Cisco Ethernet Interface statistic does not count CRC (4 Bytes). why?

[Won Lee]

Hi,

While working on lab testing, i have noticed that the ASR9K ethernet interface (GE) Rx Byte count does not match with the Test Gear (IXIA)'s Tx Byte count. Forther testing, it looks to me that Cisco interface statistic seems not including the 4 Bytes of CRC in its count.

Can anyone confirm this? if not, point me to correct information and Cisco Link if avaiable?

ASR9K monitoring ==================================

GigabitEthernet0/1/0/12 is up, line protocol is up

Encapsulation ARPA

Traffic Stats:(2 second rates)                                     Delta

  Input  Packets:                        10                            0

  Input  pps:                             0

  Input  Bytes:                       15140                            0

WireShark Capture===================================

Frame 1 (1518 bytes on wire, 1518 bytes captured)

--snip--

Capture Length: 1518 bytes

Above, ASR9K port shows 10 Frames and total 15140 Bytes Rx count. But, Directly connected Test Gear port have Tx'd 10 of frames with the size of 1518. I was expecting the ASR statistic of 15180, instead of 15140. I have ran packet capture and checked with Wireshark and confirmed the original frame size is 1518. any explanation?

Thanks in advance.

Intopian.

[Rivalino Tamaela]

Hello Won Lee,

Input bytes in show interface statistic excludes the FCS bytes. This is per design of the IOS-XR. However, SNMP IF-MIB does include FCS byte in ifInOctets, ifOutOctets, ifHCInOctets, ifHCOutOctets. This behavior is documented on ddts CSCth69314: [NOT-A-BUG] IPv4 and IPv6 GE/10GE stats use incorrect layer2 length

HTH,

Rivalino

[Won Lee]

Hello Rivalino.

Thanks for the information! Reading the CSCth69314, bring me a couple more questions.

1. The link discuss for 10GE interface. Does this apply only to 10G or same for all other Ethernet variation, like GE/40/100GE as well?

2. Does this apply to legacy IOS devices as well?

3. What is the theory behind this, excluding the FCS? Only Cisco or is the industry standard in general?

Thanks in advance!

[Rivalino Tamaela]

Hello Won Lee,

To answer your questions:

1. This applies to all various ethernet interfaces.

2. Yes this applies to IOS as well, at least to 7200/7300 series (so far i know).

3. There is no industry standard that mandate this, this is Cisco standard internally. For SNMP, IF-MIB (RFC 2819) definition includes the FCS byte.

regards,

Rivalino