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31-3
Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 31 Configuring IPv6 Neighbor Discovery
Information About IPv6 Neighbor Discovery
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 Reachable Time
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.
Router Advertisement Messages
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. Figure 31-2 shows the router advertisement messages that are sent from IPv6
configured interfaces on the ASA.
Figure 31-2 IPv6 Neighbor Discovery—Router Advertisement Message
Router advertisement messages typically include the following information:
• One or more IPv6 prefix 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.
• The amount of time between neighbor solicitation message retransmissions on a given link.
• The amount of time a node considers a neighbor reachable.
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
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Router advertisement packet definitions:
ICMPv6 Type = 134
Src = router link-local address
Dst = all-nodes multicast address
Data = options, prefix, lifetime, autoconfig flag
Router
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