Configuring X.25 interface supplementary parameters

It is necessary to configure certain supplementary X.25 parameters in some special network environments. The following sections are related to the following supplementary parameters.

• X.25 layer 3 delay timer

  X.25 protocol defines a series of timers to facilitate its procedure. After X.25 sends a control message, if it does not receive the response before the timer expires, X.25 protocol will take measures to handle this abnormal event. Table 5 lists the names and procedures of these timers.

  Table 5: X.25 Layer 3 timer

T28 is a "Registration request sending" timer which is only defined on DTE for dynamically requesting the network for optional services or stopping these services. Its default value is 300 seconds, which cannot be changed.

• Attributes related to X.25 address

  To establish an SVC with a call, X.25 address is needed, which adopts the address format specified in ITU-T Recommendation X.121. An X.121 address is a string of 1 to 15 digits. Some attributes related to X.121 address are as follows:

• Alias of interface

  When an X.25 call is forwarded across multiple networks, different networks will likely make some modifications on the called address as needed, such as adding or deleting the prefix. In such cases, the destination address of a call that reaches X.25 interface might be inconsistent with X.121 address of the destination interface (because the destination address of this call is modified within the network); still the interface should accept this call. For this purpose, one or more alias names must be specified for this interface.

  To meet the requirements of different networks, X.25 defines nine match types and their relevant alias string formats, as shown in Table 6.

  Table 6: Alias match modes and meanings

• Attributes related to the address code block in calling or called packets

  As defined in the X.25 protocol, a call packet must carry the information set of both the calling DTE address (source address) and the called DTE address (destination address). This address information set is called "address code block." In a call accept packet, some networks require that both (the calling DTE address and the called DTE address) be carried, some networks require that only one of the two be carried, and some others require that neither should be carried. To adapt to the differences between various networks, make selections as required.

• Default upper layer protocol that X.25 bears

  An X.25 call request packet includes a CUD field indicating the upper layer protocol type carried over the X.25 protocol. When receiving an X.25 call, the device will check the CUD field in the packet. If receiving a call carrying an unidentifiable CUD field, the router will deny it. However, an upper layer protocol can be specified as the default protocol on the X.25. When X.25 receives a call with an unrecognizable CUD, it will treat it with the customized default upper layer protocol.

• The user facility adding/removing function

  The user facility adding and removing functions are applicable to only main interfaces. They work as follows on a main interface:

  • The user facility adding function is applicable to only the incoming packets. With the function, the main interface checks incoming call setup negotiation packets for the window size and maximum packet size fields. If the two fields are carried, the packets are not processed; if not, the two fields are added to the packets.

  • The user facility removing function is applicable to only the outgoing packets. With the function, the main interface checks outgoing call-setup negotiation packets for the window size and maximum packet size fields. If the two fields are not carried, the packets are not processed; if the two fields are carried, they are removed before the packets are sent out.

To configure X.25 interface supplementary parameters: