layer 2 connection, such as a Frame Relay data-link connection identifier (DLCI), is pushed
into the label stack between the tunnel label and the layer 2 data.
A service-specific control word may be placed between the layer 2 data and the VC label.
The control word is used for frame sequencing and carrying service-specific information,
such as Frame Relay forward explicit congestion notification (FECN) and backward
explicit congestion notification (BECN) information. At the tunnel end, the VC label is
used to find the layer 2 interface over which the traffic is sent. The control word, if present,
is used to convert the encapsulated layer 2 traffic into its native format.
Because MPLS labels are unidirectional, two VC labels are required for each layer 2
connection. The VC labels are distributed by the Label Distribution Protocol (LDP) in
downstream-unsolicited (DU) mode between the two routers. The layer 2 connection
status signaling may be emulated by advertising and withdrawing the VC labels. For
example, if the Frame Relay subinterface between customer premises equipment (CPE)
and a provider edge (PE) router goes down, the corresponding VC label is withdrawn by
the PE router. When the remote PE router at the other end receives the label withdrawal,
it translates the label withdrawal into LMI notifications to its CPE. When the Frame Relay
subinterface comes back, a VC label is advertised, and the remote PE router again
translates it into LMI notifications.
Related Topics Layer 2 Services over MPLS Overview on page 509•
• Configuring Frame Relay Layer 2 Services on page 530
• Configuring Interoperation with Legacy Frame Relay Layer 2 Services on page 530
• Configuring Ethernet/VLAN Layer 2 Services on page 531
• Configuring S-VLAN Tunnels for Layer 2 Services on page 532
Local Cross-Connects Between Layer 2 Interfaces Using MPLS Overview
You can configure layer 2 services over MPLS to transmit data between two layer 2
interfaces that reside on the same E Series router. In this configuration, which is referred
to as a local cross-connect, traffic that arrives at the router’s ingress interface is switched
out the egress interface, instead of going through an MPLS core network.
A local cross-connect enables the router to function as a layer 2 switch. It operates with
any supported layer 2 service. To configure local cross-connects, you must use the
mpls-relay command.
Related Topics For a list of supported layer 2 services, see Layer 2 Services over MPLS Overview on
page 509
•
• For a configuration example that shows how to create local cross-connects between
Ethernet/VLAN interfaces, see Configuring Local Cross-Connects Between
Ethernet/VLAN Interfaces on page 533
513Copyright © 2010, Juniper Networks, Inc.
Chapter 8: Layer 2 Services over MPLS Overview
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