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.
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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.