1. Frame Relay is a way of sending information over a WAN by dividing data into packets. Each packet travels through a series of switches in a Frame Relay network to reach its destination. It operates at the physical and data link layers of the OSI reference model, but it relies on upper-layer protocols such as TCP for error correction. Frame Relay is an industry-standard, switched data link-layer protocol that handles multiple virtual circuits using HDLC (High-Level Data Link Control) encapsulation between connected devices. Frame Relay uses virtual circuits to make connections through a connection-oriented service.
2. Following are some terms that are used in this chapter to discuss Frame Relay:
· Access rate -- The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud. It is the rate at which data travels into or out of the network.
· Data-link connection identifier (DLCI) – A DLCI is a number that identifies the end point in a Frame Relay network. This number has significance only to the local network. The Frame Relay switch maps the DLCIs between a pair of routers to create a permanent virtual circuit (path for data).
· Local management interface (LMI) -- A signaling standard between the customer premises equipment (CPE) device and the Frame Relay switch that is responsible for managing the connection and maintaining status between the devices. Three types of LMIs are supported: cisco, ansi, and q933a.
· Committed information rate (CIR) -- The CIR is the guaranteed rate, in bits per second, that the service provider commits to providing.
· Committed burst -- The maximum number of bits that the switch agrees to transfer during a time interval. (It is noted Bc)
· Excess burst -- The maximum number of uncommitted bits that the Frame Relay switch attempts to transfer beyond the CIR. Excess burst is dependent on the service offerings available by the vendor, but is typically limited to the port speed of the local access loop.
· Forward explicit congestion notification (FECN) -- A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. When a Frame Relay switch recognizes congestion in the network, it sends a FECN packet to the destination device, indicating that congestion has occurred.
· Backward explicit congestion notification (BECN) -- A bit set in a frame that notifies a DTE that congestion avoidance procedures should be initiated by the receiving device. When a Frame Relay switch recognizes congestion in the network, it sends a BECN packet to the source router, instructing the router to reduce the rate at which it is sending packets. If the router receives any BECNs during the current time interval, it decreases the transmit rate by 25%.
· Discard eligibility (DE) indicator -- A set bit that indicates the frame may be discarded in preference to other frames if congestion occurs. When the router detects network congestion, the Frame Relay switch will drop packets with the DE bit set first. The DE bit is set on the oversubscribed traffic (that is, the traffic that was received after the CIR was met).
3. As an interface between user and network equipment, Frame Relay provides a means for multiplexing many logical data conversations, referred to as virtual circuits, through a shared physical medium by assigning DLCIs to each DTE/DCE pair of devices.
4. Frame Relay standards address permanent virtual circuits (PVCs) that are administratively configured and managed in a Frame Relay network. Frame Relay PVCs are identified by DLCIs. Frame Relay DLCIs have local significance. That is, the values themselves are not unique in the Frame Relay WAN. Two DTE devices connected by a virtual circuit might use a different DLCI value to refer to the same connection.
5. Frame Relay provides a means for multiplexing many logical data conversations. The service provider's switching equipment constructs a table mapping DLCI values to outbound ports. When a frame is received, the switching device analyzes the connection identifier and delivers the frame to the associated outbound port. The complete path to the destination is established before the first frame is sent.
6. Frame Relay extensions are referred to as LMIs (Local Management Interfaces).
7. The main functions of local management interface process are:
· To determine the operational status of the various PVCs that the router knows about
· To transmit keepalive packets to ensure that the PVC stays up and does not shut down due to inactivity
· To tell the router what PVCs are available
8. With Cisco IOS release 11.2 or later, the LMI type set is autosensing.
9. What are the possible connection states of a frame relay map table?
10. Three LMI types can be invoked by the router: ansi, cisco, and q933a. cisco is the default type.
11. The Inverse ARP mechanism allows the router to automatically build the Frame Relay map. The router learns the DLCIs that are in use from the switch during the initial LMI exchange.
12. The router next-hop address determined from the routing table must be resolved to a Frame Relay DLCI. The resolution is done through a data structure called a Frame Relay map.
13. To enable the sending of complete routing updates in a Frame Relay network, you can configure the router with logically assigned interfaces called subinterfaces. These are logical subdivisions of a physical interface. In this configuration, each PVC can be configured as a point-to-point connection, which allows it to act as a dedicated line.
14.
15. You can configure subinterfaces to support the following connection types:
· Point-to-point -- A single subinterface is used to establish one PVC connection to another physical interface or subinterface on a remote router. In this case, the interfaces would be in the same subnet, and each interface would have a single DLCI. Each point-to-point connection is its own subnet. In this environment, broadcasts are not a problem because the routers are point-to-point and act like a leased line.
· Multipoint -- A single subinterface is used to establish multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. In this case, all the participating interfaces would be in the same subnet, and each interface would have its own local DLCI. In this environment, because the subinterface is acting like a regular Frame Relay network, routing updates are subject to split horizon.
16. When configuring an interface for Frame Relay, you set the bandwidth for each interface so that protocols can calculate a metric number. This allows the protocol to assess the best path.
17. Complete the following table.
|
Commands |
Description |
|
show interfaces serial |
Displays information about multicast DLCI, the DLCIs used on the frame relay configured serial interface and the LMI DLCI used for the LMI. |
|
show frame-relay pvc |
Displays the status of each configured connection as well as traffic statistics. This command is also useful for viewing the number of BECN and FECN packets received by the router. |
|
show frame-relay map |
Displays the network-layer address and the associated DLCI for each remote destination that the local router is connected to. |
|
show frame-relay lmi |
Displays the LMI traffic statistics. For example, it shows the number of status messages exchanged between the local router and the frame relay switch. |