MTU of the EX switch when Q-in-Q (802.1Q) tunneling is enabled

This article provides information about the MTU of the EX switch, when Q-in-Q tunneling is enabled.

  • Q-in-Q tunneling adds a service VLAN tag, before the customer’s 802.1Q VLAN tags.
  • Depending on the interface configuration, the MTU value of the related ports is adjusted to accommodate the 4 bytes, which are used for the tag that is added by Q-in-Q tunneling.

Test topology:

EX1: ping size 1469

The size parameter is assigned the payload of the ICMP packet; here it is 1469.

Ping test result :

When the ICMP payload is set to 1469, the packet size should be:

This exceeds the maximum trunk interface MTU definition of 1518; so packets will be dropped.

When the size is changed from 1469 to 1468, the ping is successful:

  • Reduce the MTU on the access links by at least 4 bytes, so that the frames do not exceed the MTU of the trunk link, when S-VLAN tags are added.
  • Increase the MTU on the trunk link, so that the link can handle the larger frame size.
  • For example, enable Jumbo frame on the related interface.

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