Overview

Multiprotocol Label Switching (MPLS) is a label switching technology that provides the ability to set up connection-oriented paths over a connectionless IP network. MPLS facilitates network traffic flow and provides a mechanism to engineer network traffic patterns independently from routing tables. MPLS sets up a specific path for a sequence of packets. The packets are identified by a label stack inserted into each packet.

MPLS requires a set of procedures to enhance network-layer packets with label stacks, which then turns them into labeled packets. Routers that support MPLS are known as Label Switching Routers (LSRs). To transmit a labeled packet on a particular data link, an LSR must support the encoding technique.

In MPLS, packets can carry not only one label but a set of labels in a stack. An LSR can swap the label at the top of the stack, pop the label, or swap the label and push one or more labels into the stack. The processing of a labeled packet is completely independent of the level of hierarchy. The processing is always based on the top label, without regard for the possibility that some number of other labels may have been above it in the past, or that some number of other labels may be below it at present.

Label Distribution Protocol (LDP) is a protocol used to distribute MPLS labels in non-traffic-engineered applications. LDP allows routers to establish LSPs through a network by mapping network-layer routing information directly to data link layer-switched paths.

An LSP is defined by the set of labels from the ingress LER to the egress LER. LDP associates a Forwarding Equivalence Class (FEC) with each LSP it establishes. A FEC is a collection of common actions associated with a class of packets. When an LSR assigns a label to a FEC, it must allow other LSRs in the path to know about the label. LDP helps to establish the LSP by providing a set of procedures that LSRs can use to distribute labels.

The FEC associated with an LSP specifies which packets are mapped to that LSP. LSPs are extended through a network as each LSR splices incoming labels for a FEC to the outgoing label assigned to the next-hop for the specific FEC.

LDP performs the label distribution only in MPLS environments. The LDP operation begins with a hello discovery process to find LDP peers in the network. LDP peers are two LSRs that use LDP to exchange label/FEC mapping information. An LDP session is created between LDP peers. A single LDP session allows each peer to learn the other’s label mappings (LDP is bidirectional) and to exchange label binding information.

LDP signaling works with the MPLS label manager to manage the relationships between labels and the corresponding FEC. An MPLS label identifies a set of actions that the forwarding plane performs on an incoming packet before discarding it. The FEC is identified through the signaling protocol (in this case, LDP) and allocated a label. The mapping between the label and the FEC is communicated to the forwarding plane.

When an unlabeled packet ingresses the router, classification policies associate it with a FEC. The appropriate label is imposed on the packet, and the packet is forwarded.

SR Linux extends the LDP control plane and data plane to support LDP IPv6 adjacencies and sessions using 128-bit LSR IDs.

LDP IPv6 is supported for both link and targeted LDP.

The implementation allows for concurrent support of independent LDP IPv4 (32-bit LSR ID) and IPv6 (128-bit LSR ID) adjacencies and sessions between peer LSRs.

In the current release, SR Linux supports the following MPLS and LDP functionality:

MPLS

• Statically configured MPLS forwarding entries

• Configurable label range

• MPLS label manager that shares the MPLS label space among client applications that require MPLS labels

LDP

• LDPv4 implementation compliant with RFC 5036

• LDPv6 implementation DM> compliant with RFC?

• LDP support in the default network-instance only

• Label distribution using DU (downstream unsolicited), ordered control

• Platform label space only

• Configurable label range (dynamic, non-shared label-block)

• Support for overload TLV when label-block has no free entries

• Configurable timers (hello-interval, hello-holdtime, keepalive-interval)

• Ingress LER, transit LSR, and egress LER roles for /32 IPv4 or /128 IPv6 FECs

• Automatic FEC origination of the systemaddress

• /32 IPv4 FEC resolution using IGP routes, with longest prefix match option

• /128 IPv6 FEC resolution using IGP routes, with longest prefix match option

• ECMP support with configurable next-hop limit (up to 64)

• Automatic installation of all LDP /32 IPv4 and /128 IPv6 prefix FECs into TTM

• Per-peer configurable FEC limit

• Graceful restart helper capability

• BGP shortcuts over LDP tunnels

• Protocol debug/trace-options

• LDP-IGP synchronization

• Advertise address mapping message for primary IPv4 or IPv6 address of the adjacent subinterface only.

• Non-configurable capability advertisement in INITIALIZATION messages only claiming support for:

  • State advertisement control (SAC) with interest in IPv4 prefix FECs only

  • Fault tolerance (Graceful Restart)

  • Nokia overload TLV

  • Unrecognized notification

• Split-horizon support: A label-mapping message is not advertised to a peer if the FEC matches an address sent by that peer in an Address Mapping message.