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withdrawn by the PE router.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.
Local Cross-Connects Between Layer 2 Interfaces Using MPLS
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
â–
â– 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
MPLS Shim Interfaces for Layer 2 Services over MPLS
An MPLS shim interface is stacked on a layer 2 interface to do either of the following:
â– Create a layer 2 circuit by cross-connecting the layer 2 interface to an LSP
corresponding to the VC label.
â– Create a local cross-connect by cross-connecting the layer 2 interface to another
layer 2 interface.
You can create or remove MPLS shim interfaces with the mpls-relay or route
interface commands. Shim interfaces are also created in or removed implicitly from
a load-balancing group by the member interface command when you configure the
group. Each MPLS shim interface exists in a particular virtual router.
Local Cross-Connects Between Layer 2 Interfaces Using MPLS â– 513
Chapter 6: Layer 2 Services over MPLS Overview
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