Reading through MPLS and vpn architechture volumn 1. Come up a question about the way ATM allocates label. It uses ordered control. My question is since the label is the vpi/vci number, instead of use ordered control to ask downsteam the lable number(actually the current vpi/vci),why it just use its vpi/vci instead? I think I miss sth here. Anybody can explain? Thanks
ATM also uses downstream-on-demand label allocation. It uses this when building an LSP. ATM has to build a end-to-end unidirection LSP. Because VPI/VCI are a scare resource ATM LSP must request all Labels in the LSP in order to build it.
Remember Labes are kind of like next hop address. In the CEF table it essesitally takes the next hop IP address and replaces this with a label and populates the LFIB. The switch does use it's own VPI/VCI but its really for the next hop router. My best guess at why it doesn't use it's own label is because the LSP is unidirectional, and therefore so are the label
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