Configuring X.25 load sharing

With the X.25 hunt group feature, network providers can load balance traffic across DTEs or links on a DTE to prevent a link from being overwhelmed by excessive accesses.

In an X.25 network, load sharing is provided by DCEs. Skip this section, if your device is operating as a DTE.

The following shows how X.25 load sharing is implemented:

• Configure a set of DTE/DCE interfaces (synchronous serial interfaces or XOT channels) as a hunt group on the remote DCE, and assign an X.121 address for this hunt group.

• When a device accesses a DTE in the hunt group, the device calls the hunt group address.

• After receiving the call request, the remote DCE selects a line from the hunt group in the round-robin or VC-number approach, and send the incoming call.

• The DCE distributes calls across the lines in the hunt group to implement load sharing.

X.25 load sharing is available only for SVCs, because PVCs do not have the call setup or clearing phase. Line selection in an X.25 hunt group only occurs at the virtual circuit establishment phase. After a virtual circuit is established, the DCE follows the normal procedure to transfer data on the virtual circuit.

All DTEs in an X.25 hunt group use the same X.121 address. They can call DTEs outside the hunt group in the normal call procedure. When a device accesses a hunt group, it does not know which device it is accessing. Line selection is controlled by the DCE.

The address of the DTEs in a hunt group can be the same as the hunt group address. X.25 hunt groups support source address and destination address substitution. With destination address substitution, you can hide the DTE address in a hunt group from DTEs outside the hunt group for security. In this approach, the DTEs outside the hunt group know only the hunt group address. With source address substitution, you can hide the DTE addresses outside the hunt group from the DTEs inside the hunt group for user privacy protection. In this approach, the DTEs inside the hunt group know only the substitution address.

In Figure 128, server A and server B, which are providing identical services, are assigned to hunt group HG 1. Server A and server B use the address 9999, and the hunt group uses the address 8888. Enable destination address substitution on Router A to replace the address 8888 with the address 9999. When a user calls the destination address 8888, Router A redirects the call to the address 9999 and then transmits it to server A or server B. Router A processes all calls to the address 8888 in this way to load balance the calls between server A and server B, preventing a single server from being overloaded when the other is idle.

You can configure an X.25 hunt group to select lines in either round-robin mode or vc-number mode:

• The round-robin mode uses a cyclic selection method to select the next interface or XOT channel inside the hunt group for each call. For example, in Figure 128, if hunt group HG 1 uses the round-robin mode, the call will be sent in turn to server A or server B.

• The vc-number mode selects the interface with the maximum idle logical channels inside the hunt group for each call. For example, in Figure 128, if the hunt group HG 1 uses the vc-number mode, the remaining logical channels of the lines between server A and DCE are 500, and those of the lines between server B and DCE are 300. The first 200 calls will be sent to server A, and the subsequent calls will be sent in turn to server A or server B.

An X.25 hunt group can comprise both synchronous serial interfaces and XOT channels. It can select a line from them indiscriminately. However, you cannot assign XOT channels to a hunt group in vc-number mode, because the device cannot calculate the number of logical channels.

If your device is operating as a DCE, you can configure it to perform load sharing.

To perform load sharing, you must set the call destination on the source DTE as a hunt group address.

A hunt group can contain up to 10 synchronous serial interfaces, Annex G DLCIs, or XOT channels.

You cannot add XOT channels to a hunt group configured with the vc-number mode.

To configure X.25 load sharing on a DCE: