MPLS applications on the frame relay network (1)

1 Introduction

The structure of MPLSMultiprotocol Label Switching protocol is described in [1. A label exchange router can be used for Frame Relay switching. Frame relay switches the routing algorithms at the network layer, such as OSPF, IS-IS, and forward data based on these routing algorithms. You do not need to select a specific frame relay path.
Frame Relay switching applies to the top-level current tags in the first DLCIData Link Connection Identifier Field of the frame relay data Link layer during tag switching. Frame Relay switching does not process additional information carried together with top-level labels. If there are packages with Multiple labels, additional information is carried together with other labels in the common MPLS encapsulation defined in file [3.
Allows you to configure a permanent virtual circuit (PVCs) for Frame Relay. DLCIs should be used when both MPLS labels and frame relay switches are suitable for frame relay switches. According to this specification, MPLS services should be encapsulated and forward transmission based on network layer routing information.
2. Relay switching features
The structure of the label exchange allows considerable flexibility in the execution of LSRLabel Switching Router, and may be pre-existent) Hardware has the ability to constrain the FR-LSR, multi-protocol interconnection utilizes the same frame format as frame relay standards. Due to such constraints, FR-LSRs requires some special procedures.
Some of the main features of Frame Relay switching that affect its performance, such as LSRs) are:
* The label exchange function is implemented on the DLCI field in the header of the frame relay data link. This specifies the length and position of the label in the package. The length of the DLCI field may be 10 by default) or 23 bits and it can be expanded to 2 or 4 bytes in the header.
* Generally, the "TTLTime To Live)-reduction" function cannot be implemented when the IP address header is executed on the vro.
* Each node passing parameters on the established circuit performs blocking control. You can set the mark in the frame header as the blocking result or exceed the agreed circuit parameters.
* Although multiple input DLCIs can be configured for one output DLCI in the Standard switch, the multi-point to one-point circuit generally does not fully support multi-point to multi-point VCs.
3. Label encapsulation and stack
3.1 tag Encapsulation
In the default status, all packages with tags should be sent using standard label encapsulation. The Frame Relay null encapsulation mechanism is used:
"N" is the address length of Q.922, which can be 2 or 4 bytes.
The Q.922 [ITU] representation of DLCI is in the standard order. the rightmost bit of a byte in the memory is stored in the first place:
The use of Frame Relay null encapsulation means that the label implies the type of the encoding network protocol.
The file [3] describes the tag stack structure and error messages returned to the frame source.
For a deep "n" tag stack, the general encapsulation includes the "n" tag. In the top-layer stack entry, it carries meaningful values for the EXP, S, and TTL fields, instead of labels, tags are more suitable for bearing in the DLCI field of the frame relay data link header encoded in Q.922 format.
3.2 tag Stack 
Each tag stack entry is represented in 4 bytes.
Label: 20 bits label value; EXP: 3 bits for the test; S: 1 bits at the bottom of the stack; TTL: 8 bits.