LDP Overview
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LDP
The Label Distribution Protocol (LDP) is used to establish MPLS transport LSPs when
traffic engineering is not required. It establishes LSPs that follow the existing IP routing table,
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.
- In unsolicited mode, the egress routers broadcast label mappings for each external link
to all of their neighbors. These broadcasts are fanned 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.
The main advantage of LDP over
RSVP 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
Layer 2 and
Layer 3 VPNs.
LDP for MPLS Services
LDP is used to establish MPLS service LSPs. The basic service is a pseudowire, which as its
name implies, can be used to simulate any type of wired service. A pseudowire can carry almost
any kind of traffic, Layer 2 packets (for example Ethernet), L3 packets, ATM cells, Frame Relay,
TDM circuits etc. Given this flexibility, pseudowires are widely used in
Carrier Ethernet and
Mobile Backhaul.
Simple pseudowires are bi-directional point-to-point. However, they can be extended simply
to provide point-to-multipoint capability. For example, a Layer 2 VPN is created by establishing
a mesh of pseudowires between VPN end points and using Ethernet switching to route packets into
the correct pseudowire.
Metaswitch's MPLS software,
DC-MPLS, fully supports LDP in both solicited and unsolicited modes.
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For inquiries about Metaswitch's MPLS products and expertise contact
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