Ref: 13930002 Title: How to Use Rotaries on the GS/X.25-XNS date: 09-27-89 Copyright 3Com Corporation, 1991. All rights reserved. This article explains the versatile function of the rotary. Several examples demonstrate its capabilities when used in combination with the other features of the X.25 gateway. Normally each port can be addressed individually for X.25 calls. For outgoing calls, the port number is specified explicitly in the connect syntax: C %080002001234!25#123456789 where: 080002001234 is the Ethernet address of the gateway 25 is the port number 123456789 is the X.25 address of the remote host When receiving an X.25 incoming call, the gateway compares the called address (inside the incoming call indication packet) to the X.25 local address as configured in the GS/X.25-XNS. The subaddress digits, if any, indicate which gateway port to use. A specific set of ports can be assigned to a rotary. When the rotary is specified as subaddress digits, the gateway will use the next available port number in the set assigned to the rotary. A port-to-line mapping table allows the network manager to distribute the ports among the various X.25 lines. Notes: a) Ports must be mapped to a line to allow connections to be made on this line. b) For outgoing calls, the gateway uses the port-to-line mapping table to determine on which X.25 line to place the call. On a rotary, ports are allocated on a round robin basis. c) For incoming calls, the gateway uses the lowest numbered available port assigned to the X.25 line in the port-to-line mapping table. Examples: I. Configuration with one X.25 line All ports must be mapped to this line. Rotaries are used to pick up ports with different configurations and to differentiate between departments for security or billing purposes. II. Configuration with two X.25 lines If the lines will be used for different purposes, ports must be shared among the lines. This case is similar to the configuration with one X.25 line. If the lines are going to the same destination, follow one of the two procedures below as appropriate. 1. If line costs are equivalent--pseudo "load balancing" can be achieved by the following: a. Map all even ports to line 0. b. Map all odd ports to line 1. c. Create a rotary which contains all ports. To place an outgoing call, use the rotary previously defined; the first call will use line 0, the second will use line 1 and so on. NOTE: Load balancing works for outgoing calls only since it is the active end's responsibility to place the call to the appropriate line. This is pseudo load balancing since routing is done at call setup. Even if the call does not last, the port will be available when the rotary has made a full round. 2. If line cost is different--for example, a private line is used to connect two remote locations and the PDN is used as a backup: a. Choose the maximum number of connections that can use the private line. Additional connections will be routed through the PDN. b. Assign this number of ports to the private line, by defining a rotary which contains all these ports. c. Name this rotary "private_line". d. Map the remaining ports to the line connected to the PDN; define a second rotary and name it "pdn_line". e. Define a global rotary and name it "x25." This name will include both the private-line and pdn_line names as: "NAme x25=private_line,pdn_line." f. Use the "x25" name to place the calls. The calls will be routed through the private line first and when saturation is reached, will be routed through the PDN. If the private line is down, calls will be automatically routed through the PDN. III. Configuration with several gateways This case is an extension of the configuration with two X.25 lines, above, since the global rotary concept can span across the LAN. Careful use of the features described above--direct port addressing, rotary, global rotary and port-to-line mapping--allows the network designer to build a cost effective, secure, and redundant network when using a X.25 public network.