Information About Long Distance
In case of congestion, the long distance ingress ports sends a pause frame to its peer to pause all data transmission on the link for a specified period of time in order to absorb the in-flight packets.
When a pause frame is generated from a switch, it takes time to reach the peer switch. Until the packet reaches to peers all the in-flight packets need to be buffered and should not be dropped.
If the distance between the switches is longer, time taken by a pause frame to reach the peer switch is more and the number of the in-flight packets becomes higher. All these in-flights packets need to be buffered and should not be dropped. To absorb the large numbers of in-flight packets, the buffer size needs to be larger.
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Lower threshold—Pause: The lower threshold is to trigger the generation of pause packets. When the buffer reaches the pause threshold, pause frames are generated.
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High threshold—Stop: This threshold is to stop generating the pause frames. When the buffer reaches the stop threshold, the pause frames stop getting generated.
To absorb a large number of in-flight packets, the difference between the pause threshold and the stop threshold should to be more. The pause threshold should be lower and the stop threshold should be higher. If the difference between the pause and stop thresholds are higher, the number of in-flight packets are absorbed in ingress buffer. If the pause threshold is lower, a pause frame is generated even for small buffer utilization.
On Cisco MDS 9000 switches, there is only one queue, the 7e template and all buffers are allocated to this queue. The pause threshold and the stop threshold are marked at same level. With this limitation, it cannot handle the long distance functionality.
With introduction of the long distance feature for Cisco MDS 9000 Series, you can enable the long distance feature on certain ports where peer switches are at a longer distance.