Understand and take: How frame-relay works

Understand and take: How frame-relay works

Corresponding Teaching Video in: http://edu.51cto.com/lecturer/user_id-7648423.html

9.9 release, which can be viewed after the Administrator's 24-hour review. The course name is Cisco CCNA-certified Frame Relay analysis 200-120) 9 lessons.

Frame Relay (Frame Relay) is an interface standard between a network and a data terminal device (DTE). It is a computer-oriented WAN connection for group switching. Because of the high cost performance and stability, most telecom operators provide the Frame Relay Service, which is also a method based on OSI data link layer technology to establish a high-performance WAN connection.

Learning tips:It is difficult to understand the working principle and persistence of Frame Relay simply by defining the above text. Therefore, I suggest the following learning methods:

N understand the packet switching feature of Frame Relay

N understand the DLCI Number of Frame Relay

N compared with leased lines, it is highly cost-effective to understand frame relay.

N components for understanding Frame Relay

N understand the signaling Management of Frame Relay and analyze the data frame of LMS

N understand the type of Frame Relay LMS

N understand and collect evidence of frame Frames

N understand the network shape of Frame Relay

N configure the Frame Relay Network

Understand the packet switching feature of Frame Relay:

Multiple virtual links in logical link group switching are carried by one physical link, as shown in Figure 8.41. This is a typical line multiplexing technology based on group switching. Communication channels from R1 to R2 and from R1 to R3 are placed on the same physical link. After the communication channels from R1 to R2 and R1 to R3 are put on the same physical link, how does R1 distinguish which one is the logical channel to R2 and which one is the logical channel to R3? How can we ensure that R1 will not confuse the two logical channels. A dlci number is the key to distinguishing a logical link. a dlci number identifies a logical channel, so that R1 can use a logical link with a DLCI Number of 102 to reach R2; use the logical link with the DLCI number 103 to reach R3. In this way, R1 will not confuse the two logics.

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Note: The Logical Link carried by the physical link is called VC (virtual circuit ). Virtual circuits are divided into permanent virtual circuits (PVC) and switched virtual circuits (SVC ). Permanent virtual circuit (PVC): No matter when the user uses it or even does not use it, the communication channel will be permanently reserved for the user and will be charged for monthly rent. Switch virtual circuit (SVC): the channel is created temporarily when the user needs it. When the user does not need it, the logical channel can be closed. Billing is based on the pay-as-you-go billing method.

Understand the DLCI Number of Frame Relay:

Data Link Connection Identifier) is an Identifier for the virtual circuit in the frame relay network. The frame relay switch identifies different virtual circuits based on the DLCI number and determines the forwarding path of the virtual circuit. Generally, the DLCI number is only meaningful locally, as shown in Figure 8.42, the frame relay switch of a telecom operator maintains a table of the relationship between the forwarding interface and DLCI. For example, router R1 sends data to interface 1 of the frame relay switch, in general, the DLCI number will have a ing relationship with the IP address. If the IP address is 192.168.1.2, the corresponding DLCI number is 201, and the interface of the frame relay switch corresponding to the DLCI Number 201 is interface 2, therefore, the data frame is output to the target router R2. The DLCI number only takes effect locally. It indicates that the DLCI number takes effect only between R1 and interface 1 of the same-Side Frame Relay switch in the user's frontend device ratio. Generally, in the frame relay environment, the user obtains the physical link and completes the communication to the target. The user obtains a physical link, which only indicates that the user's front-end device can be connected to the carrier's frame relay switch. To complete the communication with the target, the user must obtain the DLCI number allocated by the carrier, this indicates that a virtual circuit is obtained.

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Compared with leased lines, it is more cost-effective to understand Frame Relay:

If an enterprise has four office sites in different geographical locations, the four office sites need to communicate with each other through the WAN link to meet the needs of the business platform. If you want to use a point-to-point leased line connection, as shown in Figure 8.43 in the following figure, to complete communications between office sites in four different geographic locations, you must pay the price: Each router must have three serial interfaces, connect the other three routers. There are 6 Leased lines and 6 physical links in total. The conclusion is that the cost of leased line connection is too high.

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If frame relay is used to design this environment, there are still four office sites in different geographical locations. The frame relay design is used for interconnection between two sites, as shown in Figure 8.44 below. R1 has only one link to the frame relay switch of the telecom operator, the communication channels to R2, R3, and R4 are carried by the physical links of R1, and the selected logical links are identified by the DLCI, so that no error is sent. The same is true for R2, R3, and R4, because the communication link between R1 and these routers is a virtual link of the group switch, which is carried by a physical link. The cost of using frame relay connections: Each router only needs one physical serial port, which carries three logical links to R2, R3, and R4. There are 6 PVC and 4 physical links in total. The conclusion is that frame relay is completed using the line multiplexing technology of the group switch. Cost-effective and cost-effective.

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Understand the Frame Relay components:

Components of the frame relay device: the DCE device of the frame relay switch) and the DTE device of the user data terminal ). DCEDataCircuit-Terminal Equipment, it is actually a group switch on the WAN link. This device is usually controlled by a telecom operator. DTEDataTermainal Equipment) A data terminal device, usually an internal device of an enterprise, such as a router, which is used for access to a frame relay switch. In short, a DTE device is a user's front-end device that is used and operated by the enterprise and has full control over the device. See Figure 8.45.

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Understand the signaling Management of Frame Relay and analyze the data frame of LMS:

A certain signaling protocol must be used between the DTE of Frame Relay and the DCE of the frame relay switch to exchange important management information, such as exchanging keep live and transmitting regulatory information, and adding or deleting PVC information, this information is transmitted only between the user end and the frame relay switch, and is not carried into the virtual circuit. What transmits these signaling messages is the local management interface (LMS) of Frame Relay ). The MnS used to maintain Frame Relay signaling can be divided into State query enquiry and State response Stauts. They are completed at an interval every 10 seconds. The periodic query of data frames is shown in Figure 8.46. The structure of the LMS packet is shown in Figure 8.47.

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NFlag:Including the start and end signs of Frame Relay, used to mark the start and end of a data frame.

NLc-dlci:It indicates the DLCI number used by the LMS signaling of Frame Relay. Note that the lc-dlci number is not the DLCI number used by the virtual circuit VC. It is related to the type of lc-signaling, if the MNS signaling type adopts the cisco LCM type, the DLCI number is 1023. If the ANSI or ITUq33a type is used, the DLCI number is 0. The types of LMS are described in detail later.

NUnnumbered UII ):The unnumbered flag is used to set the last polling bit to 0.

NProtocol identifier NLPID ):The Protocol Identifier Field indicates that the data frame is a frame with Frame Relay (LMIs.

NCall Reference call Ref ):Call reference is not currently used. Its value is generally 0.

NMessage Type messagetype ):There are two possibilities for this field: one is to indicate the status query enquiry) and the other is the status response Stauts ).

NMessage Element informationElement ):Contains a specified number of independent information elements, such as the IE identifier and IE length.

NFCS:The data frame verification sequence is used for data verification to ensure the integrity of data transmission.

As shown in figure 8.48 below, we use protocol analyzer to collect evidence of frame frames through ll-signaling data frames during real-time communication. Through the analysis of the data frame, we can see that it is the frame (LMS). It indicates the start and end mark of the 8-byte data frame respectively. The lc-dlci Number of the LMS is 1023. Both the DLCI number 0 and the DLCI number 1023 are reserved for the LMIS signaling. When the LMIS is Cisco, 1023 is used, and when it is Ansi, The DLCI number used is 0, in this frame, it indicates the use of cisco's LCM type; then it can be seen that the unnumbered indicator field is "0x00", which is usually the default value of this field; the Protocol interpreter field indicates that the frame is an LMS protocol, the call reference field is usually "0x00", and the message type field indicates the LMS Status inquiry message; in the message element, it indicates that DTE and DCE use the keep Alive form to monitor the LMS status.

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Understand the types of Frame Relay LMS signaling:

The ll-signaling protocol must be consistent between the DCE end and the DTE end. To ensure the normal operation of the frame relay link, in other words, the same type of LMS must be used for the enterprise front-end devices. Generally, LMS provides the following three criteria. You can run the frame-relaylmi-type command in the router interface mode to specify the LMS type manually.

NAnsi: defined by American National Standards Commission ANSI.

NQ.933: ITU-TQ.933 limit a standard.

NCisco: MnS is jointly defined by cisco, digital equipment, Nortel, and StrataCom.

Understanding and collecting data frame relay evidence:

The structure of the frame relay message is shown in Figure 8.49, and the data frame for forensics is shown in Figure 8.50. Please understand the definitions of fields in the context of the packet structure against the data frame of forensic evidence:

NStart Flag:It indicates the start of the frame, also known as the first address of Frame Relay, which is represented by an eight-bit group.

NDLCI:The DLCI number used to identify the arrival of the frame relay network to the target.

NC/R:(Command/Response) Command Response. This field is usually not used.

NEA:The extended address is used to indicate whether the field containing the EA field is the last frame mark. If the value of this field is 1, it indicates that this is the last end frame mark, because frame relay has two frame marks, one start Flag and one end Flag. It appears in two places: Start Flag and end Flag. Therefore, when the EA Flag appears in the Start Flag part of Figure 9, its value is 0; in the end part, the value is 1.

NFECN:Forward Explicit Congestion Notification. This field is used when the frame relay network is congested, the DCE device of the frame relay switch sets this field to 1 and sends it to the target access device DTE of Frame Relay ), indicates network congestion. devices that receive FECN will implement appropriate traffic control measures.

NBECN:Backward Explicit Congestion Notification. This field is used when the frame relay network is congested, the DCE device of the frame relay switch sets this field to 1 and sends it to the Source Access Device DTE of Frame Relay ), indicates network congestion. devices that receive BECN will reduce the sending rate by about 25%.

NDE:The relay flag can be discarded. If this field is set to 1, the frame relay data packet will be discarded in case of network congestion. It should be noted that the mark is generally set by the access device DTE of frame relay, and the data configured with the DE mark is not discarded immediately, but will be discarded preferentially when the network is congested.

NData:Indicates the upper layer data encapsulated in Frame Relay.

NEnd Flag:Indicates the end of the frame, also known as the second address of Frame Relay, which is represented by an eight-bit group.

NFCS:Used to detect transmission errors.

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Understand the network shape of Frame Relay:

Understanding the network shape of frame relay is a necessary task to clearly understand the Frame Relay architecture. The network shape of frame relay is related to the design and implementation of the routing protocol under different network shapes, if you cannot clearly understand the characteristics of different network shapes of frame relay, the normal operation of the routing protocol in the frame relay network will be directly affected.

Semi-mesh: Semi-mesh frame frames are generally used to connect multiple remote branches to the headquarters. The semi-mesh anti-frame service branches do not need to communicate with each other. Almost all communication traffic is generated between the branches and the headquarters. If there is occasional communication traffic between branches, the occasional communication traffic will be forwarded through the headquarters center. See Figure 8.51.

Features:There is a VC between the Headquarters and the branch, and there is no VC between the branch. By default, R2 and R3 cannot communicate with each other. If there is occasional communication, traffic is forwarded by the central point of the Headquarters. The cost is low and the traffic pressure on the central point is high.

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Full mesh:The full-mesh Frame Relay indicator shows the Access Point of Frame Relay. Each of them has an independent VC. Generally, the full-mesh frame relay is applied to environments with frequent communication traffic between Headquarters and branches, as well as between branches and branches, as shown in Figure 8.51.

Features: Each communication point has an independent VC. The cost is higher than that of the semi-mesh, and the traffic pressure is less than that of the semi-mesh.

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