i used at as A pppoe server i have used QINQ secend tag in rotuer

interface TenGigabitEthernet2/0/0.904
 description Cust1-QINQ
 encapsulation dot1Q 904 second-dot1q any
 pppoe enable group PPPOE-SERVER3

interface Virtual-Template2
 **mtu 1492**
 ip unnumbered Loopback0
 ip nat inside
 no logging event link-status
 peer default ip address pool interface localpool2
 keepalive 15
 ppp mtu adaptive
 ppp authentication chap ms-chap ms-chap-v2 eap pap
 ppp ipcp dns
 ppp timeout idle 172800 either

the used is connected but they have some website not open and some application not working too i think the issue with MTU because i used double tag (qinq) so what MTU size must be ?

1 Answer 1


the basic way is to use ping with the don't fragment bit set to ping with various packet size until you find the correct value:

On a Mac OS system use

ping -D -s <packetsize> <pingable IP accross the tunnel>

On Windows

ping -f -l <packetsize> <pingable IP accross the tunnel>

On linux, the Don't Fragment bit is set by default:

ping -s <packetsize> <pingable IP accross the tunnel>

Once you find the maximum value that respond, add 28 for the ICMP (8) and IP (20) headers and you have your MTU.

  • hello , 1480 is max size i get ping reachable Commented Apr 25, 2020 at 20:58
  • my interface is 1500 , if we add 28 to 1480 is 1508 , so what value i must add Commented Apr 25, 2020 at 20:59

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