messages (ICMPv6 Type 134) are periodically sent out each IPv6 configured interface of the ASA. The router advertisement messages are sent to the all-nodes multicast address.
Router advertisement messages typically include the following information:
Router advertisements are also sent in response to router solicitation messages (ICMPv6 Type 133). Router solicitation messages are sent by hosts at system startup so that the host can immediately autoconfigure without needing to wait for the next scheduled router advertisement message. Because router solicitation messages are usually sent by hosts at system startup, and the host does not have a configured unicast address, the source address in router solicitation messages is usually the unspecified IPv6 address (0:0:0:0:0:0:0:0). If the host has a configured unicast address, the unicast address of the interface sending the router solicitation message is used as the source address in the message. The destination address in router solicitation messages is the all-routers multicast address with a scope of the link. When a router advertisement is sent in response to a router solicitation, the destination address in the router advertisement message is the unicast address of the source of the router solicitation message.
You can configure the following settings for router advertisement messages:
Unless otherwise noted, the router advertisement message settings are specific to an interface and are entered in interface configuration mode.
You can manually define a neighbor in the IPv6 neighbor cache. If an entry for the specified IPv6 address already exists in the neighbor discovery cache—learned through the IPv6 neighbor discovery process—the entry is automatically converted to a static entry. Static entries in the IPv6 neighbor discovery cache are not modified by the neighbor discovery process.
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Configure IPv6 addressing according to the Configuring IPv6 Addressing.
This section includes the guidelines and limitations for this feature.
Supported in single and multiple context mode.
Supported in routed mode only. Transparent mode is not supported.
Additional Guidelines and Limitations
The following guidelines and limitations apply for configuring a static IPv6 neighbor:
Table 33-1 lists the default settings for IPv6 Neighbor Discovery.
Configure neighbor discovery settings per interface. To enter interface configuration mode, perform the following steps.
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To configure the interval between IPv6 neighbor solicitation retransmissions on an interface, enter the following command.
The following example configures an IPv6 neighbor solicitation transmission interval of 9000 milliseconds for GigabitEthernet 0/0:
To configure the amount of time that a remote IPv6 node is considered reachable after a reachability confirmation event has occurred, enter the following command.
The following example configures an IPv6 reachable time of 1700000 milliseconds for the selected interface, GigabitEthernet 0/0:
To configure the interval between IPv6 router advertisement transmissions on an interface, enter the following command.
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Sets the interval between IPv6 router advertisement transmissions. The optional msec keyword indicates that the value provided is in milliseconds. If this keyword is not present, the value provided is in seconds. Valid values for the value argument range from 3 to 1800 seconds or from 500 to 1800000 milliseconds if the msec keyword is provided. The interval between transmissions should be less than or equal to the IPv6 router advertisement lifetime if the ASA is configured as a default router. For more information, see Configuring the Router Lifetime Value. To prevent synchronization with other IPv6 nodes, randomly adjust the actual value used to within 20 percent of the desired value. |
The following example configures an IPv6 router advertisement interval of 201 seconds for the selected interface, GigabitEthernet 0/0:
To configure the router lifetime value in IPv6 router advertisements on an interface, enter the following command.
The following example configures an IPv6 router lifetime value of 2000 seconds for the selected interface, GigabitEthernet 0/0:
To specify DAD settings on the interface, enter the following command.
The following example configures a DAD attempt value of 20 for the selected interface, GigabitEthernet 0/0:
Router advertisement messages are automatically sent in response to router solicitation messages. You may want to disable these messages on any interface for which you do not want the ASA to supply the IPv6 prefix (for example, the outside interface).
To suppress the router lifetime value in IPv6 router advertisements on an interface, enter the following command.
The following example suppresses an IPv6 router advertisement transmission for the specified interface, which is GigabitEthernet 0/0:
You can add a flag to IPv6 router advertisements to inform IPv6 autoconfiguration clients to use DHCPv6 to obtain an IPv6 address and/or additional information such as the DNS server address.
To configure the which IPv6 prefixes are included in IPv6 router advertisements, enter the following command.
The following example includes the IPv6 prefix 2001:DB8::/32, with a valid lifetime of 1000 seconds and a preferred lifetime of 900 seconds, in router advertisements sent out on the specified interface, which is GigabitEthernet 0/0:
To configure a static entry in the IPv6 neighbor discovery cache, enter the following command.
The following example adds a static entry for an inside host with an IPv6 address of 3001:1::45A and a MAC address of 002.7D1a.9472 to the neighbor discovery cache:
To monitor IPv6 neighbor discovery parameters, enter the following command:
For additional information related to implementing IPv6 prefixes, see the following topics:
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Table 33-2 lists each feature change and the platform release in which it was implemented.