CCNP Route Lab 3-3, OSPF Virtual Links and Area Summarization

CCNP Route Lab 3-3, OSPF Virtual Links and Area Summarization




  • Configure multi-area OSPF on a router.
  • Verify multi-area behavior.
  • Create an OSPF virtual link.
  • Summarize an area.
  • Generate a default route into OSPF.

You are responsible for configuring the new network to connect your company’s engineering, marketing, and accounting departments, represented by loopback interfaces on each of the three routers. The physical devices have just been installed and connected by serial cables. Configure multiple-area OSPF to allow full connectivity between all departments.

In addition, R1 has a loopback interface representing a connection to the Internet. This connection will not be added into OSPF. R3 will have four additional loopback interfaces representing connections to branch offices.

Note: This lab uses Cisco 1841 routers with Cisco IOS Release 12.4(24)T1 and the Advanced IP Services image c1841 -advipservicesk9-mz.124-24.T1 .bin. You can use other routers (such as a 2801 or 2811) and Cisco IOS Software versions if they have comparable capabilities and features. Depending on the router model and Cisco IOS Software version, the commands available and output produced might vary from what is shown in this lab.

Required Resources

  • 3 routers (Cisco 1841 with Cisco IOS Release 12.4(24)T1 Advanced IP Services or comparable)
  • Serial and console cables

Step 1: Configure addressing and loopbacks.
Using the addressing scheme in the diagram, apply IP addresses to the serial interfaces on R1, R2, and R3. Create loopbacks on R1, R2, and R3, and address them according to the diagram.

Step 2: Add interfaces into OSPF.
a. Create OSPF process 1 on all three routers. Using the network command, configure the subnet of the serial link between R1 and R2 to be in OSPF area 0. Add loopback 1 on R1 and loopback 2 on R2 into OSPF area 0.

Note: The default behavior of OSPF for loopback interfaces is to advertise a 32-bit host route. To ensure that the full /24 network is advertised, use the ip ospf network point-to-point command. Change the network type on the loopback interfaces so that they are advertised with the correct subnet.

b. Verify that you can see OSPF neighbors in the show ip ospf neighbors output on both routers. Verify that the routers can see each other’s loopback with the show ip route command.

c. Add the subnet between R2 and R3 into OSPF area 23 using the network command. Add loopback 3 on R3 into area 23.

d. Verify that this neighbor relationship comes up with the show ip ospf neighbors command.

e. Using a Tcl script, verify connectivity to all interfaces from any router, with the exception of loopback 30 on R1, and R3 loopbacks 100 through 103.

Step 3: Create a virtual link.
a. Add loopbacks 100 through 103 on R3 to the OSPF process in area 100 using the network command. Change the network type to advertise the correct subnet mask.

b. Look at the output of the show ip route command on R2. Notice that the routes to those networks do not appear. The reason for this behavior is that area 100 on R3 is not connected to the backbone. It is only connected to area 23. If an area is not connected to the backbone, its routes are not advertised outside of its area.

What would happen if routes could pass between areas without going through the backbone?
Routing loops might occur because any route could get advertised to different areas. By passing through the backbone, type 3 LSAs are generated by their respective areas and not sent back in. You can get around this situation by creating a virtual link. A virtual link is an OSPF feature that creates a logical extension of the backbone area across a regular area, without actually adding any physical interfaces into area 0.

Note: Prior to creating a virtual link you need to identify the OSPF router ID for the routers involved (R2 and R3), using a command such as show ip ospf, show ip protocols or show ip ospf interface. The output for the show ip ospf command on R1 and R3 is shown below.

c. Create a virtual link using the area transit_area virtual-link router-id OSPF configuration command on both R2 and R3.

Note: To ensure that the router ID of the virtual link endpoints remains constant, you can statically configure the OSPF router ID of the virtual link endpoints using the router-id command.

d. After you see the adjacency over the virtual interface come up, issue the show ip route command on R2 and see the routes from area 100. You can verify the virtual link with the show ip ospf neighbor and show ip ospf interface commands.

When are virtual links useful?
Virtual links are useful when there needs to be a temporary extension of the backbone, either because the backbone became discontiguous or a new area got added onto an existing area.

Why are virtual links a poor long-term solution?
Virtual links are a poor long-term solution because they add processing overhead and basically extend the backbone area onto routers where it might not belong. They can also add a lot of complexity to troubleshooting.

Step 4: Summarize an area.
Loopbacks 100 through 103 can be summarized into one supernet of /22. You can configure area 100 to be represented by this single summary route.

a. Configure R3 (the ABR) to summarize this area using the area area range network mask command.

b. You can see the summary route on R2 with the show ip route and show ip ospf database commands.

c. Notice on R3 that OSPF has generated a summary route pointing toward Null0.

This behavior is known as sending unknown traffic to the “bit bucket.” This means that if the router advertising the summary route receives a packet destined for something covered by that summary but not in the routing table, it drops it.

What is the reasoning behind this behavior?
The reason that summaries generate local routes to Null0 is that when a router creates a summary address, it should have routes to all the existent more-specific routes. If the router lacks a more-specific route for a prefix within the summary, it is assumed that the route does not exist, and packets destined for that prefix should be dropped. If the route did not exist, bandwidth could be wasted if this router has a less specific route (such as a default route) and forwards the packet to the route until it is dropped further down the line.

The discard route also solves another problem. Depending on the contents of the routing table, a routing loop can be formed between two routers, one receiving a summary route from the second one, while the second one uses the first one as its default gateway. If a packet for a nonexistent component of the summary route was received and there was no discard route installed in the second router, the packet would loop between the routers until its TTL was decremented to 0.

Step 5: Generate a default route into OSPF.
You can simulate loopback 30 on R1 to be a connection to the Internet. You do not need to advertise this specific network to the rest of the network. Instead, you can just have a default route for all unknown traffic to go to R1 .

a. To have R1 generate a default route, use the OSPF configuration command default-information originate always. The always keyword is necessary for generating a default route in this scenario.Without this keyword, a default route is generated only into OSPF if one exists in the routing table.

b. Verify that the default route appears on R2 and R3 with the show ip route command.

c. You should be able to ping the interface connecting to the Internet from R2 or R3, despite never being advertised into OSPF.

d. Use the following Tcl script to verify connectivity to all addresses in the topology.

Challenge: Configure OSPF Authentication
Configure OSPF MD5 authentication on the link between R2 and R3, using key ID 1 and the password cisco. Record the commands used below.
Enter the following configuration commands on R2 and R3:

Router Interface Summary Table

Router Interface Summary
Router Model Ethernet Interface
Ethernet Interface
Serial Interface
Serial Interface
1700 Fast Ethernet 0
Fast Ethernet 1
Serial 0 (S0) Serial 0/0/1
1800 Fast Ethernet 0/0
Fast Ethernet 0/1
Serial 0/0/0
Serial 0/0/1
2600 Fast Ethernet 0/0
Fast Ethernet 0/1
Serial 0/0 (S0/0) Serial 0/1 (S0/1)
2800 Fast Ethernet 0/0
Fast Ethernet 0/1
Serial 0/0/0
Serial 0/0/1
Note: To find out how the router is configured, look at the interfaces to identify the type of router and how many interfaces the router has. Rather than list all combinations of configurations for each router class, this table includes identifiers for the possible combinations of Ethernet and serial interfaces in the device. The table does not include any other type of interface, even though a specific router might contain one. For example, for an ISDN BRI interface, the string in parenthesis is the legal abbreviation that can be used in Cisco IOS commands to represent the interface.

Device Configurations (Instructor version)
Router R1

Router R2

Router R3

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