MPLS - LDP and LSP

In this section, we will see about LDP protocol and MPLS LSP and how it is formed using LDP.

LDP overview-

The Label Distribution Protocol (LDP) is used to establish MPLS and to form LSPs .It establishes LSPs that follow the existing IP routing table made by IGP, and is particularly well suited for establishing a full mesh of 

LSPs between all of the routers on the network.

LDP can operate in many modes to suit different requirements; however the most common usage is unsolicited mode, which sets up a full mesh of tunnels between routers.

• In solicited mode, the ingress router sends an LDP label request to the next hop router, as determined from its IP routing table. This request is forwarded on through the network hop-by-hop by each router. Once the request reaches the egress router, a return message is generated. This message confirms the LSP and tells each router the label mapping to use on each link for that LSP.

Solicited Distribution

• In unsolicited mode, the egress routers broadcast label mappings for each external link to all of their neighbors. These broadcasts are spread across every link through the network until they reach the ingress routers. Across each hop, they inform the upstream router of the label mapping to use for each external link, and by flooding the network they establish LSPs between all of the external links.

Unsolicited Distribution

The main advantage of LDP is the ease of setting up a full mesh of tunnels using unsolicited mode, so it is most often used in this mode to set up the underlying mesh of tunnels needed by MPLS Layer 3 VPNs.

LSP – Label switched path

•  A Label Switched Path (LSP) is a sequence of LSRs that forward the packets of a certain forwarding equivalence class.
• MPLS unicast IP forwarding builds LSPs based on the output of IP routing protocols.
• LDP advertises the labels only for individual segments in the LSP.
• LSPs are unidirectional
• Return traffic uses different LSP (usually reverse path because the most routing protocol provide symmetrical routing)

An LSP can take a different path from the one chosen by an IP routing protocol.(MPLS TE)           

The IP Routing protocols determine the path of LSP.

LSP is made by IGP

• In MPLS network each router will have LIB (label information base) and LFIB.    In LIB all label are installed then by calculating best next hop. Best labels is installed in LFIB to process the labels for particular network, In this way LSP builds

• LDP propagates the labels to converge the path to LSP.

• We also have other label information which is in LIB, So In this way we can converge fast in failure of link. Since we are having the alternative label information with us. This is known as Liberal retention mode.

 Hope this post will help you to understanding LDP and LSP . 

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