CCNP Route Lab 4-3, Manipulating Administrative Distances

CCNP Route Lab 4-3, Manipulating Administrative Distances

Topology

Objectives

• Configure RIP on a router.
• Configure OSPF on a router.
• Manipulate administrative distances.
• Compare routing protocol behavior.
  

Background
In this lab, you will compare the RIP and OSPF routing protocols based on how efficient they are at selecting routes, as well as what happens when you manipulate administrative distances in the routing table.

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), switches (such as 2950), 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

• 3 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 : Review default administrative distances.
Fill in the following table with all the administrative distances you can recall from your reading.

Of the interior gateway protocols (IGPs) that you have studied, which one is considered the most trusted on a
Cisco router and why?
Currently, EIGRP is considered the most trusted IGP on Cisco routers with an administrative distance of 90.

Step 2: Configure router loopbacks and addressing.
Configure all loopback interfaces on the three routers in the diagram. Configure the serial interface with the IP addresses, bring them up, and set a clock rate where appropriate.

Step 3: Configure switch VLANs.
a. Configure the switch VLANs, and place the correct access ports in each VLAN.

Note: The switch ports used are not important as long as the ports connecting to R1 Fa0/0 and R2 Fa0/0 are in VLAN 12 and the ports connecting to R3 Fa0/0 and R2 Fa0/1 are in VLAN 23.

b. Verify that you can ping across the local subnets.

Step 4: Configure RIP.
a. Configure RIPv2 on all three routers for the major networks. Disable automatic summarization.

b. Verify the configuration using the show ip route rip command on each router.

c. Verify that each router is receiving RIP routes from other routers using the show ip protocols command.

Step 5: Configure OSPF.
a. Configure OSPF on all routers. Include the entire major network in area 0 on all three routers. Remember to change the network type on the loopback interfaces.

b. Verify the configuration using the show ip ospf neighbors and show ip route commands on each router.

What is the best next hop on R1 for 172.16.3.1 with only RIP running?
On R1, the best next hop to 172.16.3.1 is the R3 serial 0/0/0 interface with an IP address of 172.16.13.3.

What is the best next hop on R1 for 172.16.3.1 with OSPF running?
On R1, the best next hop to 172.16.3.1 is the R2 Fast Ethernet 0/0 interface with an IP address of 172.16.12.2. On R1, the best next hop for the R3 loopback is now through the VLAN between R1 and R2. This is because the sum of the costs for the two Ethernet links is still less than that of the single low-bandwidth (64 kb/s) serial link. This is one of the reasons why RIP’s metric of a hop count is not very effective.

Which metric does R1 use to make routing decisions about whether to cross the serial link to R3 to reach R3’s 172.16.3.1?
The metric R1 receives for the loopback 3 network on R3 via the serial link is 1562, which is not preferred by
R1.

Use the following information for your answer.

Step 6: Modify the routing protocol distance.
The distance command is a protocol-independent way to manipulate routing protocol distances. This command is different from the routing protocol-specific commands such as distance ospf and distance eigrp. This command lets you globally change a routing protocol’s distances, change only routes from a certain neighbor or those matching an access list, or a combination of any two of these three options.

Try applying the distance distance command, which changes the distance of every route. The previous output of the show ip route command shows that OSPF marks routes it injects into the routing table with a default administrative distance of 110. RIP injects routes into the routing table with a default administrative
distance of 120.

What would happen if the administrative distance on each router for RIP were set to 100?
All RIP routes would be preferred in the routing tables over OSPF routes.

a. On all three routers, change the distance of RIP to 100.

b. Examine the output of the show ip route command. Notice that all the routes have become RIP routes because RIP now has a lower distance than OSPF.

c. You can display the new default distance for RIP using the show ip protocols command.

Step 7: Modify distance based on route source.
You can also modify administrative distance based on route source using the distance distance address wildcard command, where address and wildcard represent the peer advertising the route. For OSPF, the address is the router ID.

a. On all three routers, change the OSPF administrative distance to 85 for any routes being advertised from routers with IDs in the range of 192.168.100.0/21.

b. Verify the change with the show ip protocols and show ip route commands.

Step 8: Modify distance based on an access list
You can also modify administrative distance based on which routes match an access list using the distance distance address wildcard acl command. The way you list routes in an access list which will be used to modify distance is similar to how you list them when the access list is used to filter routes. For this lab, create an access list containing all the subnets of 172.16.0.0/16. Then associate the access list with the distance command, setting the address and wildcard to be any IP address (i.e., any route source).

a. On all three routers, change the distances of the affected routes to 65.

b. Verify the change with the show ip protocols and show ip route commands.

c. Verify full connectivity with the following Tcl script.

Challenge
Attempt this exercise based on what you know about OSPF, Dijkstra’s algorithm, and the distance command. Using only the distance command, write out the commands necessary to confuse the routers in this topology

so that packets destined for 172.16.3.1 would continually bounce between R1 to R2?
A permanent routing loop between R1 and R2 can be created by leaving the administrative distances at their default values on R1 and, on R2, by setting the administrative distance of RIP-discovered routes from R1 to a value lower than 110 to favor them more than OSPF-discovered routes.

Because it is possible to intentionally break routing in this way, what degree of caution should be exercised when manipulating administrative distances in a production network?
Extreme caution should be used when manipulating administrative distances.

Router Interface Summary Table

Device Configurations (Instructor version)

Router R1