WTD vs WRED

[torontoyuxiang]

Hi,

From my understanding, WRED is better than WTD. WRED can avoid discarding big trunk of packets. However, the new switch platforms (3560, 3750, 4948 etc) all support Srr with WTD. Is there any specific reason for srr using WTD instead of WRED? I couldn't find any explanation on Cisco website.

Thanks in advance,

Sean

[Joseph W. Doherty]

"WRED is better than WTD"

Not always. Depends much on the type of traffic and what you're trying to accomplish.

As to why WTD is supported on switches where WRED isn't, likely due to it being easier to implement especially working in hardware. With WTD, when queue reaches certain size drop frames/packets. With WRED, you're likely dealing with a complex moving average queue size and drop probabilities at the current average queue depth.

[torontoyuxiang]

Thanks Joseph for your answer. In theory, WRED should be better than WTD, that's what I meant. My confusion is that Cisco's old switches like 3550, 6500 series are all using WRR with WRED. There must be some specific reasons that Cisco is using WTD in the newer platform.

[John Curtis Gibson]

You got your question all wrong. The WRED was not supported on the high density ports in the "old" switches, either. The WRED is not supported on the high density ports in the new switches. There is no "trend", that you feel or imagine, of going from WRED to WTD.

It has always been this way, the high density ports always use WTD. Why? WRED needs the supervisor system motherboard to generate a random number every time any port wants to drop one packet. Can you imagine 48 ports  all need to request the central CPU to run the complicated algorithm to generate a random number that the per-port ASIC does not provide? What if you need to drop 10 packets per port per second. Calling system board service 480 times per second is like opening 480 disk files per second then close all immediately, then repeat. That is not going to happen. Your little switch is either fried by this kind of grueling punishment or it just stops working all together. Most likely that by the time a port is served a random number by the central CPU, the entire buffer in that port's ASIC is flooded to the tail, which forces tail drops. Then what is the point of enabling WRED when it will be forced to behave like WTD? 

3750 supports at lease 2 uplink ports for WRED. There are very few uplink ports, so WRED is no problem. Routers have 4-8 ports for each motherboard CPU, so routers can afford to run WRED on every port.