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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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MPLS MTU in the MPLS Backbone

In AToM networks, if the two PE routers are not directly connected, at least two labels are added to each frame that is transported across the pseudowire. This increases the packet size with at least 8 bytes. Another 4 bytes are added if the control word is used. The payload that is transported with AToM is not just an IP packet as with MPLS VPN, but a Layer 2 frame. Therefore, when calculating the biggest possible AToM packet, you must add the number of bytes for the Layer 2 frame header. Take Ethernet over MPLS as an example. If the transported frame is an Ethernet II frame carrying an IP packet of 1500 bytes, the AToM payload is 1514 bytes. The added 14 bytes are the 6 bytes for Source and Destination MAC Address ...

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