■When a TCP segment for a TCP connection forwarded through the default router is retransmitted three times, dead gateway detection changes the destination cache entry for that remote IPv6 address to use the next default router in the list as its next-hop address. A destination cache entry is an entry in the destination cache, which stores the next-hop IPv6 address for a destination IPv6 address. When one-fourth of the TCP connections routed through the default router have had their destination cache entries adjusted to the next default router, dead gateway detection informs IPv6 to change the computer’s default router to the one that the adjusted connections are now using. If TCP connections continue to fail, dead gateway detection attempts to use the next default router in the list, returning to the first default router after cycling through the entire list.

TCP/IP in Windows Server 2008 and Windows Vista also supports fail-back behavior for default router changes. The lack of fail-back support for default routers can cause throughput problems on a subnet containing two routers: a high-capacity primary router and a lowercapacity backup router. The hosts on the subnet have the high-capacity router as their first default router and the backup router as their second default router. If the high-capacity router has a temporary failure, hosts on the subnet will switch over to the backup router. When the high-capacity router becomes available again, none of the hosts on the network use it because they have switched to the backup router.

TCP/IP in Windows Server 2008 and Windows Vista performs fail-back for default router changes by periodically attempting to send TCP traffic through the previous router. If the TCP traffic sent through the previous router is successful, TCP/IP in Windows Server 2008 and Windows Vista switches the default router to the previous router.

In the example with the high-capacity router and backup router, if the neighboring highcapacity router becomes unavailable, the hosts on the subnet use Neighbor Unreachability Detection to switch their default routers to the backup router. The hosts then periodically attempt to send TCP traffic through the high-capacity router. When the high-capacity router becomes available and the hosts determine that TCP traffic sent through the high-capacity router is successful, the hosts switch their default router back to the high-capacity router.

Support for fail-back to primary default routers can provide faster throughput by sending traffic through the primary default router on the subnet.

References

The following references were cited in this chapter:

■RFC 1723 — “RIP Version 2”

■RFC 2080 — “RIPng for IPv6”

■RFC 2328 — “OSPF Version 2”

■RFC 2545 — “Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing”

256Understanding IPv6, Second Edition

RFC 2740 — “OSPF for IPv6”

RFC 4191 — “Default Router Preferences and More-Specific Routes”

RFC 4271 — “A Border Gateway Protocol 4 (BGP-4)”

RFC 4760 — “Multiprotocol Extensions for BGP-4”

Internet draft — “Routing IPv6 with IS-IS”

ISO 10589 — “Intermediate system to Intermediate system intra-domain routing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service”

You can obtain these RFCs from the \RFCs_and_Drafts folder on the companion CD-ROM or from http://www.ietf.org/rfc.html.

Testing for Understanding

To test your understanding of IPv6 routing, answer the following questions. See Appendix D, “Testing for Understanding Answers,” to check your answers.

1.How does IPv6 on a router determine the single route in the routing table to use when forwarding a packet? How is the process different for an IPv6 sending host?

2.Describe the conditions that would cause a router to send the following ICMPv6 error messages:

ICMPv6 Packet Too Big

ICMPv6 Destination Unreachable—Address Unreachable

ICMPv6 Time Exceeded—Hop Limit Exceeded in Transit

ICMPv6 Destination Unreachable—Port Unreachable

ICMPv6 Destination Unreachable—No Route to Destination

ICMPv6 Parameter Problem—Unrecognized IPv6 Option Encountered

3.A host running Windows Vista receives a Router Advertisement message from a router advertising itself as a default router with the link-local address of fe80::2aa:ff:fe45:a431:2c5d, and containing a Prefix Information option to autoconfigure an address with the prefix 2001:db8:0:952a::/64 and a Route Information option with the prefix 2001:db8:0:952c::/64. Fill in the expected entries for the host based on this Router Advertisement message in the following abbreviated routing table:

4.Describe the differences between distance vector, link state, and path vector routing protocol technologies in terms of convergence time, ability to scale, ease of deployment, and appropriate use (intranet vs. Internet).

5.A static IPv6 router running Windows Server 2008 is configured with the following commands.

netsh interface ipv6 set interface “Local Area Connection” forwarding=enabled advertise=enabled

netsh interface ipv6 set interface “Local Area Connection 2” forwarding=enabled advertise=enabled

netsh interface ipv6 add route 2001:db8:0:1a4c::/64 “Local Area Connection” publish=yes

netsh interface ipv6 add route 2001:db8:0:90b5::/64 “Local Area Connection 2” publish=yes

With just these commands, will a host on the 2001:db8:0:90b5::/64 subnet have a default route? Why or why not? With just these commands, can a host on the 2001:db8:0:90b5::/64 subnet reach a host on the 2001:db8:0:1a4c::/64 subnet? If so, how?