Chapter 32. Implementing IPv6 Routing
This chapter covers the following exam topics:
3.0 Routing Technologies
3.6 Configure, verify, and troubleshoot IPv4 and IPv6 static routing
3.6.a Default route
3.6.b Network route
3.6.c Host route
3.6.d Floating static
The one remaining piece of the IPv6 story for this book is how routers learn IPv6 routes. Interestingly, Cisco chose to keep IPv6 routing protocols in the ICND2 half of the CCNA R&S exam, leaving only static IPv6 routes for discussion in the ICND1 half. So this chapter discusses the simplest and most straightforward ways for a router to create IPv6 routes: connected, local, and static routes.
The chapter breaks the topics into two main sections. The first section of this chapter walks you through the details of how IPv6, similar to IPv4, adds both connected and local routes based on each interface IPv6 address. The second major section of this chapter then looks at how to configure static IPv6 routes by typing in commands, in this case using the ipv6 route command instead of IPv4’s ip route command.
“Do I Know This Already?” Quiz
Take the quiz (either here, or use the PTP software) if you want to use the score to help you decide how much time to spend on this chapter. The answers are at the bottom of the page following the quiz, and the explanations are in DVD Appendix C and in the PTP software.
Refer to the following figure for questions 1, 3, and 4.
1. A router has been configured with the ipv6 address 2000:1:2:3::1/64 command on its G0/1 interface as shown in the figure. The router creates a link-local address of FE80::FF:FE00:1 as well. The interface is working. Which of the following routes will the router add to its IPv6 routing table? (Choose two answers.)
a. A route for 2000:1:2:3::/64
b. A route for FE80::FF:FE00:1/64
c. A route for 2000:1:2:3::1/128
d. A route for FE80::FF:FE00:1/128
2. A router has been configured with the ipv6 address 3111:1:1:1::1/64 command on its G0/1 interface and ipv6 address 3222:2:2:2::1/64 on its G0/2 interface. Both interfaces are working. Which of the following routes would you expect to see in the output of the show ipv6 route connected command? (Choose two answers.)
a. A route for 3111:1:1:1::/64
b. A route for 3111:1:1:1::1/64
c. A route for 3222:2:2:2::/64
d. A route for 3222:2:2:2::2/128
3. An engineer needs to add a static IPv6 route for prefix 2000:1:2:3::/64 to Router R5’s configuration, in the figure shown with question 1. Which of the following answers shows a valid static IPv6 route for that subnet, on Router R5?
a. ipv6 route 2000:1:2:3::/64 S0/1/1
b. ipv6 route 2000:1:2:3::/64 S0/1/0
c. ip route 2000:1:2:3::/64 S0/1/1
d. ip route 2000:1:2:3::/64 S0/1/0
4. An engineer needs to add a static IPv6 route for prefix 2000:1:2:3::/64 to Router R5 in the figure shown with question 1. Which of the following answers shows a valid static IPv6 route for that subnet on Router R5?
a. ipv6 route 2000:1:2:3::/64 2000:1:2:56::5
b. ipv6 route 2000:1:2:3::/64 2000:1:2:56::6
c. ipv6 route 2000:1:2:3::/64 FE80::FF:FE00:5
d. ipv6 route 2000:1:2:3::/64 FE80::FF:FE00:6
5. An engineer types the command ipv6 route 2001:DB8:8:8::/64 2001:DB8:9:9::9 129 in configuration mode of Router R1 and presses Enter. Later, a show ipv6 route command does not list any route for subnet 2001:DB8:8:8::/64. Which of the following could have caused the route to not be in the IPv6 routing table?
a. The command should be using a next-hop link-local address instead of a global unicast.
b. The command is missing an outgoing interface parameter, so IOS rejected the ipv6 route command.
c. The router has no routes that match 2001:DB8:9:9::9.
d. A route for 2001:DB8:8:8::/64 with administrative distance 110 already exists.
6. The command output shows two routes from the longer output of the show ipv6 route command. Which answers are true about the output? (Choose two answers.)
R1# show ipv6 route static
! Legend omitted for brevity
S 2001:DB8:2:2::/64 [1/0]
via 2001:DB8:4:4::4
S ::/0 [1/0]
via Serial0/0/1, directly connected
a. The route to ::/0 is added because of an ipv6 route global command.
b. The administrative distance of the route to 2001:DB8:2:2::/64 is 1.
c. The route to ::/0 is added because of an ipv6 address interface subcommand.
d. The route to 2001:DB8:2:2::/64 is added because of an IPv6 routing protocol.
Answers to the “Do I Know This Already?” quiz:
1 A, C 2 A, C 3 A 4 B 5 C 6 A, B
Foundation Topics
Connected and Local IPv6 Routes
A Cisco router adds IPv6 routes to its IPv6 routing table for several reasons. Many of you could predict those reasons at this point in your reading, in part because the logic mirrors the logic routers use for IPv4. Specifically, a router adds IPv6 routes based on the following:
The configuration of IPv6 addresses on working interfaces (connected and local routes)
The direct configuration of a static route (static routes)
The configuration of a routing protocol, like OSPFv3, on routers that share the same data link (dynamic routes)
This chapter examines the first of these two topics.
Note
Cisco leaves all IPv6 routing protocols until the ICND2 half of the CCNA R&S exam topics, so Part VI of that book discusses IPv6 routing protocols. If you do not have the ICND2 book and want a little background info about IPv6 routing protocols, check Appendix Q in this book (on the DVD), for the section titled “Dynamic Routes with OSPFv3,” for OSPFv3 material from an earlier edition of this book.
Rules for Connected and Local Routes
Routers add and remove connected routes and local routes, based on the interface configuration and the interface state. First, the router looks for any configured unicast addresses on any interfaces by looking for the ipv6 address command. Then, if the interface is working—if the interface has a “line status is up, protocol status is up” notice in the output of the show interfaces command—the router adds both a connected and local route.
Note
Routers do not create IPv6 routes for link-local addresses.
The connected and local routes themselves follow the same general logic as with IPv4. The connected route represents the subnet connected to the interface, whereas the local route is a host route for only the specific IPv6 address configured on the interface.
As an example, consider a router, with a working interface, configured with the ipv6 address 2000:1:1:1::1/64 command. The router will calculate the subnet ID based on this address and prefix list, and it will place a connected route for that subnet (2000:1:1:1::/64) into the routing table. The router also takes the listed IPv6 address and creates a host route for that address, with a /128 prefix length. (With IPv4, host routes have a /32 prefix length, while IPv6 uses a /128 prefix length, meaning “exactly this one address.”)
The following list summarizes the rules about how routers create routes based on the configuration of an interface IPv6 unicast address, for easier review and study:
1. Routers create IPv6 routes based on each unicast IPv6 address on an interface, as configured with the ipv6 address command, as follows:
A. The router creates a route for the subnet (a connected route).
B. The router creates a host route (/128 prefix length) for the router IPv6 address (a local route).
2. Routers do not create routes based on the link-local addresses associated with the interface.
3. Routers remove the connected and local routes for an interface if the interface fails, and they re-add these routes when the interface is again in a working (up/up) state.
Example of Connected IPv6 Routes
While the concept of connected and local IPv6 routes works much like IPv4 routes, seeing a few examples can certainly help. To show some sample routes, Figure 32-1 gives the details of one sample internetwork used in this chapter. The figure shows the IPv6 subnet IDs. The upcoming examples focus on the connected and local routes on Router R1.
To clarify the notes in Figure 32-1, note that the figure shows IPv6 prefixes (subnets), with a shorthand notation for the interface IPv6 addresses. The figure shows only the abbreviated interface ID portion of each interface address near each interface. For example, R1’s G0/0 interface address would begin with subnet ID value 2001:DB8:1111:1, added to ::1, for 2001:DB8:1111:1::1.
Now on to the example of connected routes. To begin, consider the configuration of Router R1 from Figure 32-1, as shown in Example 32-1. The excerpt from the show running-config command on R1 shows three interfaces, all of which are working. Also note that no static route or routing protocol configuration exists.
Example 32-1 IPv6 Addressing Configuration on Router R1
ipv6 unicast-routing
!
interface serial0/0/0
ipv6 address 2001:db8:1111:4::1/64
!
interface serial0/0/1
ipv6 address 2001:db8:1111:5::1/64
!
interface gigabitethernet0/0
ipv6 address 2001:db8:1111:1::1/64
Based on Figure 32-1 and Example 32-1, R1 should have three connected IPv6 routes, as highlighted in Example 32-2.
Example 32-2 Routes on Router R1 Before Adding Static Routes or Routing Protocols
R1# show ipv6 route
IPv6 Routing Table - default - 7 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2
IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external
ND - Neighbor Discovery, l - LISP
O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
C 2001:DB8:1111:1::/64 [0/0]
via GigabitEthernet0/0, directly connected
L 2001:DB8:1111:1::1/128 [0/0]
via GigabitEthernet0/0, receive
C 2001:DB8:1111:4::/64 [0/0]
via Serial0/0/0, directly connected
L 2001:DB8:1111:4::1/128 [0/0]
via Serial0/0/0, receive
C 2001:DB8:1111:5::/64 [0/0]
via Serial0/0/1, directly connected
L 2001:DB8:1111:5::1/128 [0/0]
via Serial0/0/1, receive
L FF00::/8 [0/0]
via Null0, receive
All three highlighted routes show the same basic kinds of information, so for discussion, focus on the first pair of highlighted lines, which detail the connected route for subnet 2001:DB8:1111:1::/64. The first pair of highlighted lines state: The route is a “directly connected” route; the interface ID is GigabitEthernet0/0; and the prefix/length is 2001:DB8:1111:1::/64. At the far left, the code letter “C” identifies the route as a connected route (per the legend above). Also note that the numbers in brackets mirror the same ideas as IPv4’s show ip route command: The first number represents the administrative distance, and the second is the metric.
Examples of Local IPv6 Routes
Continuing this same example, three local routes should exist on R1 for the same three interfaces as the connected routes. Indeed, that is the case, with one extra local route for other purposes. Example 32-3 shows only the local routes, as listed by the show ipv6 route local command, with highlights of one particular local route for discussion.
Example 32-3 Local IPv6 Routes on Router R1
R1# show ipv6 route local
! Legend omitted for brevity
L 2001:DB8:1111:1::1/128 [0/0]
via GigabitEthernet0/0, receive
L 2001:DB8:1111:4::1/128 [0/0]
via Serial0/0/0, receive
L 2001:DB8:1111:5::1/128 [0/0]
via Serial0/0/1, receive
L FF00::/8 [0/0]
via Null0, receive
For the highlighted local route, look for a couple of quick facts. First, look back to R1’s configuration in Example 32-1, and note R1’s IPv6 address on its G0/0 interface. This local route lists the exact same address. Also note the /128 prefix length, meaning this route matches packets sent to that address (2001:DB8:1111:1::1), and only that address.
Note
While the show ipv6 route local command shows all local IPv6 routes, the show ipv6 route connected command shows all connected routes.
Static IPv6 Routes
While routers automatically add connected and local routes based on the interface configuration, static routes require direct configuration with the ipv6 route command. Simply put, someone configures the command, and the router places the details from the command into a route in the IPv6 routing table.
The ipv6 route command follows the same general logic as does IPv4’s ip route command, as discussed in Chapter 18, “Configuring IPv4 Addresses and Static Routes.” For IPv4, the ip route command starts by listing the subnet ID and mask, so for IPv6, the ipv6 route command begins with the prefix and prefix length. Then the respective commands list the directions of how this router should forward packets toward that destination subnet or prefix by listing the outgoing interface or the address of the next-hop router.
Figure 32-2 shows the concepts behind a single ipv6 route command, demonstrating the concepts behind a static route on Router R1 for the subnet on the right (subnet 2, or 2001:DB8:1111:2::/64). A static route on R1, for this subnet, will begin with ipv6 route 2001:DB8:1111:2::/64, followed by either the outgoing interface (S0/0/0) or the next-hop IPv6 address, or both.
Now that you understand the big ideas with IPv6 static routes, the next few pages walk you through a series of examples. In particular, the examples look at configuring static routes with an outgoing interface, then with a next-hop global unicast address, and then with a next-hop link-local address. This section ends with a discussion of static IPv6 default routes.
Static Routes Using the Outgoing Interface
This first IPv6 static route example uses the outgoing interface option. As a reminder, for both IPv4 and IPv6 static routes, when the command references an interface, the interface is a local interface. That is, it is an interface on the router where the command is added. In this case, as shown in Figure 32-2, R1’s ipv6 route command would use interface S0/0/0, as shown in Example 32-4.
Example 32-4 Static IPv6 Routes on Router R1
! Static route on router R1
R1(config)# ipv6 route 2001:db8:1111:2::/64 s0/0/0
While Example 32-4 shows the correct syntax of the route, if using static routes throughout this internetwork, more static routes are needed. For example, to support traffic between hosts A and B, R1 is now prepared. Host A will forward all its IPv6 packets to its default router (R1), and R1 can now route those packets out S0/0/0 to R2 next. However, Router R2 does not yet have a route back to host A’s subnet, subnet 1 (2001:DB8:1111:1::/64), so a complete solution requires more routes.
Example 32-5 solves this problem by giving Router R2 a static route for subnet 1 (2001:DB8:1111:1::/64). After adding this route, hosts A and B should be able to ping each other.
Example 32-5 Static IPv6 Routes on Router R2
! Static route on router R2
R2(config)# ipv6 route 2001:db8:1111:1::/64 s0/0/1
Many options exist for verifying the existence of the static route and testing whether hosts can use the route. ping and traceroute, as discussed in Chapter 31, “Implementing IPv6 Addressing on Hosts,” can test connectivity. From the router command line, the show ipv6 route command will list all the IPv6 routes. The shorter output of the show ipv6 route static command, which lists only static routes, could also be used; Example 32-6 shows that output, with the legend omitted.
Example 32-6 Verification of Static Routes Only on R1
R1# show ipv6 route static
! Legend omitted for brevity
S 2001:DB8:1111:2::/64 [1/0]
via Serial0/0/0, directly connected
This command lists many facts about the one static route on R1. First, the code “S” in the left column does identify the route as a static route. (However, the later phrase “directly connected” might mislead you to think this is a connected route; trust the “S” code.) Note that the prefix (2001:DB8:1111:2::/64) matches the configuration (in Example 32-4), as does the outgoing interface (S0/0/0).
While this command lists basic information about each static route, it does not state whether this route would be used when forwarding packets to a particular destination. For example, if host A sent an IPv6 packet to host B (2001:DB8:1111:2::22), would R1 use this static route? As it turns out, R1 would use that route, as confirmed by the show ipv6 route 2001:DB8:1111:2::22 command. This command asks the router to list the route the router would use when forwarding packets to that particular address. Example 32-7 shows an example.
Example 32-7 Displaying the Route R1 Uses to Forward to Host B
R1# show ipv6 route 2001:db8:1111:2::22
Routing entry for 2001:DB8:1111:2::/64
Known via "static", distance 1, metric 0
Route count is 1/1, share count 0
Routing paths:
directly connected via Serial0/0/0
Last updated 00:01:29 ago
Static Routes Using Next-Hop IPv6 Address
Static IPv6 routes that refer to a next-hop address have two options: the unicast address on the neighboring router (global unicast or unique local) or the link-local address of that same neighboring router. Figure 32-3 spells out those two options with an updated version of Figure 32-2, this time showing Router R2’s global unicast as well as R2’s link-local address.
The next few pages walk you through examples, first with a global unicast as a next-hop and then with a link-local as a next-hop.
Example Static Route with a Global Unicast Next-Hop Address
This example uses the internetwork shown in Figure 32-3, but with the earlier static routes removed. That is, both routers have only connected and local routes to begin the example.
In Example 32-8, both R1 and R2 add static routes that refer to the neighbor’s global unicast address. R1 adds a route for subnet 2 (on the right), while R2 adds a route for subnet 1 (on the left). Note that the example shows routes in both directions so that the two hosts can send packets to each other.
Example 32-8 Static IPv6 Routes Using Global Unicast Addresses
! The first command is on router R1, listing R2's global unicast address
R1(config)# ipv6 route 2001:db8:1111:2::/64 2001:DB8:1111:4::2
! The next command is on router R2, listing R1's global unicast address
R2(config)# ipv6 route 2001:db8:1111:1::/64 2001:db8:1111:4::1
The ipv6 route command itself is relatively straightforward. Focus on R1’s route, which matches the logic shown in Figure 32-3. The command lists subnet 2 (2001:DB8:1111:2::/64). It then lists R2’s global unicast address (ending in 4::2).
The verification commands on R1, as shown in Example 32-9, list the usual information. Example 32-9 shows two commands, first listing R1’s only static route (the one configured in Example 32-8). The end of the example lists the show ipv6 route 2001:DB8:1111:2::22 command, which lists the route R1 uses when forwarding packets to Host B, proving that R1 uses this new static route when forwarding packets to that host.
Example 32-9 Verification of Static Routes to a Next-Hop Global Unicast Address
R1# show ipv6 route static
! Legend omitted for brevity
S 2001:DB8:1111:2::/64 [1/0]
via 2001:DB8:1111:4::2
R1# show ipv6 route 2001:db8:1111:2::22/64
Routing entry for 2001:DB8:1111:2::/64
Known via "static", distance 1, metric 0
Backup from "ospf 1 [110]"
Route count is 1/1, share count 0
Routing paths:
2001:DB8:1111:4::2
Last updated 00:07:43 ago
Example Static Route with a Link-Local Next-Hop Address
Static routes that refer to a neighbor’s link-local address work a little like both of the preceding two styles of static routes. First, the ipv6 route command refers to a next-hop address, namely a link-local address. However, the command must also refer to the router’s local outgoing interface. Why both? The ipv6 route command cannot simply refer to a link-local next-hop address by itself, because the link-local address does not, by itself, tell the local router which outgoing interface to use.
Interestingly, when the ipv6 route command refers to a global unicast next-hop address, the router can deduce the outgoing interface. For example, the earlier example on R1, as shown in Example 32-8, shows R1 with a static IPv6 route with a next-hop IPv6 address of 2001:DB8:1111:4::2. R1 can look at its IPv6 routing table, see its connected route that includes this 2001:DB8:1111:4::2 address, and see a connected route off R1’s S0/0/0. As a result, with a next-hop global unicast address, R1 can deduce the correct outgoing interface (R1’s S0/0/0).
With a link-local next-hop address, a router cannot work through this same logic, so the outgoing interface must also be configured. Example 32-10 shows the configuration of static routes on R1 and R2, replacements for the two routes previously configured in Example 32-8.
Example 32-10 Static IPv6 Routes Using Link-Local Neighbor Addresses
! The first command is on router R1, listing R2's link-local address
R1(config)# ipv6 route 2001:db8:1111:2::/64 S0/0/0 FE80::FF:FE00:2
! The next command is on router R2, listing R1's link-local address
R2(config)# ipv6 route 2001:db8:1111:1::/64 S0/0/1 FE80::FF:FE00:1
Example 32-11 verifies the configuration in Example 32-10 by repeating the show ipv6 route static and show ipv6 route 2001:DB8:1111:2::22 commands used in Example 32-9. Note that the output from both commands differs slightly in regard to the forwarding details. Because the new commands list both the next-hop address and outgoing interface, the show commands also list both the next-hop (link-local) address and the outgoing interface. If you refer back to Example 32-9, you will see only a next-hop address listed.
Example 32-11 Verification of Static Routes to a Next-Hop Link-Local Address
R1# show ipv6 route static
! Legend omitted for brevity
S 2001:DB8:1111:2::/64 [1/0]
via FE80::FF:FE00:2, Serial0/0/0
R1# show ipv6 route 2001:db8:1111:2::22
Routing entry for 2001:DB8:1111:2::/64
Known via "static", distance 1, metric 0
Backup from "ospf 1 [110]"
Route count is 1/1, share count 0
Routing paths:
FE80::FF:FE00:2, Serial0/0/0
Last updated 00:08:10 ago
Static Default Routes
IPv6 supports a default route concept, similar to IPv4. The default route tells the router what to do with an IPv6 packet when the packet matches no other IPv6 route. The logic is pretty basic:
With no default route, the router discards the IPv6 packet.
With a default route, the router forwards the IPv6 packet based on the default route.
Default routes can be particularly useful in a couple of network design cases. For example, with an enterprise network design that uses a single router at each branch office, with one WAN link to each branch, the branch routers have only one possible path over which to forward packets. In a large network, when using a routing protocol, the branch router could learn thousands of routes—all of which point back toward the core of the network over that one WAN link.
Branch routers could use default routes instead of a routing protocol. The branch router would forward all traffic to the core of the network. Figure 32-4 shows just such an example, with two sample branch routers on the right and a core site router on the left.
To configure a static default route, use the same rules already discussed in this section of the chapter, but use a specific value to note the route as a default route: ::/0. Taken literally, the double colon (::) is the IPv6 abbreviation for all 0s, and the /0 means the prefix length is 0. This idea mirrors the IPv4 convention to refer to the default route as 0.0.0.0/0. Otherwise, just configure the ipv6 route command as normal.
Example 32-12 shows one such sample static default route on Router B1 from Figure 32-4. This example uses the outgoing interface option.
Example 32-12 Static Default Route for Branch Router B1
!Forward out B1's S0/0/1 local interface...
B1(config)# ipv6 route ::/0 S0/0/1
With IPv6, the router displays the default a little more cleanly than with IPv4. The show ipv6 route command simply includes the route in the output of the command, along with the other routes. Example 32-13 shows an example, with “::/0” listed to denote this route as the default route.
Example 32-13 Router B1’s Static Default Route (Using Outgoing Interface)
B1# show ipv6 route static
IPv6 Routing Table - default - 10 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2
IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external
ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect
O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
S ::/0 [1/0]
via Serial0/0/1, directly connected
Static IPv6 Host Routes
Both IPv4 and IPv6 allow the definition of static host routes; that is, a route to a single host IP address. With IPv4, those routes use a /32 mask, which identifies a single IPv4 address in the ip route command; with IPv6, a /128 mask identifies that single host in the ipv6 route command.
A host route follows the same rules as a route for any other IPv6 subnet. For instance, in Figure 32-3, host B sits on the right side of the figure. Earlier examples showed R1’s static routes for the subnet in which host B resides; for example, the routes for Router R1 in Examples 32-8 and 32-10. To create a host route on R1, referring to host B’s specific IPv6 address, just change those commands to refer to host B’s entire IPv6 address (2001:DB8:1111:2::22), with prefix length /128.
Example 32-14 shows two sample host routes. Both define a host route to host B’s IPv6 address on Router R1 in Figure 32-3. One uses Router R2’s link-local address as the next-hop address, and one uses R2’s global unicast address as the next-hop address.
Example 32-14 Static Host IPv6 Routes on R1, for Host B
! The first command lists host B's address, prefix length /128,
! with R2's link-local address as next-hop, with an outgoing interface.
R1(config)# ipv6 route 2001:db8:1111:2::22/128 S0/0/0 FE80::FF:FE00:2
! The next command also lists host B's address, prefix length /128,
! but with R2's global unicast address as next-hop, and no outgoing interface.
R1(config)# ipv6 route 2001:db8:1111:2::22/128 2001:DB8:1111:4::2
Floating Static IPv6 Routes
Next, consider the case in which a static route competes with other static routes or routes learned by a routing protocol. For example, consider the topology shown in Figure 32-5, which shows a branch office with two WAN links: one very fast Gigabit Ethernet link and one rather slow (but cheap) T1. In this design, the network uses OSPFv3 to learn IPv6 routes over the primary link, learning a route for subnet 2001:DB8:1111:7::/64. R1 also defines a static route over the backup link to that exact same subnet, so R1 must choose whether to use the static route or the OSPF-learned route.
IOS considers static routes better than OSPF-learned routes by default due to administrative distance. IOS uses the same administrative distance concept and default values for IPv6 as it does for IPv4. As a result, a static IPv6 over the lower route would be given an administrative distance of 1, and an OSPFv3-learned route over the top path would be given an administrative distance of 110. R1 would use the lower path to reach subnet 2001:DB8:1111:7::/64 in this case, which is not the intended design. Instead, the engineer prefers to use the OSPF-learned routes over the much-faster primary link and use the static route over the backup link only as needed when the primary link fails.
To instead prefer the OSPF routes, the configuration would need to change the administrative distance settings and use what many networkers call a floating static route. Like an IPv4 floating static route, an IPv6 floating static route floats or moves into and out of the IPv6 routing table depending on whether the better (lower) administrative distance route learned by the routing protocol happens to exist currently. Basically, the router ignores the static route during times when the better routing protocol route is known.
To implement an IPv6 floating static route, just override the default administrative distance on the static route, making the value larger than the default administrative distance of the routing protocol. For example, the ipv6 route 3444:4:4:4::/64 3444:2:2:2::2 130 command on R1 would do exactly that, setting the static route’s administrative distance to 130. As long as the primary link (G0/0) stays up, and OSPFv3 on R1 learns a route for 3444:4:4:4::/64 with OSPF’s default administrative distance of 110, R1 ignores the static route whose administrative distance is explicitly configured as 130.
Finally, note that both the show ipv6 route and show ipv6 route 3444:4:4:4::/64 command list the administrative distance. Example 32-15 shows a sample matching this most recent example. Note that in this case, the static route is in use in the IPv6 routing table.
Example 32-15 Displaying the Administrative Distance of the Static Route
R1# show ipv6 route static
! Legend omitted for brevity
S 3444:4:4:4::/64 [130/0]
via 3444:2:2:2::2
R1# show ipv6 route 3444:4:4:4::/64
Routing entry for 3444:4:4:4::/64
Known via "static", distance 130, metric 0
Route count is 1/1, share count 0
Routing paths:
3444:2:2:2::2
Last updated 00:00:58 ago
Table 32-2 lists some of the default administrative distance values used with IPv6.
Default Routes with SLAAC on Router Interfaces
The section, “Learning a Default Route Using DHCP” in Chapter 19 discusses how routers can use DHCP on their own interface and learn their IP address, mask, and even a default IPv4 route. In particular, that process can be useful on a router that connects to the Internet. The enterprise router uses DHCP as a client, learning its own IPv4 address with DHCP and adding a default route pointing to the ISP’s router as the next-hop IPv4 address.
Routers can accomplish the same goals with IPv6, just with a few different protocols and methods. As with IPv4, the IPv6 enterprise router can dynamically learn its IPv6 address and dynamically create a default IPv6 route to the ISP’s router. This section shows the details, with the enterprise router using SLAAC to learn its address and the information needed to create a default route.
First, the enterprise router that connects to the ISP, like Router R1 in Figure 32-6, requires the configuration of the interface subcommand ipv6 address autoconfig default. This command tells the router that, on that interface, use SLAAC to build its own IPv6 address. R1 would act like any host that uses SLAAC, as shown in Step 2 of the figure, and send an NDP RS message over the link. As noted at Step 3, the ISP router would send back an RA message, announcing router ISP1’s IPv6 address and the IPv6 prefix used on the link.
When R1 receives the NDP RA message, it does the following:
Interface address: Builds its own interface IPv6 address using the SLAAC process, based on the prefix in the RA.
Local /128 Route: Adds a local (/128) IPv6 route for the address, as it would for any interface IPv6 address.
Connected Route for Prefix: Adds a connected (/64) route for the prefix learned in the NDP RA message.
Default route: R1 adds a default route, to destination ::/0, with the next-hop address of ISP’s link local address, as learned in the RA sent by router ISP1.
Note that the router can be configured to add this default route or not. As shown in the figure, the router builds a default route. Using the ipv6 address autoconfig subcommand without the default keyword causes the router to build its address with SLAAC but not add a default route.
Example 32-16 shows the three IPv6 routes on Router R1 just mentioned in the list. In particular, note the codes for the connected route and the default route; both codes begin with ND, meaning the route was learned with NDP. In particular, as highlighted in the legend part of the output, ND refers to an NDP-learned default route, and NDp refers to an NDP-learned prefix (as listed in the NDP RA message in Figure 32-6 in this case). Note also that these same two routes have an administrative distance of 2, which is the default administrative distance of IPv6 routes learned with NDP.
Example 32-16 Learning an Address and Default Static Route with DHCP
R1# show ipv6 route
IPv6 Routing Table - default - 4 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, HA - Home Agent, MR - Mobile Router, R - RIP
H - NHRP, I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea
IS - ISIS summary, D - EIGRP, EX - EIGRP external, NM - NEMO
ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect
O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2, la - LISP alt
lr - LISP site-registrations, ld - LISP dyn-eid, a - Application
ND ::/0 [2/0]
via FE80::22FF:FE22:2222, Serial0/0/0
NDp 2001:DB8:1:12::/64 [2/0]
via Serial0/0/0, directly connected
L 2001:DB8:1:12:32F7:DFF:FE29:8560/128 [0/0]
via Serial0/0/0, receive
! lines omitted for brevity
Troubleshooting Static IPv6 Routes
The current set of CCENT and CCNA R&S exam topics includes troubleshooting of both IPv4 and IPv6 static routes. Chapter 18 already discussed how to troubleshoot IPv4 static routes, and many of those same concepts apply to IPv6 static routes. However, IPv6 static routes do have a few small differences. This last part of the chapter looks at troubleshooting IPv6 static routes, reviewing many of the same troubleshooting rules applied to IPv4 static routes, while focusing on the details specific to IPv6.
This topic breaks static route troubleshooting into two perspectives: the route is in the routing table but is incorrect and cases in which the route is not in the routing table.
Troubleshooting Incorrect Static Routes That Appear in the IPv6 Routing Table
A static route is only as good as the input typed into the ipv6 route command. IOS checks the syntax of the command, of course. However, IOS cannot tell if you choose the incorrect outgoing interface, incorrect next-hop address, or incorrect prefix/prefix-length in a static route. If the parameters pass the syntax checks, IOS places the ipv6 route command into the running-config file. Then, if no other problem exists (as discussed at the next heading), IOS puts the route into the IP routing table—even though the route may not work because of the poorly chosen parameters.
For instance, an exam question might show a figure with Router R1 having an address of 2001:1:1:1::1 and neighboring Router R2 with an address of 2001:1:1:1::2. If R1 lists a static route with the command ipv6 route 3333::/64 2001:1:1:1::1, the command would be accepted by IOS with correct syntax, but it would not be effective as a route. R1 cannot use its own IPv6 address as a next-hop address. IOS does not prevent the configuration of the command, however; it allows the command and adds the route to the IPv6 routing table, but the route cannot possibly forward packets correctly.
When you see an exam question that has static routes, and you see them in the output of show ipv6 route, remember that the routes may have incorrect parameters. Check for these types of mistakes:
Step 1. Prefix/Length: Does the ipv6 route command reference the correct subnet ID (prefix) and mask (prefix length)?
Step 2. If using a next-hop IPv6 address that is a link-local address:
A. Is the link-local address an address on the correct neighboring router? (It should be an address on another router on a shared link.)
B. Does the ipv6 route command also refer to the correct outgoing interface on the local router?
Step 3. If using a next-hop IPv6 address that is a global unicast or unique local address, is the address the correct unicast address of the neighboring router?
Step 4. If referencing an outgoing interface, does the ipv6 route command reference the interface on the local router (that is, the same router where the static route is configured)?
This troubleshooting checklist works through the various cases in which IOS would accept the configuration of the static IPv6 route, but the route would not work because of the incorrect parameters in context. It helps to see a few examples. Figure 32-7 shows a sample network to use for the examples; all the examples focus on routes added to Router R1, for the subnet on the far right.
Example 32-17 shows five ipv6 route commands. All have correct syntax, but all have one incorrect value; that is, the route will not work because of the types of problems in the troubleshooting checklist. Look for the short comment at the end of each configuration command to see why each is incorrect.
Example 32-17 ipv6 route Commands with Correct Syntax but Incorrect Ideas
ipv6 route 2001:DB8:9:33::/64 2001:DB8:9:2::2 ! Step 1: Wrong prefix
ipv6 route 2001:DB8:9:3::/64 G0/2 FE80::AAA9 ! Step 2A: Wrong neighbor link local
ipv6 route 2001:DB8:9:3::/64 FE80::2 ! Step 2B: Missing outgoing interface
ipv6 route 2001:DB8:9:3::/64 2001:DB8:9:2::1 ! Step 3: Wrong neighbor address
ipv6 route 2001:DB8:9:3::/64 G0/1 FE80::2 ! Step 4: Wrong interface on R1
All these incorrect examples have correct syntax and would be added to R1’s IPv6 routing table if configured on R1. However, all have flaws. Working through the examples in order:
Step 1. The prefix (2001:DB8:9:33::) has a typo in the fourth quartet (33 instead of 3).
Step 2A. The figure shows R2’s G0/1 with link-local address FE80::2, but the command uses FE80::AAA9.
Step 2B. The command uses the correct link-local address on R2’s address on the common link (FE80::2 per the figure), but it omits the outgoing interface of R1’s G0/2 interface. (See the next example for more detail.)
Step 3. The figure shows the subnet in the center as 2001:DB8:9:2::/64, with R1 using the ::1 address and R2 using ::2. For the fourth command, R1’s command should use R2’s address 2001:DB8:9:2::2, but it uses R1’s own 2001:DB8:9:2::1 address instead.
Step 4. As a command on R1, the outgoing interface references R1’s own interfaces. R1’s G0/1 is the interface on the left, whereas R1 should use its G0/2 interface on the right when forwarding packets to subnet 2001:DB8:9:3::/64.
The key takeaway for this section is to know that a route in the IPv6 routing table may be incorrect due to poor choices for the parameters. The parameters should always include the neighboring router’s IPv6 addresses, but the local router’s interface type/number, and in all cases, the correct prefix/length. The fact that a route is in the IPv6 routing table, particularly a static route, does not mean it is a correct route.
Note that of the five example commands in Example 32-17, IOS would accept all of them except the third one. IOS can notice the case of omitting the outgoing interface if the next-hop address is a link-local address. Example 32-18 shows a sample of the error message from IOS.
Example 32-18 IOS Rejects the ipv6 route Command with Link-Local and No Outgoing Interface
R1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)# ipv6 route 2001:DB8:9:3::/64 FE80::2
% Interface has to be specified for a link-local nexthop
R1(config)# ^Z
R1#
R1# show running-config | include ipv6 route
R1#
The Static Route Does Not Appear in the IPv6 Routing Table
The preceding few pages focused on IPv6 static routes that show up in the IPv6 routing table but unfortunately have incorrect parameters. The next page looks at IPv6 routes that have correct parameters, but IOS does not place them into the IPv6 routing table.
When you add an ipv6 route command to the configuration, and the syntax is correct, IOS considers that route to be added to the IPv6 routing table. IOS makes the following checks before adding the route; note that IOS uses this same kind of logic for IPv4 static routes:
For ipv6 route commands that list an outgoing interface, that interface must be in an up/up state.
For ipv6 route commands that list a global unicast or unique local next-hop IP address (that is, not a link-local address), the local router must have a route to reach that next-hop address.
If another IPv6 route exists for that exact same prefix/prefix-length, the static route must have a better (lower) administrative distance.
For example, Router R1, again from Figure 32-7, has been configured with IPv6 addresses. Example 32-19 shows the addition of an ipv6 route command for remote subnet 2001:DB8:9:3::/64, but with incorrect next-hop address 2001:DB8:9:3::2. That address is on R2, but it is the address on the far side of R2, on R2’s G0/2 interface.
Example 32-19 No Route for Next-Hop IPv6 Address in Static Route
R1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)# ipv6 route 2001:DB8:9:3::/64 2001:DB8:9:3::2
R1(config)# ^Z
R1# show ipv6 route
IPv6 Routing Table - default - 5 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, HA - Home Agent, MR - Mobile Router, R - RIP
H - NHRP, I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea
IS - ISIS summary, D - EIGRP, EX - EIGRP external, NM - NEMO
ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect
RL - RPL, O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1
OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
la - LISP alt, lr - LISP site-registrations, ld - LISP dyn-eid
a - Application
C 2001:DB8:9:1::/64 [0/0]
via GigabitEthernet0/1, directly connected
L 2001:DB8:9:1::1/128 [0/0]
via GigabitEthernet0/1, receive
C 2001:DB8:9:2::/64 [0/0]
via GigabitEthernet0/2, directly connected
L 2001:DB8:9:2::1/128 [0/0]
via GigabitEthernet0/2, receive
L FF00::/8 [0/0]
via Null0, receive
Chapter Review
One key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book, DVD, or interactive tools for the same material found on the book’s companion website. Refer to the “Your Study Plan” element for more details. Table 32-3 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column.
Command References
Tables 32-5 and 32-6 list configuration and verification commands used in this chapter. As an easy review exercise, cover the left column in a table, read the right column, and try to recall the command without looking. Then repeat the exercise, covering the right column, and try to recall what the command does.
Table 32-5 Chapter 32 Configuration Command Reference
Table 32-6 Chapter 32 EXEC Command Reference