How to configure queues on router 3662

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Oct 2nd, 2008
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What is the diference between tx-lmit command and hold queue command?

Actually I have two routers connected through a FR cloud. I make a ping to ROUTER_B's serial interface from ROUTER_A and I have icmp request timout during the day. I noticed from several commands outputs (find them attached) that I have many drop packets and delayed packets.

Could you take a look and help me?

tahnks in advance!

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Overall Rating: 4.5 (2 ratings)
Joseph W. Doherty Thu, 10/02/2008 - 04:25
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"What is the diference between tx-lmit command and hold queue command?"

Tx-ring-limit controls the size/depth of the FIFO queue on the controller. Hold queue controls the size/depth of the software (FIFO) queue. (The latter only obtains packets when the former overflows.)

Congested links usually drop packets. Frame-relay can also drop packets in the cloud (especially if you exceed your CIR).

Generally two things to try, first insure if one side's FR cloud connection is larger than the other, or there are multiple PVCs, regulate it (them) so as to not to overrun FR egress. Second, try to implement fair-queue instead of FIFO. The former tends to provide all flows more more consistent performance and bandwidth hog flow packets tend to be dropped first.

jgtheodor Thu, 10/02/2008 - 05:18
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I had WFQ configured on the interfaces but I had the same symptom, about 2500 dropped packets for every 20000 output packets. After configuring the serial interface with hold queue 900 (I tried several values before this) and now I have about 50 dropped packets every 60000 output packets.

The serial link to FR Cloud is never overloaded during the day. The CIR is the same on both sides 512. from the command outputs I have posted what does the "pkts delayed" field actually mean?

The hold queue command affect only CPU switched packets or all the packets? The number in the tx-ring-limit & hold queue commands what are representing packets or bytes? Is there any specific value for tx-ring that I can use for testing.

Thanks for any help

Joseph W. Doherty Fri, 10/17/2008 - 15:06
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Sorry, for the long, long delay. Misplaced your response questions.

Unclear how the FR cloud isn't overloaded but you're seeing all these drops. You shouldn't see drops unless the interface is congested.

Hold queue applies to software queued packets. Hold queue, I believe, always count packets, tx-ring-limit I believe usually counts packets but on some platforms I recall it counts particles. (Neither should be in bytes.)

For a tx-ring-limit, I either allow the router to default the value unless I'm supporting voice where I'll set it to 2 or 3. (Believe I've read the very latest IOSs are much "smarter" about setting the default tx-ring-limit.)

Don't know what "pkts delayed" specically means unless it indicates packets delayed by the shaper.

The CIR is 512 but's what the physical link speed? If the physical link is faster, wonder whether the shaper is holding your bandwidth to 512. If so, and if these links are in North America, you could likely run them at full port speed.

jgtheodor Sat, 10/18/2008 - 04:42
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Actually the interface access-rate is 768Kbps and the CIR is 512Kbps. I configured the software pvc hold queue for a value 90 packets and now router does not drop packets at all. However I think that we must proceed with a new CIR contract with our ISP.


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