Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 549)

Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide 525
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How MPLS VLL works
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How MPLS VLL works
The following diagram illustrates how packets are forwarded over an MPLS VLL.
FIGURE 57 Forwarding packets over an MPLS VLL
Packets are forwarded over an MPLS VLL as described below.
1. A Customer Edge (CE) device forwards a packet to a Label Edge Router (LER) serving as a
Provider Edge (PE) router at the edge of the MPLS domain.
2. The PE router assigns the packet to an RSVP-signalled LSP whose destination is an LER (also
serving as a PE router) that is connected to a CE device at the far end of the MPLS domain. The
PE router at the other end of the MPLS domain is known as this PE router’s VLL peer. The
RSVP-signaled LSP used to reach the VLL peer is known as the tunnel LSP. Alternatively, an
LDP-signaled, tunneled LSP can be used.
When a Class of Service (COS) value is set for the VLL, the device selects a tunnel LSP that also
has this COS value, when one is available. When no tunnel LSP with this COS value is available,
the device selects a tunnel LSP with the highest configured COS value (although never higher
than the COS setting for the VLL). Refer to
“QoS for VLL traffic” on page 527 for more
information.
When there are multiple tunnel LSPs that can be used to reach the VLL peer, the PE router
selects one of the tunnel LSPs by using a round-robin method.
The PE router pushes two labels onto the packet:
• The inner VC label is used for determining what happens to the packet once it reaches the
VLL peer. This label is significant only to the VLL peer
• The outer tunnel label is used for forwarding the packet through the MPLS domain. This
label corresponds to an RSVP-signalled tunnel LSP
Refer to “MPLS VLL packet encoding” on page 526 for information on the structure of packets
forwarded along an MPLS VLL. After applying the two labels to the packet, the PE router
forwards it to the next LSR in the tunnel LSP.
3. The penultimate LSR in the tunnel LSP removes the tunnel label and forwards the packet (now
with the VC label as the top label) to the PE router at the other edge of the MPLS domain.