comments by ‘zhihua’
对于ASR9000来说 ：
“show interfaces” and “show policy-map interfaces” show L2 frame size.
“show interfaces” does not include FCS bytes, but “show policy-map interfaces” includes FCS bytes(SIP700是个例外)
L1 frame size 和 L2 frame size的区别
如上图就是一个以太网帧的物理层具体所包含的内容。
- Preamble: 前同步码 7 bytes
- SFD: start frame delimiter 1 byte 帧开始定界符
- Dst MAC: 6 bytes
- Src MAC: 6 bytes
- EtherType: 2 bytes
- FCS(CRC): 4 bytes
除此之外，每个帧之前还会存在12 bytes 的最小帧间隙 （Inter Frame Gap min）
- L1 Size = L2 frame Size + IFG min + Preamble + SFD = L2 frame Size + 20 Bytes
- L2 Size = Dst MAC+ Src MAC + Ether Type + Payload + FCS
Show interface计数
我们拿测试仪构造一个66字节的帧.
Packet size 66 Bytes = Ethernet II Header(14)+ IPv4 (20) + Payload (28) + FCS (4)
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#show int tenGigE 0/0/0/0 | in packets input
Tue Feb 14 12:21:00.065 UTC
58992573918 packets input, 4860381127998 bytes, 0 total input drops
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#show int tenGigE 0/0/0/0 | in packets input
Tue Feb 14 12:21:03.400 UTC
58992573951 packets input, 4860381130044 bytes, 0 total input drops
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#
计算接口数据包的计数:
(4860381130044-4860381127998)/(58992573951-58992573918) = 62 Bytes
即show interface 显示的计数为L2 frame 并且不包含4字节的FCS
show policy-map计数
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#show policy-map interface tenGigE 0/0/0/1 output | in Matched
Tue Feb 14 12:23:49.045 UTC
Matched : 1342020745/88573369170 5
Matched : 0/0 0
Matched : 0/0 0
Matched : 0/0 0
Matched : 0/0 0
Matched : 0/0 0
Matched : 54037745345/4375790026504 0
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#show policy-map interface tenGigE 0/0/0/1 output | in Matched
Tue Feb 14 12:24:01.514 UTC
Matched : 1342020905/88573379730 5
Matched : 0/0 0
Matched : 0/0 0
Matched : 0/0 0
Matched : 0/0 0
Matched : 0/0 0
Matched : 54037745345/4375790026504 0
计算show policy-map的输出：
(88573379730-88573369170)/(1342020905-1342020745)=66 Bytes
即show policy-map显示的计数为L2 frame 并且包含4字节对FCS。
关于端口速率
正是因为show interface的输出显示的L2 frame size，而L1 size 会比L2 size 多20 字节，这样就会导致我们看接口的rate往往跑不满实际的接口带宽，这个rate会因为以太网帧的大小而进行浮动，以太网帧越小，show interface 端口速率显示就越少（L1 速率保持一致的情况下）。
以下是以相同的L1 速率10Gbps向ASR9K的TenG接口打流，不同frame size大小show interface rate的显示:
1400字节frame
Protocol:General
Interface In(bps) Out(bps) InBytes/Delta OutBytes/Delta
Te0/0/0/0 9.8G/ 98% 0/ 0% 1.3T/0 22780/0
Te0/0/0/1 0/ 0% 9.8G/ 98% 127.2G/0 1.1T/0
Quit='q', Clear='c', Freeze='f', Thaw='t',
Next set='n', Prev set='p', Bytes='y', Packets='k'
(General='g', IPv4 Uni='4u', IPv4 Multi='4m', IPv6 Uni='6u', IPv6 Multi='6m')
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#show int tenGigE 0/0/0/1 | in rate
Tue Feb 14 09:10:07.706 UTC
30 second input rate 0 bits/sec, 0 packets/sec
30 second output rate 9830632000 bits/sec, 880250 packets/sec
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#
66字节frame
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#show int tenGigE 0/0/0/1 | in rate
Tue Feb 14 09:21:45.425 UTC
30 second input rate 0 bits/sec, 0 packets/sec
30 second output rate 7209227000 bits/sec, 14534730 packets/sec
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#
RP/0/RSP1/CPU0:513E.B.31.ASR9K-V2-2#monitor int tenGigE 0/0/0/0 tenGigE 0/0/0/1
Tue Feb 14 09:22:18.975 UTC
513E.B.31.ASR9K Monitor Time: 00:00:02 SysUptime: 15:52:49
Protocol:General
Interface In(bps) Out(bps) InBytes/Delta OutBytes/Delta
Te0/0/0/0 7.2G/ 72% 0/ 0% 2.0T/1.8G 22780/0
Te0/0/0/1 0/ 0% 7.2G/ 72% 0/0 99.7G/1.8G
Quit='q', Clear='c', Freeze='f', Thaw='t',
Next set='n', Prev set='p', Bytes='y', Packets='k'
(General='g', IPv4 Uni='4u', IPv4 Multi='4m', IPv6 Uni='6u', IPv6 Multi='6m')
ifHCInOctets和IfHCOutOctets
这里补充一点，ifHCInOctets 和 IfHCOutOctets SNMP采集到的计数也是L2 frame size ，但是包含FCS。
参考
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