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The Label Distribution Protocol (LDP) is 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.
These LSPs might have an endpoint at a directly attached neighbor (comparable to IP hop-by-hop forwarding), or at a network egress node, enabling switching through all intermediary nodes. LSPs established by LDP can also traverse traffic-engineered LSPs created by RSVP.
LDP associates a forwarding equivalence class (FEC) with each LSP it creates. The FEC associated with an LSP specifies which packets are mapped to that LSP. LSPs are extended through a network as each router chooses the label advertised by the next hop for the FEC and splices it to the label it advertises to all other routers. This process forms a tree of LSPs that converge on the egress router.
Understanding the LDP Signaling Protocol
LDP is a signaling protocol that runs on a device configured for MPLS support. The successful configuration of both MPLS and LDP initiates the exchange of TCP packets across the LDP interfaces. The packets establish TCP-based LDP sessions for the exchange of MPLS information within the network. Enabling both MPLS and LDP on the appropriate interfaces is sufficient to establish LSPs.
LDP is a simple, fast-acting signaling protocol that automatically establishes LSP adjacencies within an MPLS network. Routers then share LSP updates such as hello packets and LSP advertisements across the adjacencies. Because LDP runs on top of an IGP such as IS-IS or OSPF, you must configure LDP and the IGP on the same set of interfaces. After both are configured, LDP begins transmitting and receiving LDP messages through all LDP-enabled interfaces. Because of LDP's simplicity, it cannot perform the true traffic engineering which RSVP can perform. LDP does not support bandwidth reservation or traffic constraints.
When you configure LDP on a label-switching router (LSR), the router begins sending LDP discovery messages out all LDP-enabled interfaces. When an adjacent LSR receives LDP discovery messages, it establishes an underlying TCP session. An LDP session is then created on top of the TCP session. The TCP three-way handshake ensures that the LDP session has bidirectional connectivity. After they establish the LDP session, the LDP neighbors maintain, and terminate, the session by exchanging messages. LDP advertisement messages allow LSRs to exchange label information to determine the next hops within a particular LSP. Any topology changes, such as a router failure, generate LDP notifications that can terminate the LDP session or generate additional LDP advertisements to propagate an LSP change.
Example: Configuring LDP-Signaled LSPs
This example shows how to create and configure LDP instances within an MPLS network.
Before you begin:
Configure network interfaces. See Interfaces User Guide for Security Devices.
Configure an IGP across your network. (The LDP configuration is added to the existing IGP configuration and included in the MPLS configuration.)
Configure a network to use LDP for LSP establishment by enabling MPLS on all transit interfaces in the MPLS network.
Because LDP runs on top of an IGP such as IS-IS or OSPF, you must configure LDP and the IGP on the same set of interfaces.
To configure LDP-signaled LSPs, you must enable the MPLS family on all transit interfaces in the MPLS network and include all the transit interfaces under the [protocols mpls] and [protocols ldp] hierarchy levels.
In this example, you enable the MPLS family and create an LDP instance on all the transit interfaces. Additionally, you enable the MPLS process on all the transit interfaces in the MPLS network. In this example, you configure a sample network as shown in Figure 1.
CLI Quick Configuration
To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the  hierarchy level and then enter commit from configuration mode.
For Router R1, perform the following:
For Router R2, perform the following:
For Router R3, perform the following:
To enable LDP instances within an MPLS network:
- Enable the MPLS family on the transit interface on Router
R1. user@R1# set interfaces ge-0/0/0 unit 0 family mpls
- Enable the MPLS process on the transit interface. user@R1# set protocols mpls interface ge-0/0/0 unit 0
- Create the LDP instance on the transit interface. user@R1# set protocols ldp interface ge-0/0/0 unit 0
Confirm your configuration by entering the show command from configuration mode. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.
For brevity, this show output includes only the configuration that is relevant to this example. Any other configuration on the system has been replaced with ellipses (...).
If you are done configuring the device, enter the commit command from the configuration mode to activate the configuration.
Junos OS LDP Protocol Implementation
The Junos OS implementation of LDP supports LDP version 1. The Junos OS supports a simple mechanism for tunneling between routers in an interior gateway protocol (IGP), to eliminate the required distribution of external routes within the core. The Junos OS allows an MPLS tunnel next hop to all egress routers in the network, with only an IGP running in the core to distribute routes to egress routers. Edge routers run BGP but do not distribute external routes to the core. Instead, the recursive route lookup at the edge resolves to an LSP switched to the egress router. No external routes are necessary on the transit LDP routers.
You must configure LDP for each interface on which you want LDP to run. LDP creates LSP trees rooted at each egress router for the router ID address that is the subsequent BGP next hop. The ingress point is at every router running LDP. This process provides an inet.3 route to every egress router. If BGP is running, it will attempt to resolve next hops by using the inet.3 table first, which binds most, if not all, of the BGP routes to MPLS tunnel next hops.
Two adjacent routers running LDP become neighbors. If the two routers are connected by more than one interface, they become neighbors on each interface. When LDP routers become neighbors, they establish an LDP session to exchange label information. If per-router labels are in use on both routers, only one LDP session is established between them, even if they are neighbors on multiple interfaces. For this reason, an LDP session is not related to a particular interface.
LDP operates in conjunction with a unicast routing protocol. LDP installs LSPs only when both LDP and the routing protocol are enabled. For this reason, you must enable both LDP and the routing protocol on the same set of interfaces. If this is not done, LSPs might not be established between each egress router and all ingress routers, which might result in loss of BGP-routed traffic.
You can apply policy filters to labels received from and distributed to other routers through LDP. Policy filters provide you with a mechanism to control the establishment of LSPs.
LDP Message Types
LDP uses the message types described in the following sections to establish and remove mappings and to report errors. All LDP messages have a common structure that uses a type, length, and value (TLV) encoding scheme.
Discovery messages announce and maintain the presence of a router in a network. Routers indicate their presence in a network by sending hello messages periodically. Hello messages are transmitted as UDP packets to the LDP port at the group multicast address for all routers on the subnet.
LDP uses the following discovery procedures:
Basic discovery—A router periodically sends LDP link hello messages through an interface. LDP link hello messages are sent as UDP packets addressed to the LDP discovery port. Receipt of an LDP link hello message on an interface identifies an adjacency with the LDP peer router.
Extended discovery—LDP sessions between routers not directly connected are supported by LDP extended discovery. A router periodically sends LDP targeted hello messages to a specific address. Targeted hello messages are sent as UDP packets addressed to the LDP discovery port at the specific address. The targeted router decides whether to respond to or ignore the targeted hello message. A targeted router that chooses to respond does so by periodically sending targeted hello messages to the initiating router.
Session messages establish, maintain, and terminate sessions between LDP peers. When a router establishes a session with another router learned through the hello message, it uses the LDP initialization procedure over TCP transport. When the initialization procedure is completed successfully, the two routers are LDP peers and can exchange advertisement messages.
Advertisement messages create, change, and delete label mappings for forwarding equivalence classes (FECs). Requesting a label or advertising a label mapping to a peer is a decision made by the local router. In general, the router requests a label mapping from a neighboring router when it needs one and advertises a label mapping to a neighboring router when it wants the neighbor to use a label.
Notification messages provide advisory information and signal error information. LDP sends notification messages to report errors and other events of interest. There are two kinds of LDP notification messages:
Error notifications, which signal fatal errors. If a router receives an error notification from a peer for an LDP session, it terminates the LDP session by closing the TCP transport connection for the session and discarding all label mappings learned through the session.
Advisory notifications, which pass information to a router about the LDP session or the status of some previous message received from the peer.
Tunneling LDP LSPs in RSVP LSPs
You can tunnel LDP LSPs over RSVP LSPs. The following sections describe how tunneling of LDP LSPs in RSVP LSPs works:
Tunneling LDP LSPs in RSVP LSPs Overview
If you are using RSVP for traffic engineering, you can run LDP simultaneously to eliminate the distribution of external routes in the core. The LSPs established by LDP are tunneled through the LSPs established by RSVP. LDP effectively treats the traffic-engineered LSPs as single hops.
When you configure the router to run LDP across RSVP-established LSPs, LDP automatically establishes sessions with the router at the other end of the LSP. LDP control packets are routed hop-by-hop, rather than carried through the LSP. This routing allows you to use simplex (one-way) traffic-engineered LSPs. Traffic in the opposite direction flows through LDP-established LSPs that follow unicast routing rather than through traffic-engineered tunnels.
If you configure LDP over RSVP LSPs, you can still configure multiple OSPF areas and IS-IS levels in the traffic engineered core and in the surrounding LDP cloud.
Beginning with Junos OS Release 15.1, multi-instance support is extended to LDP over RSVP tunneling for a virtual router routing instance. This allows splitting of a single routing and MPLS domain into multiple domains so that each domain can be scaled independently. BGP labeled unicast can be used to stitch these domains for service forwarding equivalence classes (FECs). Each domain uses intra-domain LDP-over-RSVP LSP for MPLS forwarding.
With the introduction of the multi-instance support for LDP-over-RSVP LSPs, you cannot enable MPLS on an interface that is already assigned to another routing instance. Adding an interface that is part of another routing instance at the [edit protocols mpls] hierarchy level, throws a configuration error at the time of commit.
Benefits of Tunneling LDP LSPs in RSVP LSPs
Tunneling LDP LSPs in RSVP LSPs provides the following benefits:
Provides convergence of different traffic types such as IPv4, IPv6, unicast, and multicast across Layer 2 and Layer 3 VPNs.
Enables flexible access connectivity options that can accommodate multiple topologies, different protocols, and multiple administrative boundaries.
Enables secure interworking among multiple providers.
Enables provision of differentiated services on a per customer basis because RSVP-TE supports traffic engineering, bandwidth guarantees, and link and node redundancy capabilities.
Reduces the number of LSPs required in the core, which reduces the resource requirements of the protocols and routers as well as reducing convergence time.
Provides cost-efficient rollouts with minimal network disruption because the LSPs are built using point-to-point TE tunnels to directly attached neighbors. These TE tunnels only go to the next hop, not end to end. Then when LDP is run over those tunnels, the sessions are built to the directly connected neighbor. When there is a change in the network, such as adding a new node, the directly connected neighbors of the new node have RSVP and LDP sessions. Thus, the RSVP LSPs are only to the next hop, and LDP takes care of advertising labels for the new addresses.
Figure 2 depicts an LDP LSP being tunneled through an RSVP LSP. (For definitions of label operations, see MPLS Label Overview.) The shaded inner oval represents the RSVP domain, whereas the outer oval depicts the LDP domain. RSVP establishes an LSP through routers B, C, D, and E, with the sequence of labels L3, L4. LDP establishes an LSP through Routers A, B, E, F, and G, with the sequence of labels L1, L2, L5. LDP views the RSVP LSP between Routers B and E as a single hop.
When the packet arrives at Router A, it enters the LSP established by LDP, and a label (L1) is pushed onto the packet. When the packet arrives at Router B, the label (L1) is swapped with another label (L2). Because the packet is entering the traffic-engineered LSP established by RSVP, a second label (L3) is pushed onto the packet.
This outer label (L3) is swapped with a new label (L4) at the intermediate router (C) within the RSVP LSP tunnel, and when the penultimate router (D) is reached, the top label is popped. Router E swaps the label (L2) with a new label (L5), and the penultimate router for the LDP-established LSP (F) pops the last label.
Figure 3 depicts a double push label operation (L1L2). A double push label operation is used when the ingress router (A) for both the LDP LSP and the RSVP LSP tunneled through it is the same device. Note that Router D is the penultimate hop for the LDP-established LSP, so L2 is popped from the packet by Router D.
LDP Session Protection
LDP session protection is based on the LDP targeted hello functionality defined in RFC 5036, LDP Specification, and is supported by the Junos OS as well as the LDP implementations of most other vendors. It involves sending unicast User Datagram Protocol (UDP) hello packets to a remote neighbor address and receiving similar packets from the neighbor router.
If you configure LDP session protection on a router, the LDP sessions are maintained as follows:
- An LDP session is established between a router and a remote neighboring router.
- If all of the direct links between the routers go down, the LDP session remains up so long as there is IP connectivity between the routers based on another connection over the network.
- When the direct link between the routers is reestablished, the LDP session is not restarted. The routers simply exchange LDP hellos with each other over the direct link. They can then begin forwarding LDP-signaled MPLS packets using the original LDP session.
By default, LDP targeted hellos are set to the remote neighbor so long as the LDP session is up, even if there are no more link neighbors to that router. You can also specify the duration you would like to maintain the remote neighbor connection in the absence of link neighbors. When the last link neighbor for a session goes down, the Junos OS starts an LDP session protection timer. If this timer expires before any of the link neighbors come back up, the remote neighbor connection is taken down and the LDP session is terminated. If you configure a different value for the timer while it is currently running, the Junos OS updates the timer to the specified value without disrupting the current state of the LDP session.
LDP Native IPv6 Support Overview
IPv6 connectivity often relies on tunneling IPv6 over an IPv4 MPLS core with IPv4-signaled MPLS label-switched paths (LSPs). This requires the IPv4-signaled LSPs to be configured statically or established dynamically by IPv6 provider edge routers. Because of the growing demand of IPv6, it has become imperative to deploy an IPv6 MPLS core with an IPv6-signaled LSP to provide IPv6 connectivity. In Junos OS, LDP is supported in an IPv6 network only, and in an IPv6/IPv4 dual-stack network as described in RFC 7552. Apart from providing a single session for both IPv4 and IPv6 networks, Junos OS LDP supports separate IPv4 sessions for IPv4 only, and IPv6 sessions for IPv6 only.
You can configure the address family as inet for IPv4 or inet6 for IPv6, or both. If the family address is not configured, then the default address of family inet is enabled. When both IPv4 and IPv6 are configured, you can use the transport-preference statement to configure the prefered transport to be either IPv4 or IPv6. Based on the preference, LDP attempts to establish a TCP connection using IPv4 or IPv6. By default, IPv6 is selected. The dual-transport statement allows Junos OS LDP to establish the TCP connection over IPv4 with IPv4 neighbors, and over IPv6 with IPv6 neighbors as a single-stack LSR. The inet-lsr-id and inet6-lsr-id IDs are the two LSR IDs that have to be configured to establish an LDP session over IPv4 and IPv6 TCP transport. These two IDs should be non-zero and must be configured with different values.
Longest Match Support for LDP Overview
LDP is often used to establish MPLS label-switched paths (LSPs) throughout a complete network domain using an IGP such as OSPF or IS-IS. In such a network, all links in the domain have IGP adjacencies as well as LDP adjacencies. LDP establishes the LSPs on the shortest path to a destination as determined by IGP. In Junos OS, the LDP implementation does an exact match lookup on the IP address of the forwarding equivalence class (FEC) in the routing information base (RIB) or IGP routes for label mapping. This exact mapping requires MPLS end-to-end LDP endpoint IP addresses to be configured in all the label edge routers (LERs). This defeats the purpose of IP hierarchical design or default routing in access devices. Configuring longest-match allows LDP to set up LSP based on the routes aggregated or summarized across OSPF areas or IS-IS levels in the inter-domain.