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Riverstone Networks RS Switch Router User Guide Release 8.0 17-65
MPLS Configuration Configuring L2 Tunnels
17.6.2 Configuring Dynamic L2 Labels
In Figure 17-15, layer-2 frames are received at the ingress LSR R1, then transmitted to the egress LSR R2 across an
MPLS network through a tunnel LSP. At the ingress LSR, a virtual circuit (VC) label is added to the L2 frame. The
VC label is used to inform the egress LSR how to treat the received packet and the interface on which the frame is to
be output. When R1 sends the L2 frame to R2, a tunnel label is pushed onto the MPLS label stack. As the packet
traverses through the MPLS network, additional labels can be pushed onto and popped off the label stack. A number
of L2 VCs can be carried in this way across a single tunnel LSP.
Note that the process of transporting L2 frames across an MPLS network is unidirectional; you will need to repeat the
configuration in the reverse direction to provide bidirectional operation. You must configure the same VC identifiers
(for example, VLAN IDs) for each direction of a virtual circuit.
Figure 17-15 Transport of layer 2 frames across an MPLS network
Virtual Circuit Signaling
For dynamic label assignments, the VC label is distributed using LDP in downstream unsolicited mode. A remote LDP
connection must be established between the ingress and egress LSRs. You configure VLANs to carry the signaling
necessary to establish the LDP connection. In this section, these VLANs are referred to as LDP-signaling VLANs.
Tunnel LSP Signaling
Either RSVP or LDP can be used to assign and distribute the tunnel labels used within the tunnel LSP. If you use RSVP
for signaling in the tunnel LSP, you need to configure LSPs with the
mpls create
and/or
mpls set
commands. If
you use LDP for signaling in the tunnel LSP, you do not need to configure LSPs with the
mpls
commands; however,
you need to enable and start MPLS on tunnel LSRs.
R1 R2
Tunnel LSP (RSVP or LDP)
Virtual Circuit
VC label
Tunnel label
(LDP signaling over VLAN)
Layer 2 Frame
Layer 2 Frame