Answers: IPv4 Static Routes 1

certskills
By certskills May 10, 2016 09:10

 

The configuration for this latest lab is relatively simple. However, it definitely helps you solidify your understanding of IPv4 routing, and that is something you should definitely master along the way. As always, start by checking on the lab and answering it your own. Then come back here for the answers and explanation.

 

Answers

Figure 1: Simple Hub and Spoke Topology

Example 1: Core Config

 

Example 2: Branch1 Config

 

Example 3: Branch2 Config

 

Example 4: Branch3 Config

 

Commentary

When configuring static routes, it is important to ensure that you have all of the correct IPv4 subnet information, just a little difference in subnet mask can make a big difference in routing behavior.

For this lab you were tasked with configuring static routes to all remote subnets on all routers. The figure shows six different subnets. The Core router connects to three subnets, so it will need three static routes. Each branch router connects to two subnets, so each branch router will need static routes for four different subnets.

For Core, the three remote LAN subnets are 192.168.50.0/24, 192.168.51.0/24 and 192.168.52.0/24. Just to help us get our bearings, Table 1 lists the three destination subnets, with router Core’s outgoing interface and next-hop global unicast address:

Destination Subnet

Off of Router…

Core Router’s Outgoing interface

Core’s Next-Hop Address

192.168.50.0/24

Branch1

G0/1

10.100.200.2

192.168.51.0/24

Branch2

G0/2

10.100.200.6

192.168.52.0/24

Branch3

G0/3

10.100.200.10

Table 1: Data to Use in Router Core’s Static IPv4 Routes

The configuration, shown in Example 1, repeats the ip route command three times. Each time it lists the destination subnet per the first column of the table, and the next hop address per the last column. For example, the complete command to configure one static route would be ip route 192.168.50.0 255.255.255.0 10.100.200.2. (Note that the mask is in DDN format, not prefix format.)

For Branch1, before thinking about the configuration command, think about the topology in the figure. Branch1 has two connected subnets. Then, to send packets to the other four subnets, router Branch1 will always send the packets out its G0/1 interface to the Core router next. As a result, and as seen in Example 2, all four of Branch1’s routes list 10.100.200.1 as the next hop address. The four commands each list different destination subnets: 10.100.200.4/30, 192.168.51.0/24, 10.100.200.8/30, and 192.168.52.0/24.

Branch2 uses the same basic logic as router Branch1. Branch2 has two connected subnets, four remote subnets, and all four static routes for the remote subnets points to router Core next. In Branch2’s case, the next-hop address of the Core router is 10.100.200.5. As seen in Example 3, the four ip route commands list 10.100.200.5 as the next-hop address, with subnets 10.100.200.0/30, 192.168.50.0/24, 10.100.200.8/30, and 192.168.52.0/24 in the four different commands.

As you probably guess by now, Branch3 uses the same basic logic as routers Branch1 and Branch2. Branch3 has two connected subnets, four remote subnets, and all four static routes for the remote subnets points to router Core next. In Branch3’s case, the next-hop address of the Core router is 10.100.200.9. As seen in Example 4, the four ip route commands list 10.100.200.9 as the next-hop address, with subnets 10.100.200.0/30, 192.168.50.0/24, 10.100.200.4/30, and 192.168.51.0/24.

IPv4 Static Routes 1
EIGRPv6 for IPv6 #2
certskills
By certskills May 10, 2016 09:10
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11 Comments

  1. Mika July 7, 15:27

    I can’t seem to ping any of the pcs to eachother. For instance PC1 to PC2 the ping fails. Is this supposed to happen?

    Reply to this comment
  2. PedroT August 1, 11:58

    Hi, I have two questions regarding this:
    1)Could I use G0/1 instead of the next-hop IP address when configuring the routes?
    2)Why do we need to configure routes to the subnets connecting two routers? (for example, Branch 1 configuring a route to subnet 10.100.200.4 in this example).

    Best regards,
    Pedro

    Reply to this comment
    • CCENTSkills August 10, 11:07

      Hi Pedro,
      Specifically to your questions…
      1) As for syntax, yes. However, using an outgoing interface that’s a LAN interface is ambiguous, that is, any number of next-hop devices could be reachable out a LAN interface. So it’s good form to use either next-hop address, or both outgoing interface and next-hop address. See the chapter that covers IPv6 static routes for some of theologic.
      2) Those routes are not required for user traffic, but they would be useful for network management. EG, if the router’s LAN interface fails, but the WAN is still working, then you would have routes to make that WAN interface IP address reachable from any management software.
      Wendell

      Reply to this comment
  3. Peter August 7, 16:37

    Would it be incorrect to aggregate the static routes on router 3 to a /29 and a /23?

    Reply to this comment
    • CCENTSkills August 10, 11:08

      Peter,
      The math definitely works! I didn’t go there in the lab, but removing those blinders, yes for sure, that would work.
      Wendell

      Reply to this comment
  4. Nick January 25, 09:40

    Hi, Why does Branch1 need static routes to 200.4/30 and 200.8/30 if they are connected routes on Core?

    Reply to this comment
    • CCENTSkills January 25, 10:00

      Hi Nick,
      Two answers. Literally, it’s because the lab asked for them. Branch1 doesn’t have connected routes for those subnets, and the lab asked for static routes for all subnets that aren’t connected.
      If you were asking more about real life, and why bother to configure those static routes for WAN subnets, the main reason would be to simplify troubleshooting. By ensuring that all routers have routes to the WAN subnets, any ping/traceroute commands done from other routers’ CLIs won’t fail just because you didn’t bother to add the static routes.

      Of course, in real life, you’d use a routing protocol, which would learn those routes. The exercise is admittedly not as much a real life example as an excuse to practice static route config. 🙂
      Wendell

      Reply to this comment
  5. Tobias November 8, 11:41

    The branch routers can all use default routes as static routes instead of specific ones to save some CPU cycles!

    These default routes are also known as quad-zero routes. The config would be as follows:

    B1 = ip route 0.0.0.0 0.0.0.0 10.100.200.1
    B2 = ip route 0.0.0.0 0.0.0.0 10.100.200.5
    B3 = ip route 0.0.0.0 0.0.0.0 10.100.200.9

    Reply to this comment
  6. Chris November 11, 21:11

    Hi. For the branches router can i just use the last resort Gateway (0.0.0.0 0.0.0.0 serial interface that connects to core router) because every packet send to another subnet that resides out of the LAN of that router will always be sent to the core router.

    Reply to this comment
    • CCENTSkills November 27, 10:22

      Chris (and Tobias),
      I agree, you could use default routes as well. The lab asked that you use the routes shown in the answer as listed here, but absolutely, routing would work with a default route at the remote routers as well.
      Wendell

      Reply to this comment
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