What are the flag settings when a host should use the slaac only option?
 Show continued from: http://www.ccieordie.com/legend/cciers-lab/cciers-lab-2-0/cciers-lab-2-1/cciers-lab-2-1-b/2-1-b-nd-rs-ra/ Task 2 Task
3 An address from the range 12::/64 Task 4 Configure R4 as a DHCP server using the options stated in the task. To work as a DHCP server, unicast routing must be enabled: R4 configure ipv6 link-local and GUA R4(config-dhcpv6)#int s1/1 The following specifies the address range to provide in the pool: R4(config)#ipv6 dhcp pool DHCPv6 The following configuration provides the DNS server and the domain name option to DHCP clients: R4(config-dhcpv6)#dns-server 2001:1:1111::1 The following command associates the DHCP pool with the interface facing the client (R5), effectively starting the particular DHCP server instance on s1/1 R4 R4(config-if)#do sh ipv6 int s1/1 Serial interfaces do not send the Router Advertisement messages by default—this is different from Ethernet interfaces, where RA messages are sent automatically. Because they will be required later, we are configuring the s1/5 interface to send them. As opposed to IPv4, where starting a DHCP server was essentially enough for the hosts to obtain their configuration via DHCP, in IPv6, the hosts must actually be instructed to use DHCP. Without this indication, they will continue using SLAAC. This is accomplished using the Router Advertisement (RA) messages. RA messages contain two specific bits, or flags, that are used to inform hosts about the mechanism hosts should use to obtain their IPv6 settings. These flags are commonly called the M-flag and the O-flag. The M-flag, or the managed address configuration flag, tells hosts to obtain their entire IPv6 configuration using DHCP, including their address, prefix length, DNS server address, domain name, and so on. The only parameter that will still remain discovered using RA messages is the default gateway address. DHCP for IPv6 does not support conveying the default gateway address information to clients because this information can always be learned from RA messages, which must have been received by hosts in the first place; otherwise, they would not be contacting DHCP at all. The M-flag can be set in outgoing RA messages using the ipv6 nd managed-config-flag interface configuration command. The O-flag, or the other configuration flag, tells hosts to obtain their IPv6 address and gateway using SLAAC, and to acquire all
other configuration (DNS address, domain name, and so on) using DHCP. The O-flag can be set in outgoing RA messages using the ipv6 nd other-config-flag interface configuration command. It is important to note that these flags are indications only. It is entirely up to the software running on the host to honor them. Some operating systems may choose to ignore these flags and behave according to their preset configuration. This is also valid for IOS—obviously, when an interface is statically configured to obtain its configuration via DHCP, it will attempt to talk to DHCP even if the RA messages do not have the M-flag set. Nonetheless, we will set up the flags diligently—it is considered a best practice, as well as a very safe approach, to accommodate most operating systems. The following command sets the M-flag in the RA messages. This tells the hosts not to rely on SLAAC and instead to use DHCP to obtain their IPv6 configuration. (Without this command, some hosts would continue using SLAAC even if the DHCP server was running.) R4(config-if)#ipv6 nd managed Note Hosts use DHCP to obtain routable addresses. R4#sh ipv6 int s1/1 R5#sh run int s1/1 | b int R5#debug ipv6 dhcp R5(config-if)#do sh run int s1/1 | b
int Here, the ipv6 enable command is required to allow the interface to have a link-local address. Without it, the interface would be unable to send DHCP requests because it would have no source IPv6 address. Based on the following output, we can see that the local router (R5) sends a Solicit message to FF02::1:2. Because there is no broadcast in IPv6, this is a special multicast address that the clients use to communicate with a DHCP server. The local router receives an Advertise message from the link-local IPv6 address of R4, the DHCP server. The local router then sends a request to use the IPv6 address given to it by the DHCP server, and it receives a reply from the server. In IPv4 DHCP, we also had four messages: Discover, Offer, Request, and Acknowledge, all with a similar meaning. R5(config-if)#no s http://www.gtri.com/accounting-differences-dhcpv6-dhcp/ The default route in DHCPv6 environments is somewhat confusing. Because IPv6 routers are required to send RA messages, the creators of DHCPv6 decided not to have a default gateway option for DHCPv6, and instead simply rely on the hosts discovering their gateways using RA messages. As a result, even in a DHCPv6 environment, RA messages are required for hosts to discover their gateway, while DHCPv6 provides the rest of the IPv6 configuration. On Cisco IOS-based routers, having a router obtain its interface configuration through ipv6 address dhcp will cause it to obtain its address and other optional information via DHCP, but this will not make it install a default route through a neighbor sourcing RA messages. If you want to do that as well, you must add the ipv6 address autoconfig default command to the same interface to have the router install a default route discovered via SLAAC. Finally, having an interface configured both with ipv6 address dhcp and ipv6 address autoconfig default will cause the interface to have two addresses: one obtained via DHCP, the other via SLAAC.
Although this may appear to defeat the purpose of DHCP, that is not entirely true: You can configure R4’s s1/5 interface with the ipv6 nd prefix default no-advertise command, which will prevent R4 from advertising any prefix in its RA messages. Although R4 will continue to send RAs, there will be no global prefix advertised, so R5 would only install a default route through R4 without generating a SLAAC-derived address itself. An address from the range 12::/64 R1(config-if)#do sh cdp neigh s1/0 | b Dev R1(config-if)#do sh ipv6 int brie | ex admin|unass R1(config-if)#do ping ipv6 14::4 R4#sh run | b DHCPv6_2 apply the pool called “DHCPv6_2” on R4’s closest interface to R2. (as illustrated by the diagram, that must be s1/0) R4#sh run int s1/0 | b int R1’s s1/1 interface should be configured to set the M-flag, the no ipv6 nd ra suppress and then relay the Solicit messages to the DHCP server: R1(config-if)#do sh cdp neigh s1/1 | b Dev R1(config-if)#do sh run int s1/1 | b int As an aside, the configuration of IPv6 DHCP relay is much more intuitive than its IPv4 counterpart whose command, ip helper-address, does not even readily resemble anything related to DHCP. In this case, R2 is going to be the DHCP client. R2 will multicast a DHCP Solicit message. R1 will receive this message and relay it to R4, the DHCP server. The Solicit message will have the Link Address field populated with the IPv6 address of R1’s link facing R2. R4 will go through its DHCP scopes and will find one that matches the same network. Subsequently, it will lease out an IPv6 address from that scope and offer it to R2 using the Advertise message. Note, however, that the Advertise response will be unicast to R1’s address learned from the Solicit message. R1 will receive the Advertise message and will relay it down to R2. After R2 receives the Advertise message, it will continue with multicasting a DHCP Request message, which will again be relayed by R2 to R4. Then, R4 will respond with a DHCP Reply message to R2, which in turn forwards it to R1. At this point, R1 has a usable address. The entire process is very similar to IPv4 DHCP. Note If the address prefix 12::/64 is configured in the previous pool (TST), R2 will get two IPv6 addresses: one from the 12::/64 network, and the second from the 45::/64 network. A DHCP pool in IPv6 can contain multiple IPv6 prefixes at the same time. R2#debug ipv6 dhcp R2(config)#int s1/1 R2(config-if)#do sh ipv6 int s1/1 R1(config-if)#do sh ipv6 int s1/1 | i Hosts place relay, managed-config and suppression on the relay’s directly connected client interface. place the dhcp server statement on the dhcp server interface (R4 in the exercise) connected to the relay agent. R2(config)#do ping ipv6
12::1 % No valid route for destination Rapid-commit: Default: The rapid-commit option must be configured both on the DHCP client and the DHCP server to be effective. Here’s how to configure the DHCP server for rapid-commit: R4(config-if)#ipv6 dhcp serv DHCPv6 rapid R5(config)#default int s1/1 R5(config-if)#ipv6 add dhcp rapid Which flag is associated with Slaac in IPv6 for provision of network services?When the A (or autonomous address-configuration) flag is set to “1”, it specifies that the included prefix can be used for SLAAC. Thus, the Prefix Information Option message must include both a prefix and the A flag set to one for SLAAC to function properly.
What flag settings combination is used for stateless DHCP?M = 0, O = 1 : Client gets stateless address from RA. DHCPv6 only assign other configuration settings to client. This combination is known as DHCPv6 stateless.
Which flags should be set when a host should use stateless DHCPv6 for configuration?To tell the hosts to use Stateless DHCPv6 for other information, we need to set the O-flag to 1. This is done using the ipv6 nd other-config-flag command.
What type of address is Slaac helping configure?(StateLess Address Auto Configuration) The preferred method of assigning IP addresses in an IPv6 network. SLAAC devices send the router a request for the network prefix, and the device uses the prefix and its own MAC address to create an IP address.
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