Cabletron Systems CSX200 CyberSWITCH Installation Manual

Also see for CSX200 CyberSWITCH: Setup guide User guide Configuration guide User guide

Contents

Chapter 2: About the CSX2002-6CSX200 Installation GuideThis is done to replace the hidden local IP addresses from the sub network with one or more publicInterNic assigned IP addresses that can be sent over the outside network on the WAN interfaces.Once the CSX200 is assigned at least one public IP address, over 250 IP clients can share thisaddress simultaneously using NAT. This public IP address is assigned statically by the InternetService Provider (ISP).Frame Relay ProtocolFrame Relay can be defined as a “packet mode” service, organizing data into individuallyaddressed units known as “frames”. Frame Relay eliminates Layer 3 processing. Only a fewLayer 2 functions are used, such as checking for a valid, error free frame, but not requestingretransmission if an error is found. Frame Relay uses a variable length framing structure, which,depending on user data, can range from a few to more than a thousand characters.A Frame Relay Network will often be depicted as a cloud, because the Frame Relay Network isnot a single physical connection between one endpoint and another. Frame Relay Protocol is basedon the concept of “virtual circuits” (VCs). VCs are two-way, software defined data paths betweentwo ports that take the place of private lines in the network. There are two types of Frame Relayconnections; Switched Virtual Circuits (SVCs), and Permanent Virtual Circuits (PVCs).Permanent Virtual Circuits, or PVCs, are set up via a network management system, and initiallydefined as a connection between two sites, or endpoints. PVCs may be added as the demand arisesfor more bandwidth, alternate routing, or more sites, or endpoints. PVCs are fixed paths, notavailable on demand, or on a call-by-call basis. Although the actual path through the network maychange from time to time, such as when automatic rerouting takes place, the beginning and end ofthe circuit will not change.Switched Virtual Circuits, or SVCs, are available on a call-by-call basis using the SVC signalingprotocol (Q.933). The network must quickly establish the connection, and allocate bandwidthbased on the user’s request.In a Frame Relay frame, user data packets are not changed in any way. A two-byte header isappended to the frame. Contained in this header is a 10-bit number called the Data LinkConnection Identifier (DLCI). The DLCI is the “virtual circuit” number which corresponds to aparticular destination. The DLCI allows data coming into a Frame Relay switch to be sent acrossthe network using a three-step process: check the integrity of the frame and discard it if it is inerror, look up the DLCI in a table and if not intended for this link, discard the frame. If the framepasses the previous tests, relay the frame toward its destination out the port specified in the table. Ifthe frame passes the previous tests, relay the frame toward its destination out the port specified inthe table.