Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.9.xE and IOS 15.2(5)Ex

Overview

With TDR, you can check the status of copper cables on the 48-port 10/100/1000 BASE-T modules for the Catalyst 4500 series switch. TDR detects a cable fault by sending a signal through the cable and reading the signal that is reflected back. All or part of the signal can be reflected back either by cable defects or by the end of the cable.

Note Four pairs of standard category 5 cable exist. Each pair can assume one of the following states: open (not connected), broken, shorted, or terminated. The TDR test detects all four states and displays the first three as “Fault” conditions, and displays the fourth as “Terminated.” Although the CLI output is shown, the cable length is displayed only if the state is “Faulty.”

TDR feature is supported on the following modules:

WS-X4524-GB-RJ45V

WS-X4548-GB-RJ45

WS-X4548-GB-RJ45V

WS-X4548-GB-RJ45V+

WS-X4548-RJ45V+

WS-X4748-RJ45+E

WS-X4748-RJ45V+E

WS-X4748-UPOE+E

WS-X4908-10GB-RJ45 (only functions on WS-C4900M chassis)

WS-C4948E

WS-C4948E-F

TDR detects a cable fault by sending a signal along its wires and depending on the reflected signal it can determine roughly where a cable fault could be. The variations on how TDR signal is reflected back determine the results on TDR. On cat4k products, we only support cable fault types: OPEN or SHORT. We do display Terminated status in case cable is proper terminated and this is done for illustrative purpose.

Running the TDR Test

To start the TDR test, perform this task:

This example shows how to start the TDR test on port 1 on module 2:

This example shows the message that displays when the TDR test is not supported on a module:

This example shows how to display TDR test results for a port:

Note After this command is deprecated, use the diagnostic start and the show diagnostic result commands to run the TDR test and display the test results.

Note TDR is a port test; the port cannot handle traffic for the duration of the test (generally, 1 minute).

TDR Guidelines

The following guidelines apply to the use of TDR:

• If you connect a port undergoing a TDR test to an Auto-MDIX enabled port, the TDR result might be invalid. In those instances, the port on the WS-X4148-RJ45V should be administratively down before the start of the TDR test.
• If you connect a port undergoing a TDR test to a 100BASE-T port such as that on the WS-X4148-RJ45V, the unused pairs (4-5 and 7-8) is reported as faulty because the remote end does not terminate these pairs.
• Do not change the port configuration while the TDR test is running.
• Due to cable characteristics, you should run the TDR test multiple times to get accurate results.
• Do not change port status (for example, remove the cable at the near or far end) because the results might be inaccurate.
• TDR works best if the test cable is disconnected from the remote port. Otherwise, it might be difficult for you to interpret results correctly.
• TDR operates across four wires. Depending on the cable conditions, the status might show one pair is OPEN or SHORT while all other wire pairs display as faulty. This operation is acceptable because you should declare a cable faulty provided one pair of wires is either OPEN or SHORT.
• TDR intent is to determine how poorly a cable is functioning rather than to locate a faulty cable.
• When TDR locates a faulty cable, you should still use an offline cable diagnosis tool to better diagnose the problem.
• TDR results might differ between runs on different Catalyst 4500 modules because of the resolution difference of TDR implementations. When this occurs, you should refer to offline cable diagnosis tool.

Understanding How Ping Works

The ping command allows you to verify connectivity to remote hosts. If you attempt to ping a host in a different IP subnetwork, you must define a static route to the network or configure a router to route between those subnets.

The ping command is configurable from normal executive and privileged EXEC mode. Ping returns one of the following responses:

• Normal response—The normal response ( hostname is alive) occurs in 1 to 10 seconds, depending on network traffic.
• Destination does not respond—If the host does not respond, a No Answer message is returned.
• Unknown host—If the host does not exist, an Unknown Host message is returned.
• Destination unreachable—If the default gateway cannot reach the specified network, a Destination Unreachable message is returned.
• Network or host unreachable—If there is no entry in the route table for the host or network, a Network or Host Unreachable message is returned.

To stop a ping in progress, press Ctrl-C.

Running Ping

To ping another device on the network from the switch, enter this command in normal executive and privileged EXEC mode:

This example shows how to ping a remote host from normal executive mode:

This example shows how to use a ping command in privileged EXEC mode to specify the number of packets, the packet size, and the timeout period:

Understanding How IP Traceroute Works

IP traceroute allows you to identify the path that packets take through the network on a hop-by-hop basis. The command output displays all network layer (Layer 3) devices, such as routers, that the traffic passes through on the way to the destination.

Layer 2 switches can participate as the source or destination of the trace command but does not appear as a hop in the trace command output.

The trace command uses the time to live (TTL) field in the IP header to cause routers and servers to generate specific return messages. Traceroute starts by sending a User Datagram Protocol (UDP) datagram to the destination host with the TTL field set to 1. If a router finds a TTL value of 1 or 0, it drops the datagram and sends back an Internet Control Message Protocol (ICMP) Time-Exceeded message to the sender. Traceroute determines the address of the first hop by examining the source address field of the ICMP Time-Exceeded message.

To identify the next hop, traceroute sends a UDP packet with a TTL value of 2. The first router decrements the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL value of 1, discards the datagram, and returns the Time-Exceeded message to the source. This process continues until the TTL is incremented to a value large enough for the datagram to reach the destination host or until the maximum TTL is reached.

To determine when a datagram reaches its destination, traceroute sets the UDP destination port in the datagram to a very large value that the destination host is unlikely to be using. When a host receives a datagram with an unrecognized port number, it sends an ICMP Port Unreachable error message to the source. The Port Unreachable error message indicates to traceroute that the destination has been reached.

Running IP Traceroute

To trace the path that packets take through the network, enter this command in EXEC or privileged EXEC mode:

This example shows how to use the trace command to display the route a packet takes through the network to reach its destination:

Layer 2 Traceroute Usage Guidelines

These are the Layer 2 traceroute usage guidelines:

• CDP must be enabled on all the devices in the network. For Layer 2 traceroute to function properly, do not disable CDP.

If any devices in the physical path are transparent to CDP, the switch cannot identify the path through these devices.

Note For more information about enabling CDP, see Chapter31, “Configuring Cisco Discovery Protocol”

• All switches in the physical path must have IP connectivity. When a switch is reachable from another switch, you can test connectivity by using the ping command in privileged EXEC mode.
• The maximum number of hops identified in the path is ten.
• You can enter the traceroute mac or the traceroute mac ip command in privileged EXEC mode on a switch that is not in the physical path from the source device to the destination device. All switches in the path must be reachable from this switch.
• The traceroute mac command output shows the Layer 2 path only when the specified source and destination MAC addresses belong to the same VLAN. If you specify source and destination MAC addresses that belong to different VLANs, the Layer 2 path is not identified, and an error message appears.
• If you specify a multicast source or destination MAC address, the path is not identified, and an error message appears.
• If the source or destination MAC address belongs to multiple VLANs, you must specify the VLAN to which both the source and destination MAC addresses belong. If the VLAN is not specified, the path is not identified, and an error message appears.
• The traceroute mac ip command output shows the Layer 2 path when the specified source and destination IP addresses belong to the same subnet. When you specify the IP addresses, the switch uses Address Resolution Protocol (ARP) to associate the IP address with the corresponding MAC address and the VLAN ID.

– If an ARP entry exists for the specified IP address, the switch uses the associated MAC address and identifies the physical path.

– If an ARP entry does not exist, the switch sends an ARP query and tries to resolve the IP address. If the IP address is not resolved, the path is not identified, and an error message appears.

• When multiple devices are attached to one port through hubs (for example, multiple CDP neighbors are detected on a port), the Layer 2 traceroute feature is not supported. When more than one CDP neighbor is detected on a port, the Layer 2 path is not identified, and an error message appears.
• This feature is not supported in Token Ring VLANs.

Running Layer 2 Traceroute

To display the physical path that a packet takes from a source device to a destination device, enter either one of these commands:

or

These examples show how to use the traceroute mac and traceroute mac ip commands to display the physical path a packet takes through the network to reach its destination:

Enabling ICMP Protocol Unreachable Messages

If the Cisco IOS software receives a nonbroadcast packet that uses an unknown protocol, it sends an ICMP Protocol Unreachable message back to the source.

Similarly, if the software receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP Host Unreachable message to the source. This feature is enabled by default.

To enable the generation of ICMP Protocol Unreachable and Host Unreachable messages, enter the following command in interface configuration mode:

Caution If you enter the no ip unreachables command, you will break the path MTU discovery functionality. Routers in the middle of the network might be forced to fragment packets.

---

To limit the rate that Internet Control Message Protocol (ICMP) destination unreachable messages are generated, enter the following command:

Enabling ICMP Redirect Messages

Data routes are sometimes less than optimal. For example, it is possible for the router to be forced to resend a packet through the same interface on which it was received. If this occurs, the Cisco IOS software sends an ICMP Redirect message to the originator of the packet telling the originator that the router is on a subnet directly connected to the receiving device, and that it must forward the packet to another system on the same subnet. The software sends an ICMP Redirect message to the packet's originator because the originating host presumably could have sent that packet to the next hop without involving this device at all. The Redirect message instructs the sender to remove the receiving device from the route and substitute a specified device representing a more direct path. This feature is enabled by default.

However, when Hot Standby Router Protocol (HSRP) is configured on an interface, ICMP Redirect messages are disabled (by default) for the interface. For more information on HSRP, refer to the following URL:

http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_hsrp_ps6350_TSD_Products_Configuration_Guide_Chapter.html

To enable the sending of ICMP Redirect messages if the Cisco IOS software is forced to resend a packet through the same interface on which it was received, enter the following command in interface configuration mode:

Enabling ICMP Mask Reply Messages

Occasionally, network devices must know the subnet mask for a particular subnetwork in the internetwork. To obtain this information, devices can send ICMP Mask Request messages. These messages are responded to by ICMP Mask Reply messages from devices that have the requested information. The Cisco IOS software can respond to ICMP Mask Request messages if the ICMP Mask Reply function is enabled.

To have the Cisco IOS software respond to ICMP mask requests by sending ICMP Mask Reply messages, enter the following command: