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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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MPLS MTU

The maximum transmission unit (MTU) of a link determines whether a labeled packet is too big to be transmitted across a link. Note that a link has a physical MTU value and an IP MTU value. Take the example of an Ethernet link on a Cisco IOS router. The MTU value is 1500 bytes. That is the Layer 3 MTU value of the packet. Therefore, an IP packet up to 1500 bytes can be transmitted across the Ethernet link without having to be fragmented. Typically, when MPLS is used, the IP packet of 1500 bytes receives one or two labels. It might be more, such as when CsC is deployed, but two labels are the maximum for this example. Therefore, 2 labels are added to the IP packet, or 2 times 4 bytes, which makes 8 bytes. The maximum size of the labeled ...

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