The current WRED implementation prevents tail dropping of packets that are queued in the output queue by selectively dropping packets early. This avoids dropping of all packets at once when congestion occurs. With ECN functionality, routers are no longer limited to packet drops to indicate congestion. Routers can instead set the CE (Congestion Experienced) codepoint in the IP header of packets from ECN-capable transport entities. The transport endpoints use the ECE and CWR bits of the TCP header to mitigate congestion.
RFC 3168 states that the two-bit ECN field comprises an ECT bit and a CE bit that are used to implement ECN. The ECT bit and the CE bit can be combined into four possible ECN field values 00 to 11. The first number is ECT bit and the second number is the CE bit. ENC Field Values shows the possible combination.
ENC Field Values
| ECT Bit Value | CE Bit Value | Combined Inference |
|---|---|---|
| 0 | 0 |
Not ECN capable. Packet is not using ECN. |
| 0 | 1 |
Endpoints are ECN capable. Data senders use these values to indicate that the endpoints are ECN capable. Routers treat the two values identically. |
| 1 | 0 | |
| 1 | 1 | Congestion experienced. |
ECN Processing indicates the processing applied on incoming packets that are identified as drop-eligible in various ECN scenarios.
ECN Processing
| Incoming ECN Bits | ECN Configuration | Action if Packet Is Drop-Eligible | Outgoing ECN bits |
|---|---|---|---|
| 00 (Non-ECT) | On/Off | Drop. | N/A (packet dropped). |
| ECT (01 or 10) | Off | Drop. | N/A (packet dropped). |
| ECT (01 or 10) | On | Not dropped. CE bit is marked. | 11 indicates "Congestion Experienced". |
| CE (11) | Off | Drop. | N/A (packet dropped). |
| CE (11) | On | Not dropped. Packet already marked, just forward. | 11 indicates "Congestion Experienced". |
To configure ECN capability as on or off, use the following command:
configure qosprofile egress qp_num wred ecn [on | off] ports [port_list | all]
To view ECN information, use the following commands:
show wredprofile {ports [port_list | all]}
show ports port_list wred ecn {no-refresh | refresh}
To clear the ECN counters, use the following commands:
clear counters wred ecn
This feature is available on Summit X460-G2, X670-G2, X770, and ExtremeSwitching X870, X690 series switches.
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