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 (123.1.1.1)--vlan tag 100--QinQswitch1--vlan tag 100,1000--QinQswitch2--vlan tag 100--(123.1.1.2)EX2
EX1: ping 123.1.1.2 size 1469
The size parameter is assigned the payload of the ICMP packet; here it is 1469.
Ping test result :
root@EX# run ping 123.1.1.2 size 1469 rapid count 10 PING 123.1.1.2 (123.1.1.2): 1469 data bytes .......... --- 123.1.1.2 ping statistics --- 10 packets transmitted, 0 packets received, 100% packet loss
When the ICMP payload is set to 1469, the packet size should be:
14 (L2 header) + 4 (outer vlan tag) + 4 (inner vlan tag) + 20 (IP header) + 8 ( icmp header) + 1469 (icmp payload) = 1519
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:
root@EX# run ping 123.1.1.2 size 1468 rapid count 10 PING 123.1.1.2 (123.1.1.2): 1468 data bytes !!!!!!!!!! --- 123.1.1.2 ping statistics --- 10 packets transmitted, 10 packets received, 0% packet loss round-trip min/avg/max/stddev = 1.666/3.292/6.964/2.151 ms
- 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.