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ccna 200 301 ebook download.For the fifth troubleshooting scenario, we can also use the previous scenario, but assume that the enterprise uses SLAAC for IPv6 address assignment, rather than stateful DHCPv6. Recall that the engineer has discovered that host D is not informed of the IPv6 address. So, what is causing SLAAC to fail? The potential root cause will be explored in the next subsection.To understand some of the root causes of this type of problem, first examine the three steps that hosts need to perform when using SLAAC to learn and build IPv6 settings. (!) Use NDP to learn the prefixes, prefix lengths, and default router addresses of routers on the same subnet.(2) Use the local host's SLAAC rules (no network messages required) to construct the host's own IPv6 address. (3) Use stateless DHCPv6 to obtain the address of the DNS server from the DHCPv6 server.16.2 Base Content 345 The first step in the procedure can use an NDP Router Request (RS) message, and the router also sends back an NDP Router Announcement (RA) message, as shown in Figure 16-17. The RS message, which is sent to the unicast address FF02::2 of all IPv6 routers, should reach all IPv6 routers in the same VLAN as host D in the figure. In this example, Rl replies by listing out Rl's IPv6 address (which is used as the default router for host D) and the prefix/length that host D should use. ipv6 address 2001:db8:1111:1::1/64________________________ RA@Router: 2001:DB8:1111:1::1 ~Prefix/Length: 2001:DB8:1111:1::/64 - Figure 16-17 NDPRS and RA Processes Hosts using SLAAC are also expected to use the information in the RA message. So, when a host using SLAAC is unable to know and construct these three settings (including the IPv6 address), the next question should be: what is causing the NDPRS/RA process to fail? A few potential root causes are listed below. /.gjj (1) No LAN connection between the host and any router in the subnet. 1.sprawl (2) The router did not execute the ipv6address interface subcommand. (3) The router is not executing the ipv6unicast-routing global configuration command. Of these reasons, the first one is the most obvious. NDPRS and RA messages cannot be sent if the LAN cannot forward Ethernet frames from the host to the router, or vice versa. For the second reason, in order to respond to RS messages, the router must execute an ipv6address command. This command enables IPv6 on the interface, and it also defines the information that the router should list in the RA message. For example, in Figure 16-17, Rlo can be configured with the ipv6 address 2001:db8:llll:l::l/64 command, which directly lists the two parts of the information that R1 provides in the RA message, and R1 can also calculate the IPv6 prefix using the address and prefix length. The third reason is probably the most surprising: the router must enable IPv6 routing using the ipv6 unicast-routing global command. Why? Without this command, the Cisco router will not attempt to send IPv6 packets. If you forget to execute this command, the router will not consider itself an IPv6 router and will not reply to NDPRS messages with an RA.The Traceroute command displays partial next hops, but all failThe sixth troubleshooting scenario in this chapter is slightly different in that it has less to do with the host and more to do with the router, usually related to IPv6 routing problems. In this scenario, the engineer hears that the host cannot connect to the server. Obviously, the ping from the host to the server has failed, and then the engineer implements the following steps, which are discussed in this chapter: . The host IPv6 settings appear to be OK. The host IPv6 settings match the default router and DNS server as expected.. The host can ping through to its default router. To continue troubleshooting, the engineer next calls the user with the server IPv6 address and asks him to try346 Chapter 16 Troubleshooting IPv6 Routing traceroute command. traceroute shows several routers in the output, but then there is no message until the user disconnects. What is the root cause? Often (but not always), these symptoms indicate some sort of IPv6 routing problem. Over the next page or two, we'll cover the root cause of these routing problems.