MPLS Forwarding and Next-Hop Tables

An MPLS forwarding table determines how MPLS handles received MPLS packets. When an MPLS packet arrives on an MPLS major interface, MPLS looks up the outermost MPLS label of the received packet in the relevant MPLS forwarding table.

The entries in the MPLS forwarding table map labels to next hops. Each entry in the MPLS forwarding table points to an entry in the MPLS next-hop table. Each MPLS next hop points to either an interface or another MPLS next hop. The chain of MPLS next hops, which ends at an interface, informs MPLS which labels to push and where to send the MPLS packet.

For RSVP-TE tunnels, minor interfaces are created in addition to the forwarding table and next-hop table entries. One minor interface is created of each in/out segment of a tunnel. the purpose of these minor interfaces is to attach QoS and policy to an LSP.

MPLS forwarding tables consist of the following:

• One forwarding table for each MPLS virtual router. This table contains labels from the platform label space. When an MPLS packet arrives on an MPLS major interface that uses the platform label space, MPLS looks up the label in the MPLS forwarding table of the virtual router in which the major interface exists.
• One forwarding table for each MPLS major interface that uses the interface label space. This table contains labels from the interface label space of that major interface. When an MPLS packet arrives on an MPLS major interface that uses the interface label space, MPLS looks up the label in the MPLS forwarding table for that particular major interface.

The signaling protocols add entries to the MPLS forwarding tables. You cannot manually create an MPLS forwarding entry. The signaling protocols set the following attributes for each entry placed in the forwarding table:

• An MPLS in label that is matched against the outermost label of the received MPLS packet.
• The MPLS next hop, which specifies the actions to be performed on the MPLS packet. MPLS next hops can be chained together to create complex actions.
• A spoof check field that specifies the type of spoof checking is performed to determine whether the MPLS packet arrived from a legitimate source. See Spoof Checking MPLS Packets for more information.

Related Topics

• See Monitoring MPLS , for information about enabling statistics collection for MPLS forwarding table entries.
• MPLS Label Switching and Packet Forwarding
• MPLS Label Distribution Methodology
• Spoof Checking MPLS Packets
• IP and IPv6 Tunnel Routing Tables and MPLS Tunnels

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