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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter32 IPv6 Support on the Cisco ASR 901 Router
Information About IPv6 Support on the Cisco ASR 901 Router
IPv6 Duplicate Address Detection
During the stateless autoconfiguration process, duplicate address detection (DAD) 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). DAD is first
performed first on the new link-local address. When the link local address is verified as unique, then
DAD is performed on the remaining IPv6 unicast addresses on the interface.
When a duplicate address is identified, the state of the address is set to DUPLICATE and the address is
not used. If the duplicate address is the link-local address of the interface, the processing of IPv6 packets
is disabled on the interface and an error message is issued. If the duplicate address is a global address of
the interface, the address is not used and an error message is issued. 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).
IPv6 Neighbor Discovery
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 reachability
of a neighbor, and keep track of neighboring routers.
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
communicate with each other.
For more information on IPv6 Neighbor Discovery, see the Implementing IPv6 Addressing and Basic
Connectivity chapter of IPv6 Configuration Guide, at the following location:
http://www.cisco.com/en/US/docs/ios-xml/ios/ipv6/configuration/15-2mt/ip6-addrg-bsc-con.html
IPv4 and IPv6 Dual-Stack on an Interface
A dual stack means that IPv4 and IPv6 addresses coexist on the same platform and support hosts of both
types. This method is a way to transition from IPv4 to IPv6 with coexistence (IPv4 and IPv6) as a first
step.
The Cisco ASR 901 router supports the configuration of both IPv6 and IPv4 on an interface. You do not
need to enter any specific commands to do so; simply enter the IPv4 configuration commands and IPv6
configuration commands as you normally would. Make sure you configure the default route for both
IPv4 and IPv6.