关于iox上“show interface”和“show policy -map”计数的一些解释

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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
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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
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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
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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
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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
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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。

NCS55 QoS
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前端时间遇到个 NCS55 Qos 的问题，借用同事的测试环境简单测试下发现:

NCS55 入向 policy 用的是 l2 frame size
NCS55 出向 shaping 用的是 l1 frame size

如下测试，测试仪打 700M L1 rate，入向 Ten0/0/0/16 我设置的是 police 600m 然后 700m 的 l1 流量不丢。

出向 Gi0/0/0/0 设置的是 shape 600m，出现丢包了。

RP/0/RP0/CPU0:F340.13.15-NCS-540X-12Z16G-626B#show policy-map interface gigabitEthernet 0/0/0/0 output
Wed Jan 17 03:00:05.610 UTC

GigabitEthernet0/0/0/0 output: test-out

Class class-default
  Classification statistics          (packets/bytes)     (rate - kbps)
    Matched             :          1145312053/146599942784         605404
    Transmitted         :          1046153656/133907667968         547219
    Total Dropped       :            99158397/12692274816          58185
  Queueing statistics
    Queue ID                             : 1024
    Taildropped(packets/bytes)           : 99158397/12692274816
Policy Bag Stats time: 1705460386809  [Local Time: 01/17/24 02:59:46.809]
RP/0/RP0/CPU0:F340.13.15-NCS-540X-12Z16G-626B#show policy-map interface tenGigE 0/0/0/16 input
Wed Jan 17 03:00:15.729 UTC

TenGigE0/0/0/16 input: test-in

Class class-default
  Classification statistics          (packets/bytes)     (rate - kbps)
    Matched             :           233250825/29856105600          605399
    Transmitted         :           233250825/29856105600          605399
    Total Dropped       :                   0/0                    0
  Policing statistics                (packets/bytes)     (rate - kbps)
    Policed(conform)    :           233250825/29856105600          605399
    Policed(exceed)     :                   0/0                    0
    Policed(violate)    :                   0/0                    0
    Policed and dropped :                   0/0
Policy Bag Stats time: 1705460386796  [Local Time: 01/17/24 02:59:46.796]

NCS55 的 qos 配置可以自定义帧头的大小，如下设定 20 字节（设置为 20 字节的原因为 L1 Size = L2 frame Size + IFG min + Preamble + SFD = L2 frame Size + 20 Bytes)，这样我们就可以将出向的 shaping 转换为 l2 frame。

service-policy output test-out account user-defined 20

参考
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Spirent TestCenter: How do I calculate Layer 1 and Layer 2 bits per second and verify with the reported results?

Ethernet frame(wiki)

L1 frame size 和 L2 frame size #

Show interface 计数 #

show policy-map 计数 #

关于端口速率 #

1400 字节 frame #

66 字节 frame #

ifHCInOctets 和 IfHCOutOctets #

NCS55 QoS #

参考 #