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Configuration Exercise: Troubleshooting CEF-Based MLS

Complete this configuration exercise to familiarize yourself with the basic troubleshooting steps of CEF-based MLS.

Required Resources

The resources and equipment required to complete this exercise are as follows:

  • A Cisco IOS–based, Layer 3 CEF-based MLS Catalyst switch such as a Catalyst 3550, 3560, 3750, 4500, or 6500 with Supervisor II or 720 with an MSFC in a Layer 3 Building Distribution submodule or Layer 3 routing topology
  • A terminal server or workstation connected directly to the console port of the Catalyst switch

Exercise Objective

The purpose of this exercise is to demonstrate the first steps in troubleshooting CEF-based MLS on a Cisco IOS–based Catalyst switch. Troubleshooting CEF-based MLS is generally a last step in troubleshooting connectivity problems. Generally, connectivity problems are the result of a physical layer issue or configuration issue. Troubleshoot CEF-based MLS after all other troubleshooting steps have been exhausted.

Network Diagram

Figure 9-10 shows the network layout for this configuration exercise. The configuration exercise uses a Catalyst 3560 switch from the Building Distribution submodule to illustrate commands. This switch learns IP routing entries from the Campus Backbone switches.

Figure 10

Figure 9-10 Network Diagram for Configuration Exercise

Command List

In this configuration exercise, you use the commands listed in Table 9-4. These commands are in alphabetical order so that you can easily locate the information you need. Refer to this list if you need configuration command assistance during the exercise. The table includes only the specific parameters used in the example and not all the available options for the command.

Table 9-4 Command List for Configuration Exercise

Command

Description

ping ip_address

Sends ICMP echo requests to a specific IP address

show adjacency detail

Displays the CEF adjacency table and rewrite information

show ip arp ip-address

Displays the ARP table for a specific IP address

show ip cef destination-network detail

Displays the IP CEF FIB table for a specific destination network

show ip route destination-subnet

Displays the Cisco IOS IP routing table

Task 1: Establish Console (Out-of-Band) Connectivity or Telnet or SSH Connectivity to the Switch

This task is self-explanatory.

Task 2: Determine the IP Routing Entry or Subnet That Is Experiencing Connectivity Problems or a CEF-Based MLS Issue

For this configuration exercise, use the destination subnet, 10.20.30.0/24, as an example.

Task 3: Verify the Cisco IOS IP Routing Table and ARP Entries for the Route

Step 1

Verify that a route exists to the destination network in the IP routing table.

Switch#show ip route 10.20.30.0 255.255.255.0
Routing entry for 10.20.30.0/24
 Known via "ospf 3738", distance 110, metric 2, type intra area
 Last update from 10.20.248.6 on Port-channel2, 05:59:50 ago
 Routing Descriptor Blocks:
 * 10.20.248.34, from 10.20.255.14, 05:59:50 ago, via Port-channel4
  Route metric is 2, traffic share count is 1
 10.20.248.6, from 10.20.255.13, 05:59:50 ago, via Port-channel2
  Route metric is 2, traffic share count is 1

Step 2

Verify the ARP entries for the next-hop addresses for the destination subnet obtained in Step 1.

Switch#show ip arp 10.20.248.34
Protocol Address   Age (min) Hardware Addr Type Interface
Internet 10.20.248.34   1 00d0.03e5.840a ARPA Port-channel4

Switch#show ip arp 10.20.248.6
Protocol Address   Age (min) Hardware Addr Type Interface
Internet 10.20.248.6    1 00d0.03eb.b40a ARPA Port-channel2

Step 3

Using the ping command, send ICMP echoes to the next-hop address to verify connectivity.

Switch#ping 10.20.248.34

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.20.248.34, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

Switch#ping 10.20.248.6

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.20.248.6, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms 

Task 4: Verify the IP CEF FIB and Adjacency Entries for the Route

Step 1

Verify the IP CEF FIB entry for the destination subnet in question.

Switch#show ip cef 10.20.30.0 detail
10.20.30.0/24, version 122, epoch 0, per-destination sharing
0 packets, 0 bytes
 via 10.20.248.34, Port-channel4, 0 dependencies
 traffic share 1
 next hop 10.20.248.34, Port-channel4
 valid adjacency
 via 10.20.248.6, Port-channel2, 0 dependencies
 traffic share 1
 next hop 10.20.248.6, Port-channel2
 valid adjacency
 0 packets, 0 bytes switched through the prefix

Step 2

Verify the CEF adjacency entry for the next-hop address obtained from Step 1 and compare the results to Step 2 in Task 3.

Switch#show adjacency detail | begin 10.20.248.34
IP  Port-channel4    10.20.248.34(170)
         0 packets, 0 bytes
         00D003E5840A
         00D00624440A0800
         ARP  00:05:42
         Epoch: 0

Switch#show adjacency detail | begin 10.20.248.6
IP  Port-channel2    10.20.248.6(166)
         0 packets, 0 bytes
         00D003EBB40A
         00D00624440A0800
         ARP  00:04:53
         Epoch: 0

Task 5: Debug the CEF FIB and Adjacency Table's Downloads to the Centralized Switching and Distributed Switching Engines, and Verify the TCAM Contents for FIB and Adjacency Tables

This task should be performed only under the supervision of a Cisco TAC engineer.

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