About Static MPLS
Generally, label switching routers (LSRs) use a label distribution protocol to dynamically learn the labels that they should use to label-switch packets. Examples of such protocols include:
Label Distribution Protocol (LDP), the Internet Engineering Task Force (IETF) standard that is used to bind labels to network addresses
Resource Reservation Protocol (RSVP), which is used to distribute labels for traffic engineering (TE)
Border Gateway Protocol (BGP), which is used to distribute labels for 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).
The static MPLS feature enables you to statically configure the following:
The binding between a label and an IPv4 or IPv6 prefix
The action corresponding to the binding between a label and an IPv4 or IPv6 prefix (label swap or pop)
The contents of an LFIB cross-connect 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 an 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 incoming 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 reroutes the MPLS traffic to the backup link and results in a swap operation.
Static MPLS Topology
This diagram illustrates the static MPLS source routing topology. The access nodes perform the swap operation, and the aggregation nodes perform the pop operation for the primary path and the swap operation for the backup path.
Benefits of Static MPLS
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.
Static cross-connects can be configured to support MPLS label switched path (LSP) midpoints when neighbor routers do not implement either LDP or RSVP label distribution but do implement an MPLS forwarding path.
High Availability for Static MPLS
Cisco Nexus 9500 Series switches support stateful switchovers (SSOs) for static MPLS. After an SSO, static MPLS returns to the state it was in previously.
Static MPLS supports zero traffic loss during SSO. MPLS static restarts are not supported.
The Cisco Nexus 9300 Series switches do not support SSO.