CCNP Route Lab 3-4, OSPF over Frame Relay

CCNP Route Lab 3-4, OSPF over Frame Relay

Topology

ccnp-route-lab-ospf-frame-relay

Objectives

  • Configure OSPF over Frame Relay.
  • Use non-broadcast and point-to-multipoint OSPF network types.
  • Modify default OSPF timers.

Background
You are responsible for configuring the new network to connect your company’s East branch and West branch through the company headquarters represented by loopback interfaces on each of the three routers. The physical devices have just been installed and connected over Frame Relay hub-and-spoke topology. Configure OSPF to allow full connectivity between all departments.

To simulate the Frame Relay WAN connections, use a router with three serial ports to act as a Frame Relay switch. The configuration of the router as a Frame Relay switch is described in Step 2. The Fast Ethernet connection between router EAST and router WEST represents a backup link from a service provider.

In real-world Frame Relay deployments, a modem or CSU/DSU normally provides the clocking functions (DCE) for each serial link. However, in this lab, R2 interface Serial0/0/1 is configured as the DCE for compatibility with other labs. If you are uncertain which side of the connection is the DCE, use the show controllers serial interface-number command:

Note: In this lab, router R3 acts as the Frame Relay switch and requires two serial interface cards. If you are using an Adtran as a Frame Relay switch, see Appendix A for the Adtran configuration. When using the Adtran as the Frame Relay switch, the clock (DCE) is provided for each serial link.

Instructor note: You can also use Lab 3.4b from CCNP version 5 in the instructor lab manual, if available.

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. The switch is a Cisco WS-C2960-24TT-L with the Cisco IOS image c2960-lanbasek9-mz.122-46.SE.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 or switch model and Cisco IOS Software version, the commands available and output produced might vary from what is shown in this lab.

Required Resources

  • 4 routers (Cisco 1841 with Cisco IOS Release 12.4(24)T1 Advanced IP Services or comparable)
  • 1 switch (Cisco 2960 with the Cisco IOS Release 12.2(46)SE C2960-LANBASEK9-M image or comparable)
  • Serial and Ethernet cables

Step 1: Configure Frame Relay and addressing on the HQ, EAST, and WEST routers.
a. Configure the router physical interfaces with IP addresses. On the interfaces connected to the Frame Relay switch, configure Frame Relay encapsulation with Inverse ARP disabled. Inverse ARP is disabled in this lab so that you have exclusive control over the IP/DLCI mappings. Use frame relay map statements to configure local Frame Relay maps so that you can ping the router’s interface. Also set up the loopback interfaces.

Router R1 (Hostname HQ)

Router R2 (Hostname EAST)

Router R4 (Hostname WEST)

b. Verify that you have local subnet connectivity with ping.

Step 2: Configure the Frame Relay switch.
Use a fourth Cisco router with three serial interfaces as a Frame Relay switch, and cable the routers according to the diagram. Configure the Frame Relay switch to have the DLCIs indicated in the diagram between HQ and EAST and HQ and WEST.

Router R3 (FRS)

Step 3: Configure OSPF network type NBMA.
Frame Relay is inherently a non-broadcast multi-access (NBMA) network. In this step, you configure OSPF for hub-and-spoke over Frame Relay using the NBMA OSPF network type, which is the default for Frame Relay physical interfaces and multipoint-type subinterfaces. HQ is the hub; EAST and WEST are the spokes. a. Create OSPF process 1. Add the Frame Relay interfaces on each router into area 0 with the network command. Add the loopback interfaces on each router into area 0, and then change the network type to allow the correct subnet mask to be advertised.

b. On EAST and WEST, change the Frame Relay interfaces to have OSPF priority 0. This priority ensures that HQ becomes the DR.

c. No OSPF Frame Relay adjacencies will be established yet because the default network type is nonbroadcast. You can change this by adding neighbor statements. Configure neighbor statements on HQ pointing toward EAST and WEST. Only the router starting the exchange needs the statements (HQ in this case). However, it is considered best practice to also specify HQ as a neighbor on the EAST and WEST routers. Because the hello timers are longer on serial nonbroadcast links, the neighbor adjacencies might take longer to come up.

Note: Neighbor commands shown for the EAST and WEST routers will not appear in the running config. This is because the local router’s OSPF priority has been set to 0 on the interface which would be used to communicate with the designated neighbor.

d. You can verify adjacency states with the show ip ospf neighbor command.

Step 4: Change the network type to point-to-multipoint.
Point-to-multipoint is an OSPF network type that lends itself well to a hub-and-spoke topology. Point tomultipoint does not elect DRs or BDRs, so it does not need interface priorities. Instead, it treats the network as a collection of point-to-point networks and advertises host routes for any neighbors that it has.

a. To configure point-to-multipoint links, remove the previously configured neighbor statements and interface priorities..

b. Use the interface command ip ospf network point-to-multipoint and reapply the Frame Relay maps using the broadcast option. Reset the OSPF process using the clear ip ospf process command. Verify that the adjacencies are active with the show ip ospf neighbor command.

c. Observe the routing table on one of the spoke routers. Notice how the routing table has host routes in it. This is part of point-to-multipoint behavior.

d. Look at the output of the show ip ospf interface interface command on your routers. Notice that the interface type is point-to-multipoint.

Step 5: Change OSPF timers.
a. Add the Ethernet link connecting EAST and WEST to the OSPF process using the network command.

Because it is a Fast Ethernet link, the default network type is broadcast and the default network timers are those associated with a broadcast network. You might want to change the default timers to allow for better network convergence. Neighbors that go down are detected more quickly with lower dead timers. The disadvantage of lower dead timers is higher router CPU utilization and more bandwidth being consumed by hello packets.

c. Use the ip ospf hello-interval seconds command to change the default hello timer interval to 5 seconds on both sides. Change the dead timer to 15 seconds with the ip ospf dead-interval seconds command.

d. Verify the changes with the show ip ospf interface interface command.

What are some disadvantages to changing the timers if they are not tuned correctly?
If timers are not tuned correctly, there are many negative side effects. Hello packets can saturate a slower link or overload a slower router. Route instability and route flapping can occur from bad timers having an adjacency come up and down. If timers are tuned to take too long, reconvergence of a network after topology changes can be slower.

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

Challenge: Minimal Hello Intervals
Configure the Fast Ethernet link between EAST and WEST to lower the convergence time using the ip ospf dead-interval minimal hello-multiplier multiplier command. This command sets the dead interval to 1 second. Hellos will be sent at the rate of multiplier per second.

Note: The use of this command overrides hello and dead intervals configured on EAST and WEST Fa0/0 in Step 5.

a. Configure the routers to send five hellos a second.

Note: Although you could use this command on the serial links, it would create additional OSPF overhead on these relatively slow (64 Kb/s) links and could result in flapping adjacencies if a link begins to experience congestion. Using this command with high-speed serial links would be less of an issue.

b. Look at the dead time column of the show ip ospf neighbor command. Is it a different format than before
for that connection?
Now the dead timer is displayed in milliseconds, not seconds.

c. Display the OSPF information for Fa0/0 on EAST or WEST using the show ip ospf interface fastEthernet 0/0 command. What are the Hello and Dead intervals now?
The Hello interval is 200 msec (1/5 of a second) and the Dead interval is 1 second.

Router Interface Summary Table

Router Interface Summary
Router Model Ethernet Interface
#1
Ethernet Interface
#2
Serial Interface
#1
Serial Interface
#2
1700 Fast Ethernet 0
(Fa0)
Fast Ethernet 1
(Fa1)
Serial 0 (S0) Serial 0/0/1
(S0/0/1)
1800 Fast Ethernet 0/0
(Fa0/0)
Fast Ethernet 0/1
(Fa0/1)
Serial 0/0/0
(S0/0/0)
Serial 0/0/1
(S0/0/1)
2600 Fast Ethernet 0/0
(Fa0/0)
Fast Ethernet 0/1
(Fa0/1)
Serial 0/0 (S0/0) Serial 0/1 (S0/1)
2800 Fast Ethernet 0/0
(Fa0/0)
Fast Ethernet 0/1
(Fa0/1)
Serial 0/0/0
(S0/0/0)
Serial 0/0/1
(S0/0/1)
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.

If an Adtran Atlas 550 is used for the Frame Relay switch, connect the serial cable from each router interface
in the topology diagram to the Adtran interface indicated in the table below. The Adtran is preconfigured to
simulate a Frame Relay service that provides the following PVCs.

Frame Relay Switching Configuration

Connected
Router
Router
Interface
Adtran
Interface
Ingress
DLCI
Egress
DLCI
Egress
Router
HQ S0/0/1 DTE port 1/1 102 201 East
HQ S0/0/1 DTE port 1/1 103 301 West
East S0/0/1 DTE port 1/2 201 102 HQ
West S0/0/0 DTE port 2/1 301 103 HQ

The Adtran Frame Relay switch interfaces all provide the DCE clock. Be sure to use the appropriate cable between each router and the Adtran. All the router interfaces are DTE, and the cable to the Adtran interface should be serial to V.35 DCE. Use the show controllers command to verify which cable type is connected to a
given router interface.

Device Configurations (Instructor version)
Router HQ (R1)

Router EAST (R2)

Router WEST (R4)

Router FRS (R3)

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