06-27-2019 08:50 AM
06-27-2019 09:23 AM - edited 06-27-2019 09:25 AM
Hello,
on Cisco devices running IOS or IOS XE mtu value is actually the size of the L3 payload carried within the frame.
the 14 bytes ethernet header and 4 final bytes of FCS are not taken in account
So actually max frame size is 1518 bytes
When using 802.1Q tagging additional 4 bytes are added
802.1Q tagged frames of total size 1522 size should be allowed to pass.
Different for IOS XR where MTU is L2 and it skips only the 4 bytes FCS. In this case default mtu size is 1514 bytes and increases to 1518 bytes for 802.1Q tagged frames or subinterfaces.
Hope to help
Giuseppe
06-27-2019 09:23 AM - edited 06-27-2019 09:25 AM
Hello,
on Cisco devices running IOS or IOS XE mtu value is actually the size of the L3 payload carried within the frame.
the 14 bytes ethernet header and 4 final bytes of FCS are not taken in account
So actually max frame size is 1518 bytes
When using 802.1Q tagging additional 4 bytes are added
802.1Q tagged frames of total size 1522 size should be allowed to pass.
Different for IOS XR where MTU is L2 and it skips only the 4 bytes FCS. In this case default mtu size is 1514 bytes and increases to 1518 bytes for 802.1Q tagged frames or subinterfaces.
Hope to help
Giuseppe
06-27-2019 09:41 AM
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