Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MPLS and H-VPLS (Rev. 03)
Multiprotocol Label Switching
© 2008 Foundry Networks, Inc
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Label Distribution Control
The behavior of the initial setup of LSPs is determined by the LSR label distribution control mode.
There are two types of label distribution control mode as configurable options: independent and
ordered
Independent Label Distribution Control
When using independent control mode, each LSR may at any time advertise label mappings to its
neighbors. For example, when operating in independent downstream on demand mode, an LSR
may answer requests for label mappings immediately without waiting for a label mapping from the
next hop. When operating in independent downstream unsolicited mode, an LSR may advertise a
label mapping for a FEC to its neighbors whenever it is prepared to label-switch that FEC. A
consequence of using independent mode is that an upstream label can be advertised before a
downstream label is received.
Ordered Label Distribution Control
When using ordered control mode, an LSR may initiate the transmission of a label mapping only
for a FEC for which it has a label mapping for the FEC next hop, or for which the LSR is the
egress. For each FEC for which the LSR is not the egress and no mapping exists, the LSR must
wait until a label from a downstream LSR is received before mapping the FEC and passing
corresponding labels to upstream LSRs. An LSR may be an egress for some FECs and a non-
egress for others.
Virtual Leased Lines (VLL)
Virtual Leased Lines are based on draft-martini-l2circuit-trans-mpls. They provide Layer 2 point-
to-point connectivity. No MAC learning is done in the PE devices. Label Distribution Protocol
(LDP) is used to signal the service. VLLs are a logical step that comes before Virtual Private LAN
Services.
Virtual Private LAN Services (VPLS)
Virtual Private LAN Services (see
Figure 4
) is a technology for transparently connecting
geographically dispersed corporate sites over MPLS network so they appear to customers and
behave like a single bridged Ethernet LAN. Combining the simplicity of Ethernet backbone LAN
technology with the scalability and security of the MPLS core, VPLS is a viable alternative for
enterprises seeking a cost-effective VPN solution. However, VPLS adds to the requirements of the
so-called Provider Edge (PE) routers. A PE router is located at the edge of the MPLS core and is
administered by the Service Provider only, with no customer management access. In the case of
VPLS, PE routers and LERs coincide, since labels are attached once packets arrive at PEs from a
non-MPLS network. Each PE connects to Customer Edge equipment that is administered solely by
the customer.