What is ECN ?
ECN = Explicit Congestion Notification, then what is used for ?
ECN are WRED extention, ECN will mark the packet when the everages queue exceeds a specific threshold value when configured with WRED
ECN feature will marks as signal that network are congested and need to be slowdown sending the packets.
so, WRED will Drop some of traffic bassically from for freeing bandwidth.
As stated in RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP,implementing ECN requires an ECN-specific field that has two bits–the ECN-capable Transport (ECT) bit and the CE (Congestion Experienced) bit–in the IP header. The ECT bit and the CE bit can be used to make four ECN field combinations of 00 to 11. The first number is the ECT bit and the second number is the CE bit. The table below lists each of the ECT and CE bit combination settings in the ECN field and what the combinations indicate.
Table 1 | ECN Bit Setting |
ECT Bit |
CE Bit |
Combination Indicates |
0 | 0 | Not ECN-capable |
0 | 1 | Endpoints of the transport protocol are ECN-capable |
1 | 0 | Endpoints of the transport protocol are ECN-capable |
1 | 1 | Congestion experienced |
The ECN field combination 00 indicates that a packet is not using ECN.
The ECN field combinations 01 and 10–called ECT(1) and ECT(0), respectively–are set by the data sender to indicate that the endpoints of the transport protocol are ECN-capable. Routers treat these two field combinations identically. Data senders can use either one or both of these two combinations. For more information about these two field combinations, and the implications of using one over the other, refer to RFC 3168, The Addition of Explicit Congestion Notification (ECN) to IP.
The ECN field combination 11 indicates congestion to the endpoints. Packets arriving a full queue of a router will be dropped.
Sample Configuration :
R1(config)# policy-map LLQ R1(config-pmap)# class MATCH_AF21 R1(config-pmap-c)# bandwidth per 70 R1(config-pmap-c)# random-detect R1(config-pmap-c)# random-detect ecn
Router# show policy-map
Policy Map LLQ
Class MATCH_AF21
Weighted Fair Queueing
Bandwidth 70 (%)
exponential weight 9
explicit congestion notification
class min-threshold max-threshold mark-probability
———————————————————-
———————————————————-
0 – – 1/10
1 – – 1/10
2 – – 1/10
3 – – 1/10
4 – – 1/10
5 – – 1/10
6 – – 1/10
7 – – 1/10
rsvp – – 1/10
The following is sample output from the show policy-map interfacecommand. The words “explicit congestion notification” included in the output indicate that ECN has been enabled.
Router# show policy-map interface Serial0/0
Serial0/0
Service-policy output:policy_ecn
Class-map:MATCH_AF21 (match-all)
1000 packets, 125000 bytes
30 second offered rate 14000 bps, drop rate 5000 bps
Match:AF21
Weighted Fair Queueing
Output Queue:Conversation 42
Bandwidth 20 (%)
Bandwidth 100 (kbps)
(pkts matched/bytes matched) 989/123625
(depth/total drops/no-buffer drops) 0/455/0 ==> Droped Packet
exponential weight:9
explicit congestion notification
mean queue depth:0
class Transmitted Random drop Tail drop Minimum Maximum Mark
pkts/bytes pkts/bytes pkts/bytes threshold threshold probability
0 0/0 0/0 0/0 20 40 1/10
1 545/68125 0/0 0/0 22 40 1/10 => 1 drop per 10 packet
2 0/0 0/0 0/0 24 40 1/10
3 0/0 0/0 0/0 26 40 1/10
4 0/0 0/0 0/0 28 40 1/10
5 0/0 0/0 0/0 30 40 1/10
6 0/0 0/0 0/0 32 40 1/10
7 0/0 0/0 0/0 34 40 1/10
rsvp 0/0 0/0 0/0 36 40 1/10
class ECN Mark
pkts/bytes
0 0/0
1 43/5375
2 0/0
3 0/0
4 0/0
5 0/0
6 0/0
7 0/0
rsvp 0/0