一台Catalyst3550通过Gig0/2与一台UBR10012千兆互联，但是在运行中出现一个奇怪问题，就是在晚上流量高峰期，只要流量超过200M左右互联处就会产生600~800ms的延时，有时还有丢包，经检查发现千兆端口产生较多PAUSE output，show interface信息如下：
sh int gi0/2
GigabitEthernet0/2 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 000b.464e.3d00 (bia 000b.464e.3d00)
Description: "connect to uBR10012 G3/0"
Internet address is 10.0.3.38/30 MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 51/255, rxload 16/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is
input flow-control is off, output flow-control is on
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 00:42:44
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 66028000 bits/sec, 29881 packets/sec
5 minute output rate 202588000 bits/sec, 32768 packets/sec
76350176 packets input, 118647447 bytes, 0 no buffer
Received 40 broadcasts (0 IP multicast)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 40 multicast, 0 pause input
0 input packets with dribble condition detected
83758909 packets output, 4029376771 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 1134732 PAUSE output
0 output buffer failures, 0 output buffers swapped out
通过Cisco网站分析可能是flow control工作不正常导致，其分析结果如下：
INFO: Switches use flow control to inhibit the transmission of packets to the port for a specified period. Typically, if the receive buffer becomes full, the port transmits a 'pause' packet that instructs remote ports to delay sending more packets for a specified period. In addition, Gigabit Ethernet ports can receive and act upon 'pause' packets from other devices.
通过查看3550的Gig0/2端口工作模式如下：
sh int transceiver properties | b Gi0/2
Name : Gi0/2
Administrative Speed: auto
Administrative Duplex: auto
Administrative Auto-MDIX: N/A
Administrative Power Inline: n/a
Operational Speed: 1000
Operational Duplex: full
Operational Auto-MDIX: N/A
通过查看端口信息，我们可以看出3550 Gig0/2 Flow control模式为： input flow-control is off, output flow-control is on ， 另外一端UBR10K Gig3/0/0 Flow control模式为： output flow-control is XOFF, input flow-control is XOFF，从两端的工作模式来看Flowcontrol工作无效。
为了解决问题，在3550 Gig0/2端口配置flowcontrol send off强制关闭Flowcontrol，之后就工作正常了，再查看UBR10 Gig3/0/0 的Flow control模式变更为：output flow-control is XON, input flow-control is XON
Flow control 设置所产生的结果关系可以参考如下链接：
https://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/
release/12.2_25_sec/command/reference/cli1.html#wp1861670