Output drops over Bundled T1's (two)

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May 8th, 2007
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I'm using PPP Multilink to bundle 2 T1's and on my core router (cisco 6509) I'm seeing unusually high output drops on my Multilink interface. (Not my actual serial interfaces) Que'ing is fifo. Has anyone experienced output drops and if so, what are some common causes.


thanks in advance

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bjornarsb Tue, 05/08/2007 - 08:10
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Hi,


Is your interface Congested ?


When packets are processed, they are sent to the output queue of the outgoing interface. Based on the type of interface and the type of queueing configured, the number of output queue drops is not explicitly shown, because the output drops counter summarizes the output drops separately at the processing level and at the interrupt level.


Therefore, it is highly unlikely that output queue drops (drops at processing level) can occur without drops at interrupt level. Output queue drops occur only if the interface is already congested at interrupt level, so that packets cannot be pulled out of the output queue before the queue becomes full. Therefore, output drops at processing level (output queue drops) and output drops at interrupt level always occur together, and there is practically no need to distinguish between these two counters.


However, there is one exception. If the output queue is constantly full and if no packets are sent out of the interface at all, you must check for a hardware failure on the interface.


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.


Maybe you can use Weighted Random Early Detection (WRED) ?




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