Introduction to GMPLS
Traditional MPLS is designed to carry Layer 3 IP traffic using established IP-based paths and associating these paths with arbitrarily assigned labels. These labels can be configured explicitly by a network administrator, or can be dynamically assigned by means of a protocol such as LDP or RSVP.
GMPLS generalizes MPLS in that it defines labels for switching varying types of Layer 1, Layer 2, or Layer 3 traffic. GMPLS nodes can have links with one or more of the following switching capabilities:
- Fiber-switched capable (FSC)
- Lambda-switched capable (LSC)
- Time-division multiplexing (TDM) switched-capable (TSC)
- Packet-switched capable (PSC)
Label-switched paths (LSPs) must start and end on links with the same switching capability. For example, routers can establish packet-switched LSPs with other routers. The LSPs might be carried over a TDM-switched LSP between SONET add/drop multiplexers (ADMs), which in turn might be carried over a lambda-switched LSP.
The result of this extension of the MPLS protocol is an expansion in the number of devices that can participate in label switching. Lower-layer devices, such as OXCs and SONET ADMs, can now participate in GMPLS signaling and set up paths to transfer data. A router can participate in signaling optical paths across a transport network.
Two service models determine the visibility that a client node (a router, for example) has into the optical core or transport network. The first is through a user-to-network interface (UNI), which is often referred to as the overlay model. The second is known as the peer model. Juniper Networks supports both models.
 | Note: There is not necessarily a one-to-one correspondence between a physical interface and a GMPLS interface. If a GMPLS connection uses a nonchannelized physical connector, the GMPLS label can use the physical port ID. However, the label for channelized interfaces often is based on a channel or time slot. Consequently, it is best to refer to GMPLS labels as identifiers for a resource on a traffic engineering link. |
To establish LSPs, GMPLS uses the following mechanisms:
- An out-of-band control channel and a data channel—RSVP messages for LSP setup are sent over an out-of-band control network. Once the LSP setup is complete and the path is provisioned, the data channel is up and can be used to carry traffic. The Link Management Protocol (LMP) is used to define and manage the data channels between a pair of nodes. You can optionally use LMP to establish and maintain LMP control channels between peers running the same Junos OS Release.
- RSVP-TE extensions for GMPLS—RSVP-TE is already designed to signal the setup of packet LSPs. This has been extended for GMPLS to be able to request path setup for various kinds of LSPs (nonpacket) and request labels like wavelengths, time slots, and fibers as label objects.
- Bidirectional LSPs—Data can travel both ways between GMPLS devices over a single path, so nonpacket LSPs are signaled to be bidirectional.
