C9200-24T Jumbo Frame

[JustinShen95399]

Using a C9200-24T switch and configured using vlan tagging shown below:

interface range GigabitEthernet1/0/1-24
switchport access vlan 7
switchport mode access

And we have vlan7 interface defined below as:

interface Vlan7
description DATA
no ip address
no ip redirects
no ip unreachables
no ip proxy-arp
ip access-group MGMT_restrict in
no ip route-cache
no snmp trap link-status

When doing some functional testing we found that the switch allows jumbo frames with the size of 1519 bytes. So I wonder why the switch is letting that extra byte through when we don't have jumbo frames configured.

[MHM Cisco World]

the value accept is in range from 1500-1522. 
check show interface see if giant counter increase.

[balaji.bandi]

it will allow you depend on the need, but make sure you use correct MTU for the good outcome

[JustinShen95399]

So I'm checking to see if the giant counter does register the jumbo frame.

But on the point of testing, 1518 byte frames are sent through the switch and get 1519 frames out. Is it known if the switch would add an artifact which would push the frame size up?

[MHM Cisco World]

Dont Know, you meaning send 1518 and receive 1519 ?

[JustinShen95399]

Yes. That's what we're seeing.

[MHM Cisco World]

but that not possible, 
because the SW must mark the frame as bad CRC frame.

""A CRC is an error detection mechanism commonly used in computer and storage networks to identify data changed or corrupted during transmission. When a device connected to the network needs to transmit data, the device runs a computation algorithm based on cyclic codes against the data that results in a fixed-length number. This fixed-length number is called the CRC value, but colloquially, it is often called the CRC for short. This CRC value is appended to the data and transmitted through the network towards another device. This remote device runs the same cyclic code algorithm against the data and compares the resulting value with the CRC appended to the data. If both values match, then the remote device assumes the data was transmitted across the network without being corrupted. If the values do not match, then the remote device assumes the data was corrupted during transmission across the network. This corrupted data cannot be trusted and is discarded.""

[JustinShen95399]

So the test program we use does have a CRC tag to ensure the integrity of the frame and all that is fine. So I did misspoke, not getting 1519 from a 1518 frame. But checking the 'show interface' prompt, we don't see any frames flagged as a giant frame until it a frame size of 1523 or larger is sent. But I thought anything larger than 1518 is considered jumbo/giant. So is there a reason why there is a 4 byte buffer before it's flagged?

The only guess I have is that it's implemented for a vlan tagging buffer.

[MHM Cisco World]

• Jumbo: Jumbo frames are frames that are bigger than the standard Ethernet frame size, which is 1518 bytes (this includes Layer 2 (L2) header and FCS). The definition of frame size is vendor-dependent, as these are not part of the IEEE standard.

Configure Jumbo/Giant Frame Support on Catalyst Switches - Cisco

that why I was mention before range not specific value.