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Pipe and Short Pipe Models

In the pipe and short pipe models, any traffic conditioning (that is, in a pure JUNOSe environment, a change in traffic class/color combination) that is applied when traffic goes through the tunnel has no effect on the EXP bits coding in the inner header. In other words, when traffic exits an LSP (when a label is popped) or when traffic enters an LSP, the inner header’s EXP bits coding is not changed.

The pipe and short pipe models differ in the header that the tunnel egress uses when it determines the PHB of an incoming packet. With the short pipe model, the tunnel egress uses an inner header that is used for forwarding. With the pipe model, the outermost label is always used. Because of this, you cannot use PHP with the pipe model.

The pipe model is the default JUNOSe behavior, which you can configure with the mpls tunnel-model command. You cannot configure the short pipe model with this command. In fact, on ingress line modules the traffic class/color combination is always determined from the outermost label, so fabric queuing is also based on the outermost label. However, on the egress line module you can achieve the queuing behavior expected with the short pipe model by attaching IP policies to egress interfaces to reset the traffic class/color combinations based on the IP header. However, this method requires that the outgoing packets to be IP. If the outgoing packets are MPLS, then this short pipe model of queuing is not supported.

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