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L2TP — Layer 2 Tunneling Protocol. A procedure for secure communication of data across a Layer 2 network that enables users to establish PPP sessions between tunnel endpoints. L2TP uses profiles for individual user and group access to ensure secure communication that is as transparent as possible to both end users and applications. See also tunneling protocol.

label — In MPLS, a 20-bit unsigned integer from 0 through 1,048,575, used to identify a packet traveling along an LSP.

Label Distribution Protocol — See LDP.

label object — RSVP message object that contains the label value allocated to the next downstream router.

label pop operation — Function performed by an MPLS router in which the top label in a label stack is removed from the data packet.

label push operation — Function performed by an MPLS router in which a new label is added to the top of the data packet.

label request object — RSVP message object that requests each router along the path of an LSP to allocate a label for forwarding.

label swap operation — Function performed by an MPLS router in which the top label in a label stack is replaced with a new label before the data packet is forwarded to the next-hop router.

label-switched interface — See LSI.

label-switched path — See LSP.

label switching — See MPLS.

label-switching router — See LSR.

label values — 20-bit field in an MPLS header used by routers to forward data traffic along an MPLS label-switched path.

LAN PHY — Local Area Network Physical Layer Device. A physical layer device that allows 10-Gigabit Ethernet wide area links to use existing Ethernet applications. See also PHY and WAN PHY.

Layer 2 circuits — Collection of transport modes that accept a stream of ATM cells, convert them to an encapsulated Layer 2 format, then tunnel them over an MPLS or IP backbone, where a similarly configured routing platform segments these packets back into a stream of ATM cells, to be forwarded to the virtual circuit configured for the far-end routing platform. Layer 2 circuits are designed to transport Layer 2 frames between provider edge (PE) routing platforms across a Label Distribution Protocol (LDP)-signaled MPLS backbone. See also AAL5 mode, cell-relay mode, standard AAL5 mode, trunk mode.

Layer 2 Tunneling Protocol — See L2TP.

Layer 2 VPN — Provides a private network service among a set of customer sites using a service provider’s existing MPLS and IP network. A customer’s data is separated from other data using software rather than hardware. In a Layer 2 VPN, the Layer 3 routing of customer traffic occurs within the customer’s network.

Layer 3 VPN — Provides a private network service among a set of customer sites using a service provider’s existing MPLS and IP network. A customer’s routes and data are separated from other routes and data using software rather than hardware. In a Layer 3 VPN, the Layer 3 routing of customer traffic occurs within the service provider’s network.

LCC — Line-card chassis. Term used by the JUNOS command-line interface (CLI) to refer to a T640 routing node in a routing matrix.

LCP — Link Control Protocol. A traffic controller used to establish, configure, and test data-link connections for the Point-to-Point Protocol (PPP).

LDAP — Lightweight Directory Access Protocol. Software protocol used for locating resources on a public or private network.

LDP — Label Distribution Protocol. A protocol for distributing labels in non-traffic-engineered applications. LDP allows routers to establish label-switched paths (LSPs) through a network by mapping network-layer routing information directly to data-link layer switched paths.

leaf node — Terminating node of a multicast distribution tree. A router that is a leaf node only has receivers and does not forward multicast packets to other routers.

learning domain — MAC address database where MAC addresses are added based on the normalized VLAN tags.

LFI — Link fragmentation and interleaving. A method that reduces excessive delays by fragmenting long packets into smaller packets and interleaving them with real-time frames. For example, short delay-sensitive packets, such as packetized voice, can race ahead of larger delay-insensitive packets, such as common data packets.

LFM — Link fault management. A method used to detect problems on links and spans on an Ethernet network defined in IEEE 802.3ah. See also OAM.

liblicense — Library that includes messages generated for routines for software license management.

libpcap — Implementation of the pcap application programming interface. libpcap is used by a program to capture packets traveling over a network. See also pcap.

Lightweight Directory Access Protocol — See LDAP.

limited operational environment — Term used to describe the restrictions placed on FIPS-certified equipment. See FIPS.

line-card chassis — See LCC.

line loopback — Method of troubleshooting a problem with physical transmission media in which a transmission device in the network sends the data signal back to the originating router.

link — Communication path between two neighbors. A link is up when communication is possible between the two end points.

Link Control Protocol — See LCP.

link fault management — See LFM.

link fragmentation and interleaving — See LFI.

Link Management Protocol — See LMP.

link protection — Method of establishing bypass label-switched paths (LSPs) to ensure that traffic going over a specific interface to a neighboring router can continue to reach the router if that interface fails. The bypass LSP uses a different interface and path to reach the same destination.

link services intelligent queuing interfaces — See LSQ.

link-state acknowledgement — OSPF data packet used to inform a neighbor that a link-state update packet has been successfully received.

link-state advertisement — See LSA.

link-state database — All routing knowledge in a link-state network is contained in this database. Each router runs the SPF algorithm against this database to locate the best network path to each destination in the network.

link-state PDU — Packet that contains information about the state of adjacencies to neighboring systems.

link-state replication — Addition to the SONET Automatic Protection Switching (APS) functionality that helps promote redundancy of the link PICs used in LSQ configurations. If the active SONET PIC fails, links from the standby PIC are used without causing a link renegotiation. Also called interface preservation.

link-state request list — List generated by an OSPF router during the exchange of database information while forming an adjacency. Advertised information by a neighbor that the local router does not contain is placed in this list.

link-state request packet — OSPF data packet used by a router to request database information from a neighboring router.

link-state update — OSPF data packet that contains one of multiple LSAs. It is used to advertise routing knowledge into the network.

linktrace message — See LTM.

Linktrace Protocol — Protocol is used for path discovery between a pair of maintenance points. Linktrace messages are triggered by an administrator using the traceroute command to verify the path between a pair of maintenance end points (MEPs) under the same maintenance association. Linktrace messages can also be used to verify the path between an MEP and an maintenance intermediate point (MIP) under the same maintenance domain. The operation of IEEE 802.1ag linktrace request and response messages is similar to the operation of Layer 3 traceroute commands.

linktrace response — See LTR.

LLC — Logical link control. Data-link layer protocol used on a LAN. LLC1 provides connectionless data transfer, and LLC2 provides connection-oriented data transfer.

LLC frame — Unit of data that contains specific information about the LLC layer and identifies line protocols associated with the layer. See also LLC.

LMI — Local management interface. Enhancements to the basic Frame Relay specifications, providing support for the following:

A keepalive mechanism that verifies the flow of data
A multicast mechanism that provides a network server with a local DLCI and multicast DLCI
In Frame Relay networks, global addressing that gives DLCIs global instead of local significance
A status mechanism that provides a switch with ongoing status reports on known DLCIs

LMP — Link Management Protocol. Part of GMPLS, a protocol used to define a forwarding adjacency between peers and to maintain and allocate resources on the traffic engineering links.

load balancing — Process that installs all next-hop destinations for an active route in the forwarding table. You can use load balancing across multiple paths between routers. The behavior of load balancing depends on the version of the Internet Processor ASIC in the router. Also called per-packet load balancing.

loading — OSPF adjacency state in which the local router sends link-state request packets to its neighbor and waits for the appropriate link-state updates from that neighbor.

local management interface — See LMI.

local packet — Chunk of data destined for or sent by the Routing Engine.

local preference — Optional BGP path attribute carried in internal BGP update packets that indicates the degree of preference for an external route.

local significance — Concept used in an MPLS network where the label values are unique only between two neighbor routers.

local RIB — Logical software table that contains BGP routes used by the local router to forward data packets.

logical interface — On a physical interface, the configuration of one or more units which include all addressing, protocol information, and other logical interface properties that enable the physical interface to function.

logical link control — See LLC.

logical operator — Characters used in a firewall filter to represent a Boolean AND or OR operation.

logical router — Logical routing device that is partitioned from an M-series or T-series routing platform. Each logical router independently performs a subset of the tasks performed by the main router and has a unique routing table, interfaces, policies, and routing instances.

longer — JUNOS software routing policy match type that represents all routes more specific than the given subnet, but not the given subnet itself. It is similar to a mathematical greater-than operation.

loopback interface (lo0) — Interface that is always available because it is independent of any physical interfaces. When configured with an address, the loopback interface is the default address for the routing platform and any unnumbered interfaces. See also unnumbered interface.

lo0 — See loopback interface (lo0).

loose hop — In the context of traffic engineering, a path that can use any router or any number of other intermediate (transit) points to reach the next address in the path. (Definition from RFC 791, modified to fit LSPs.)

loss-priority map — Maps the loss priority of incoming packets based on code point values.

lower-speed IQ interfaces — E1, NxDS0, and T1 interfaces configured on an IQ PIC.

LPDU — LLC protocol data unit. LLC frame on a DLSw network. See LLC frame.

LSA — Link-state advertisement. OSPF data structure that is advertised in a link-state update packet. Each LSA uniquely describes a portion of the OSPF network.

LSI — Label-switched interface. A logical interface supported by the JUNOS software that provides VPN services (such as VPLS and Layer 3 VPNs) normally provided by a Tunnel Services PIC.

LSP — Label -switched path. Sequence of routers that cooperatively perform MPLS operations for a packet stream. The first router in an LSP is called the ingress router, and the last router in the path is called the egress router. An LSP is a point-to-point, half-duplex connection from the ingress router to the egress router. (The ingress and egress routers cannot be the same router.) See link-state PDU.

LSQ — Link services intelligent queuing interfaces. Interfaces configured on the Adaptive Services PIC or ASM that support MLPPP and MLFR traffic and also fully support JUNOS class-of-service (CoS) components.

LSR — Label-switching router. A router on which MPLS is enabled and that can process label-switched packets.

LTM — Linktrace message. Message used by one MEP to trace the path to another maintenance end point (MEP) or maintenance intermediate point (MIP) in the same domain. It is needed for loopback (ping). All intermediate MIPs respond back with a linktrace response to the originating MEP. After decreasing the TTL by one, intermediate MIPs forward the linktrace message until the destination MIP/MEP is reached. If the destination is a MEP, every MIP along a given maintenance association responds to the originating MEP. The originating MEP can then determine the MAC address of all MIPs along the maintenance association and their precise location with respect to the originating MEP.

LTR — Linktrace response. See LTM.

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