Answers: IPv6 Static Routes 3

This most recent lab asked you to configure a bunch of static IPv6 routes on the same router. Some had the exact same destination subnet, and some created host routes that overlapped with other static routes. And all were assigned different administrative distance values. Which ones would you expect to see in the router’s IPv6 routing table? Take a look at today’s answer post after taking your own attempt at configuring the lab per the original lab post.

Answers

Figure 1: Router Triangle Topology

Example 1: R1 Config

Commentary

This lab is focused with two primary goals in mind. First, to further practice in using IPv6 static routes to remote networks using both networks and host routes. Second, to highlight some of the differences in forwarding behavior when using multiple different administrative distances.

There are two major things you need to keep in mind when determining which route will be used to a network: First, is this the most specific route to the destination? Second, if so, does this route have the lowest administrative distance? Keep in mind that the order of these two questions is very important.

For this lab, you were tasked with configuring five different static IPv6 routes. Three of these routes offer different paths to R5’s LAN subnet. The requirements tell you to use three different administrative distances. Those three routes are:

1. The first route to configure for R5’s LAN uses R2 as a next hop and an administrative distance of 50; the LAN between R1 and R2 uses the 3000:1:2::/64 network with R2 being configured with the 3000:1:2::2 address as the next hop address. The command to configure this route would be ipv6 route 2000::/64 3000:1:2::2 50.
2. The second route to configure for R5’s LAN uses R3 as a next hop and an administrative distance of 40; the LAN between R1 and R3 uses the 3000:1:3::/64 network with R3 being configured with the 3000:1:3::2 address as the next hop address. The command to configure this route would be ipv6 route 2000::/64 3000:1:3::2 40.
3. The third and final route to configure for R5’s LAN uses R4 as a next hop and an administrative distance of 60; the LAN between R1 and R4 uses the 3000:1:4::/64 network with R4 being configured with the 3000:1:4::2 address as the next hop address. The command to configure this route would be ipv6 route 2000::/64 3000:1:4::2 60.

Ask yourself at this point, which of these routes would be preferred when selecting a path from R1 to R5’s LAN?

The other two routes requested in for this lab are host routes for PC2 and PC4 which potentially alter the routing behavior of R1. The first was a host route for PC2 using R2 as a next hop and an administrative distance of 100. The command to configure this route would be ipv6 route 2000::102/128 3000:1:2::2 100. The second was a host route for PC4 using R4 as a next hop and an administrative distance of 110; the command to configure this route would be ipv6 route 2000::104/128 3000:1:4::2 110. Note that the use of the /128 prefix length is the IPv6 equivalent of using the 255.255.255.255 subnet mask.

Ask yourself now, will either of these host routes alter the routing behavior of traffic from R1 to PC2 and PC4? The answer is yes, this is because each of these routes will be more specific than the previous three routes for the 2000::/64 subnet. R5 only considers the administrative distance if the destination of the route is exactly the same.

The two host routes cause R5 to forward packet of those two hosts over those specific routes:

• Traffic destined for PC2 will always go through R2 assuming the interface is still operational.
• Traffic destined for PC4 will always go through R4 assuming that interface is still operational.

Just to complete the idea, Example 2 lists router R1’s routing table after adding the configuration in this lab. It shows only one of the three static routes to subnet 2000:://64, the one with 3000:1:3::2 as next hop, with administrative distance 40. Why? Because those three routes have the exact same destination (2000::/64), but different administrative distance values, so router R1 adds only the router that has the best AD. However, for the two host routes, the destination of each route is unique, so the AD values of 100 and 110 were not needed by R1 to choose between multiple static routes to the same destination.

Example 2: R1 IPv6 Routing Table