Label Distribution Protocol
Encyclopedia
Label Distribution Protocol (LDP) is a protocol in which two Label Edge Router
s (LER) exchange label mapping information. The two LERs are called LDP peers and the exchange of information is bi-directional.
LDP is used to build and maintain LSP
databases that are used to forward traffic through Multiprotocol Label Switching
(MPLS) networks.
LDP can be used to distribute the inner label (VC/VPN/service label) and outer label (path label) in MPLS. For inner label distribution, targeted LDP (tLDP) is used.
LDP and tLDP discovery runs on UDP port 646 and the session is built on TCP port 646. During the discovery phase hello packets are sent on UDP port 646 to the 'all routers on this subnet' group multicast address (224.0.0.2). However, tLDP unicasts the hello packets to the targeted neighbor's address.
of distributing labels in an MPLS environment. LDP relies on the underlying routing information provided by an IGP
in order to forward label packets. The router forwarding information base
, or FIB, is responsible for determining the
hop-by-hop path through the network.
Unlike traffic engineered paths, which use constraints and explicit routes to establish end-to-end Label Switched Paths (LSPs), LDP is used only for signaling best-effort LSPs.
On a router running Timos
when an SDP is configured, automatic ingress and egress labeling (targeted LDP) is enabled by default and ingress and egress “service” labels are signaled over a TLDP connection. If signaling is turned off on an SDP, ingress and egress “service” labels must be manually configured when the SDP is bound to a service.
RSVP-TE
This method determines a path through the network based on the interior gateway protocol's
view of the network. If no constraints are applied to the LSP then the routers simply send the
request for a path to the active next hop for that destination, without explicit routing. The IGP at
each router is free to select active next hops based on the link state database.
Label Edge Router
A label edge router is a router that operates at the edge of an Multiprotocol Label Switching network....
s (LER) exchange label mapping information. The two LERs are called LDP peers and the exchange of information is bi-directional.
LDP is used to build and maintain LSP
Label Switched Path
In MPLS networking, a Label Switched Path is a path through an MPLS network, set up by a signaling protocol such as LDP, RSVP-TE, BGP or CR-LDP. The path is set up based on criteria in the forwarding equivalence class ....
databases that are used to forward traffic through Multiprotocol Label Switching
Multiprotocol Label Switching
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
(MPLS) networks.
LDP can be used to distribute the inner label (VC/VPN/service label) and outer label (path label) in MPLS. For inner label distribution, targeted LDP (tLDP) is used.
LDP and tLDP discovery runs on UDP port 646 and the session is built on TCP port 646. During the discovery phase hello packets are sent on UDP port 646 to the 'all routers on this subnet' group multicast address (224.0.0.2). However, tLDP unicasts the hello packets to the targeted neighbor's address.
LDP
The Label Distribution Protocol (LDP) is a protocol defined by the IETF (RFC 5036) for the purposeof distributing labels in an MPLS environment. LDP relies on the underlying routing information provided by an IGP
Interior gateway protocol
An interior gateway protocol is a routing protocol that is used to exchange routing information within an autonomous system ....
in order to forward label packets. The router forwarding information base
Forwarding Information Base
A forwarding information base , also known as a forwarding table, is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should send a packet to be transmitted by the router.In contrast to routing information bases , also...
, or FIB, is responsible for determining the
hop-by-hop path through the network.
Unlike traffic engineered paths, which use constraints and explicit routes to establish end-to-end Label Switched Paths (LSPs), LDP is used only for signaling best-effort LSPs.
T-LDP
Targeted LDP sessions are different because during the discovery phase hellos are unicast to the LDP peer rather than using multicast. A consequence of this is that tLDP can be set up between non-directly connected peers whereas non-targeted LDP peers must be on the same subnet. tLDP may still be used between connected peers if desired.On a router running Timos
Timos
TiMOS is a proprietary operating system used on most recent Alcatel-Lucent service routers and switches. Originally developed by Timetra, a US based startup firm. . Timetra was bought out by Alcatel-Lucent in 2004 after a period of mutual engagement in projects...
when an SDP is configured, automatic ingress and egress labeling (targeted LDP) is enabled by default and ingress and egress “service” labels are signaled over a TLDP connection. If signaling is turned off on an SDP, ingress and egress “service” labels must be manually configured when the SDP is bound to a service.
RSVP-TERSVP-TEResource Reservation Protocol - Traffic Engineering is an extension of the resource reservation protocol for traffic engineering. It supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature of the packet...
This method determines a path through the network based on the interior gateway protocol'sview of the network. If no constraints are applied to the LSP then the routers simply send the
request for a path to the active next hop for that destination, without explicit routing. The IGP at
each router is free to select active next hops based on the link state database.
See also
- Resource Reservation ProtocolResource Reservation ProtocolThe Resource Reservation Protocol is a Transport Layer protocol designed to reserve resources across a network for an integrated services Internet. RSVP operates over an IPv4 or IPv6 Internet Layer and provides receiver-initiated setup of resource reservations for multicast or unicast data flows...
- Constraint-based Routing Label Distribution ProtocolConstraint-based Routing Label Distribution ProtocolConstraint-based Routing Label Distribution Protocol is a control protocol used in some computer networks.As of February 2003, the IETF MPLS working group deprecated CR-LDP and decided to focus purely on RSVP-TE....
- CR-LDP