MPLS Fundamentals - Chapter 2 - MPLS Architecture

• An ordered sequence of LSRs is a label switched path (LSP)
• LSP is unidirectional
• A Forwarding Equivalence Class (FEC) is a group or flow of packets that receive the same forwarding treatment throughout the MPLS network.
• All packets belonging to the same FEC have the same label. However, not all packets that have the same label belong to the same FEC, because their EXP values might differ; the forwarding treatment could be different, and they could belong to a different FEC.
• Labels are local to each pair of adjacent routers. Labels have no global meaning across the network.
• For every IGP IP prefix in its IP routing table, each LSR creates a local binding—that is, it binds a label to the IPv4 prefix. The LSR then distributes this binding to all its LDP neighbors. These received bindings become remote bindings. The neighbors then store these remote and local bindings in a special table, the label information base (LIB).
• The LFIB is the table used to forward labeled packets. It is populated with the incoming and outgoing labels for the LSPs. The incoming label is the label from the local binding on the particular LSR. The outgoing label is the label from the remote binding chosen by the LSR from all possible remote bindings. All these remote bindings are found in the LIB. The LFIB chooses only one of the possible outgoing labels from all the possible remote bindings in the LIB and installs it in the LFIB.
• In the case of MPLS traffic engineering, the labels are distributed by RSVP. In the case of MPLS VPN, the VPN label is distributed by BGP.
• If per-platform label space is used, the packet is forwarded solely based on the label, independently from the incoming interface.
• If per-interface label space is used, the packet is not forwarded solely based on the label, but based on both the incoming interface and the label.
• In Cisco IOS, all Label Switching Controlled-ATM (LC-ATM) interfaces have a per-interface label space, whereas all ATM frame-based and non-ATM interfaces have a per-platform label space.
• In Cisco IOS, all interfaces except LC-ATM interfaces use the UD label distribution mode. All LC-ATM interfaces use the DoD label distribution mode.
• In Cisco IOS, the retention mode for LC-ATM interfaces is the CLR mode. It is the LLR mode for all other types of interfaces.
• Cisco IOS uses Independent LSP Control mode. ATM switches that are running Cisco IOS use Ordered LSP Control mode by default.