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Ping & MTU 101 Question

Hi

After reading several docs on MTU and overhead I'm still a little confused trying to work out the maxium supported MTU over a particular ethernet WAN circuit.  I need to establish the maxium allowed payload from the provider(should be 1600 bytes), and if the circuit currently has enough overhead to deploy services like MPLS VPN, QnQ etc

My understanding is that I will require a minimum MTU of 1522 bytes for MPLS i.e. 1500 bytes + 14(Ethernet) bytes + 8(MPLS) bytes

Currently I can ping across the circuit with a maximum packet size of 1516 bytes. 

Router1#ping 192.168.0.2 size 1516 df-bit source Gi0/1

Type escape sequence to abort.

Sending 5, 1516-byte ICMP Echos to 192.168.0.2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5)

Router1#ping 192.168.0.2 size 1517 df-bit source Gi0/1

Type escape sequence to abort.

Sending 5, 1517-byte ICMP Echos to 192.168.0.2, timeout is 2 seconds:

....

Success rate is 100 percent (5/5)

# Router 1

interface Gi0/1

Description WAN Link to Router B via Provider-ABC Ethernet Circuit

no ip address

mtu 9000

interface Gi0/1.100 point-to-point

encapsulation dot1q 100

ip mtu 1600

ip add 192.168.0.1 255.255.255.252

# Router 2

interface Gi0/1

Description WAN Link to Router A via Provider-ABC Ethernet Circuit

no ip address

mtu 9000

interface Gi0/1.100 point-to-point

encapsulation dot1q 100

ip mtu 1600

ip add 192.168.0.2 255.255.255.252

I would asume the ping above would include the following overhead -> 4bytes(dot1q) + 14bytes(Ethernet)?

Therefore 1516 + 14 + 4 = 1534 bytes

Therefore if I wanted to enable MPLS (8bytes) would this mean 1516 - 8 = 1508 MTU?  Which *should* be enough to deploy MPLS VPN's using MP-BGP?

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