How to check the loop in Cisco switch

15 pts.
Tags:
Cisco switches
Network administration
Network loop
Switches
How do I check the loop in Cisco switch?
ASKED: April 22, 2009  9:41 AM
UPDATED: April 19, 2013  4:58 PM

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I think you are talking about forwarding loops seeming they can cause all sorts of wacky behavior in various Cisco switches. You will find basic troubleshooting and testing information below and you can also find more information on dealing with it at this Cisco URL:

Forwarding loops vary greatly both in their origin (cause) and effect. Due to the wide variety of issues that can affect STP, this document can only provide general guidelines about how to troubleshoot forwarding loops.

Before you start to troubleshoot, you must obtain this information:

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An actual topology diagram that details all of the switches and bridges
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Their corresponding (interconnecting) port numbers
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STP configuration details, such as which switch is the root and backup root, which links have a non-default cost or priority, and the location of blocking ports

Generally, troubleshooting involves these steps (depending on the situation, some steps may not be necessary):

1.

Identify the loop.

When a forwarding loop has developed in the network, these are the usual symptoms:
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Loss of connectivity to, from, and through affected network regions
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High CPU utilization on routers connected to affected segments or VLANs that can lead to various symptoms, such as routing protocol neighbor flapping or Hot Standby Router Protocol (HSRP) active router flapping
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High link utilization (often 100 percent)
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High switch backplane utilization (compared to the baseline utilization)
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Syslog messages that indicate packet looping in the network (for example HSRP duplicate IP address messages)
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Syslog messages that indicate constant address relearning or MAC address flapping messages
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An increasing number of output drops on many interfaces

Note: Any of these reasons alone may indicate different issues (or no issue at all). However, when many of these are observed at the same time, it is highly probable that a forwarding loop has developed in the network.

Note: The fastest way to verify this is to check the switch backplane traffic utilization:

cat# show catalyst6000 traffic-meter

traffic meter = 13% Never cleared
peak = 14% reached at 12:08:57 CET Fri Oct 4 2002

Note: The Catalyst 4000 with Cisco IOS software does not currently support this command.

If the current traffic level is well above normal or if the baseline level is not known, check whether the peak level has been achieved recently and whether it is close to the current traffic level. For example, if the peak traffic level is 15 percent and it was reached just two minutes ago and current traffic level is 14 percent, then that would mean that the switch is working under an unusually high load.

If the traffic load is at a normal level, then that probably means that there is either no loop or that this device is not involved in the loop. However, it still could be involved in a slow loop.
2.

Discover the topology (scope) of the loop.

Once it has been established that the reason for the network outage is a forwarding loop, the highest priority is to stop the loop and restore the network operation. In order to stop the loop, you must know which ports are involved in the loop: look at the ports with the highest link utilization (packets per second). The show interface Cisco IOS software command displays the utilization for each interface.

In order to display only the utilization information and the interface name (for a quick analysis), you might use Cisco IOS software regular expression output filtering. Issue the show interface | include line|\/sec command to display only the packet per second statistics and the interface name:

 cat# show interface | include line|\/sec

GigabitEthernet2/1 is up, line protocol is down
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
GigabitEthernet2/2 is up, line protocol is down
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
GigabitEthernet2/3 is up, line protocol is up
5 minute input rate 99765230 bits/sec, 24912 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
GigabitEthernet2/4 is up, line protocol is up
5 minute input rate 1000 bits/sec, 27 packets/sec
5 minute output rate 101002134 bits/sec, 25043 packets/sec
GigabitEthernet2/5 is administratively down, line protocol is down
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
GigabitEthernet2/6 is administratively down, line protocol is down
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
GigabitEthernet2/7 is up, line protocol is down
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
GigabitEthernet2/8 is up, line protocol is up
5 minute input rate 2000 bits/sec, 41 packets/sec
5 minute output rate 99552940 bits/sec, 24892 packets/sec

Pay particular attention to the interfaces with the highest link utilization. In this example, these are interfaces g2/3, g2/4, and g2/8; they are probably the ports that are involved in the loop.

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  • mshen
    Do you mean a networking loop or a loopback interface?
    27,385 pointsBadges:
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