Routing Using Matching and Calculated LSP Next Hops

Normally, a route table prefix is associated with a gateway or next hop IP address.

Using MPLS, a route table prefix can also be associated with an LSP that can be used as the next hop. There are two types of LSP next hops that can be used to route a packet to its destination:
  • Matching LSP next hop

    An LSP is considered matching with respect to an FEC if it has been associated with that FEC via LDP or RSVP-TE. An example of this is an IPv4 prefix for which a matching label mapping has been received by LDP. Matching LSPs are supported for all route origin types.

  • Calculated LSP next hop

    An LSP is considered calculated with respect to an FEC if it has been associated with that FEC through a routing protocol. Both OSPF and BGP can perform the calculations necessary to associate a route table prefix with an LSP next hop.

The following figure illustrates the concept of matching and calculated LSPs.

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Matching and Calculated LSP Next Hops
../Graphics/MPLS_17.svg

The following table describes the label bindings in the MPLS forwarding table for LSR A that are maintained for FECs reachable via LSR A to LSR C, shown in the figure above.

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Label Bindings for LSR A

Destination Next Hop Matching LSP Next Hop Label Calculated LSP Next Hop Label
10.1.1.1/32 10.2.1.1 31 30
10.0.1.0/24 10.2.1.1 32 31
10.0.2.0/24 10.2.1.1 33 31
10.0.3.0/24 10.2.1.1 34 31