Now you the first thing that you need to remember, is that the Requesting router or RR is in most cases located at the customer's location (premises); it's the router that the customer (Site) has either purchased or has been given to them by their ISP.
In real world situations, the RR has 2 main responsibilities; obtaining the Global IPv6 prefix information (configurable parameters) from the ISP's Delegating Router (DR); and then passing that Global IPv6 prefix information (configurable parameters) along through out the customer's location (Site) by using Router Advertisements (RA).
Router
Now, in order for you to configure a Cisco router's upstream interface to act like a Requesting Router (DHCP client) you'll need to perform these 5 steps:
Router>enable
Router # configure terminal
Router(config) # interface type number Router (config-if) # ipv6 address autoconfig [default]
Router (config-if) # ipv6 dhcp client pd prefix-name [rapid-commit]
Steps Explained:
Step # 1: Router > enable
Puts the router into Privileged Exec mode
Step # 2: Router # configure terminal
Puts the router into Global Configuration mode
Step # 3: Router (config) # interface serial 0/0
Allows, the RR to enter interface configuration mode for the serial 0/0 interface.
Step # 4: Router (config) # ipv6 address autoconfig default
Allows, the RR router's serial 0/0 interface to automatically configure its own IPv6 address (es) by using Stateless Autoconfiguration. The word "default" is used to tell the serial 0/0 interface toinstall a default route into the RR's routing table; the default route indicates the upstream interface (serial 0/0) and the ISP's Delegating Router's interface.
Step # 5: Router (config-if) # ipv6 dhcp client comcast dhcp-pd-prefix rapid-commit
Enables the router's serial RR 0/0 interface to begin acting as a DHCPv6 client; and also forces the interface to start requesting an IPv6 Prefix (es) from the ISP's Delegating Router. The word "rapid-commit" is telling the interface (serial 0/0) to use the two-message DHCPv6 exchange method to ask for the IPv6 Prefix (es).
Now, just in case if you were wondering, the words "comcast-dhcp-prefix" used in the command is the name of the prefix.
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Cisco IPv6 Training-Requesting IPv6 Router (RR) Configuration