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Packet Drop

I have a question.

I have a Switch 3650 with the firmware 16.3.6.

its Port 48 is connected to an AP ,after some time[could be months ] start to drop the packet.but this problem is only with this interface. if i connect the AP to another port of switch with the same configuration i don't have this problem any more.

anyone has any idea

 
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Re: Packet Drop

Behnam,

We are for sure zeroing in on the problem. Look at the output drops in the attached picture. You're dropping 100% of your packets!

Cisco links documentation to troubleshoot output drops here: https://www.cisco.com/c/en/us/support/docs/routers/10000-series-routers/6343-queue-drops.html#anc9

From the site:

Troubleshoot Output Queue Drops

You can decrease, or even prevent, output drops if you adjust the configuration of these features:

  • Duplex mode—If the interface works in half-duplex mode, configure it (if possible) to work in full-duplex.

  • Layer 2 windowing mechanism—If x.25 encapsulation is configured on the interface, increase the x.25 window size. For more information, see Setting Default Window Sizes.

  • Distributed switching—On Cisco 7500 routers, if Versatile Interface Protocol (VIP) cards are installed in the chassis, enable distributed switching. When you do so, the incoming VIP buffers up to 1 second of traffic for the interface if the outgoing interface is congested. This is called rx-side buffering .

Note: Never increase the output queue in an attempt to prevent output drops. If packets stay too long in the output queue, TCP timers can expire and trigger retransmission. Retransmitted packets only congest the outgoing interface even more.

If output drops still occur after you adjust the configuration of the router as recommended, it means that you cannot prevent or decrease output drops. However, you can control them, and this can be as effective as prevention. There are two approaches to control output drops:

  • Congestion management

  • Congestion avoidance

Both approaches are based on traffic classification, and you can use them in parallel.

However, as you said that switching interfaces fixes the problem (Not sure if that fixes it permanently?) the issue is probably not congestion and could just be that the switchport has gone bad. Sadly, at this point I would refer you to the TAC desk to verify that the interface is bad and possibly get a replacement if the device is under a service contract.

I'm sorry I didn't have better news for you.

Please rate/mark answers if you find them helpful.

-Zac

4 REPLIES 4

Re: Packet Drop

Hi Behnam,

Can you post the running configuration of the interface along with the "show interface" output of port 48? There may be some valuable information there.

 

-Zac

Re: Packet Drop

ja 

that is it

Re: Packet Drop

Behnam,

We are for sure zeroing in on the problem. Look at the output drops in the attached picture. You're dropping 100% of your packets!

Cisco links documentation to troubleshoot output drops here: https://www.cisco.com/c/en/us/support/docs/routers/10000-series-routers/6343-queue-drops.html#anc9

From the site:

Troubleshoot Output Queue Drops

You can decrease, or even prevent, output drops if you adjust the configuration of these features:

  • Duplex mode—If the interface works in half-duplex mode, configure it (if possible) to work in full-duplex.

  • Layer 2 windowing mechanism—If x.25 encapsulation is configured on the interface, increase the x.25 window size. For more information, see Setting Default Window Sizes.

  • Distributed switching—On Cisco 7500 routers, if Versatile Interface Protocol (VIP) cards are installed in the chassis, enable distributed switching. When you do so, the incoming VIP buffers up to 1 second of traffic for the interface if the outgoing interface is congested. This is called rx-side buffering .

Note: Never increase the output queue in an attempt to prevent output drops. If packets stay too long in the output queue, TCP timers can expire and trigger retransmission. Retransmitted packets only congest the outgoing interface even more.

If output drops still occur after you adjust the configuration of the router as recommended, it means that you cannot prevent or decrease output drops. However, you can control them, and this can be as effective as prevention. There are two approaches to control output drops:

  • Congestion management

  • Congestion avoidance

Both approaches are based on traffic classification, and you can use them in parallel.

However, as you said that switching interfaces fixes the problem (Not sure if that fixes it permanently?) the issue is probably not congestion and could just be that the switchport has gone bad. Sadly, at this point I would refer you to the TAC desk to verify that the interface is bad and possibly get a replacement if the device is under a service contract.

I'm sorry I didn't have better news for you.

Please rate/mark answers if you find them helpful.

-Zac

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Re: Packet Drop

One-way-traffic: The downstream device is not "talking".
The port has negotiated to 5 Gbps, so this AP is most likely an AP4800.
Look at the Total Output Drops, it is huge. This means the switch keeps sending data downstream but the downstream device keeps sending back a "pause" frame.
Counters were cleared 2 days ago: This doesn't help because we don't know the "history".
Post the complete output to the command "sh controll e Ten 1/0/48".
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