Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 288 Frame-Relay Section 26 - Frame-Relay - Frame-Relay is a packet-switched technology, which shares bandwidth between users on the switched network. Frame-relay service providers assume that all clients will not need the full capacity of their bandwidth at all times. Thus, in general, frame-relay is less expensive than dedicated WAN lines, but customers are not guaranteed bandwidth. All locations plug into the frame relay “cloud,” which is a conglomeration of dozens or hundreds of Frame-Relay switches and routers. The cloud is the Frame provider’s network, and the customer has no control (or even knowledge) of what occurs inside that infrastructure. For communication to occur between locations, virtual circuits (VC) must be created. A VC is a one-way path through the Frame-Relay cloud. In the above example, in order to establish full communication between Detroit and Houston, we would need to create two virtual circuits: • A virtual circuit between Detroit and Houston • A separate virtual circuit between Houston and Detroit Frame-relay circuits can either be permanent (PVC), or switched (SVC). A permanent virtual circuit is always kept active, and is the most common virtual circuit. A switched virtual circuit is created only when traffic needs to be sent, and is torn down when communication is complete. Virtual circuits are identified with Data Link Connection Identifiers (DLCIs). Frame-Relay switches make decisions based on DLCIs, whereas Ethernet switches make decisions based on MAC addresses. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 289 Frame-Relay Global vs Local DLCI The difference between a globally or locally significant DLCI is all based on perspective. Remember that a DLCI identifies a one-way virtual circuit. For example, the connection between Detroit and Chicago would be considered one virtual circuit, and Chicago to Detroit would be a separate virtual circuit. To get this to work, we need to map a DLCI to an IP address. For example, on router Detroit, we’re going to create a virtual circuit to router Chicago. We’ll assign it a DLCI of “102,” and point it to Chicago’s IP address. We call this locally significant, because it only affects the interface on the Detroit router. We could, on the Chicago router, set a DLCI of “102” and point it to the IP address of the Detroit router. Because the DLCI is set on a different router (and interface), there will be no conflict. When we set a globally significant DLCI, it is really only an administrative feature. It means that an administrator has consciously decided that all virtual circuits going to Chicago will be set to DLCI 102 (or whatever DLCI number you choose), whether it is from Detroit or Houston. In essence, you are symbolically assigning the DLCI of 102 to the Chicago location. Keep in mind that you are still technically assigning the DLCI to the virtual circuits connecting to Chicago. Virtual circuits pointing to other locations will be configured with different DLCIs (Detroit could be 101; Houston could be 103, etc.). The advantage to this is that it is now easy to determine the destination of a packet, based on its DLCI. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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