Information About MPLS Static Label Binding
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Label Distribution Protocol (LDP), the Internet Engineering Task Force (IETF) standard that is used to bind labels to network addresses
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Resource Reservation Protocol (RSVP), which is used to distribute labels for traffic engineering (TE)
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Border Gateway Protocol (BGP), which is used to distribute labels for Multiprotocol Label Switching (MPLS) Virtual Private Networks (VPNs)
To use a learned label to label-switch packets, an LSR installs the label into its Label Forwarding Information Base (LFIB).
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The binding between a label and an IPv4 or IPv6 prefix
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The action corresponding to the binding between a label and an IPv4 or IPv6 prefix—Label swap or pop
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The contents of an LFIB crossconnect entry
Label Swap and Pop
As a labeled packet traverses the MPLS domain, the outermost label of the label stack is examined at each hop. Depending on the contents of the label, a swap, or pop (dispose) operation is performed on the label stack. Forwarding decisions are made by performing a MPLS table lookup for the label carried in the packet header. The packet header does not need to be reevaluated during packet transit through the network. Because the label has a fixed length and is unstructured, the MPLS forwarding table lookup process is both straightforward and fast.
In a swap operation, the label is swapped with a new label, and the packet is forwarded to the next hop that is determined by the new label.
In a pop operation, the label is removed from the packet, which may reveal an inner label below. If the popped label was the last label on the label stack, the packet exits the MPLS domain. Typically, this process occurs at the egress LSR. A failure of the primary link in the aggregator configuration reroutes the MPLS traffic from the backup link and it is a swap operation.
Benefits
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Static bindings between labels and IPv4 or IPv6 prefixes can be configured to support MPLS hop-by-hop forwarding through neighbor routers that do not implement LDP label distribution.
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Static crossconnects can be configured to support MPLS Label Switched Path (LSP) midpoints when neighbor routers do not implement either the LDP or RSVP label distribution, but do implement an MPLS forwarding path.