Cisco ASr9K MTU when connected to 3rd party device

[Shri612]

Hi Team ,

 Why do we add extra 14 bytes to Cisco ASr 9kdevice MTU when connected to third party device such as arista

For example , Arista MTU is A <> Cisco MTU is A+14

Regards,

Sri

[Giuseppe Larosa]

Hello @Shri612 ,

in Cisco IOS XR the MTU is actually an OSI layer 2 concept the offset of 14 bytes is explained in this way it is the size of the ethernet header (destination MAC address 6 bytes, source MAC address 6 bytes, ethertype 2 bytes) , the final 4 byte FCS is not counted.

the same rule applies when connecting to an IOS or IOS XE device.

If you have an 802.1Q  subinterface in IOS XR the offset becomes of 18 bytes for the 4 bytes of the 802.1Q header.

when connecting to third party devices it depends from case to case. Some vendors may use the same approach as IOS XR.

Hope to help

Giuseppe