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The chapter describes configuring X.25 on ISDN using the Always On/Dynamic ISDN (AO/DI) feature. It includes the following main sections:
AO/DI supports PPP encapsulation on switched X.25 virtual circuits (VCs) only.
The X.25 encapsulation (per RFC 1356), PPP, Bandwidth Allocation Control Protocol (BACP), and Bandwidth Allocation Protocol (BAP) modules must be present in both the AO/DI client and server.
AO/DI relies on features from X.25, PPP, and BACP modules and must be configured on both the AO/DI client and server. BAP, if negotiated, is a subset of BACP, which is responsible for bandwidth allocation for the Multilink PPP (MLP) peers. It is recommended you configure MLP with the BAP option due to the differences between the ISDN (E.164) and X.25 (X.121) numbering formats.
To implement AO/DI, you must configure the AO/DI client and server for PPP, incorporating BAP and X.25 module commands. This task involves configuring the BRI or PRI interfaces with the appropriate X.25 commands and the dialer interfaces with the necessary PPP or BAP commands.
For a complete description of the commands mentioned in this chapter, see the Cisco IOS Dial Services Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
AO/DI functionality is based on the technology modules described in the following sections:
AO/DI is an on-demand service that is designed to optimize the use of an existing ISDN signalling channel (D channel) to transport X.25 traffic. The X.25 D-channel call is placed from the subscriber to the packet data service provider. The use of PPP allows protocols to be encapsulated within the X.25 logical circuit carried by the D channel. The bearer channels (B channels) use the multilink protocol without the standard Q.922 and X.25 encapsulations, and invoke additional bandwidth as needed. Optionally, BACP and BAP can be used to negotiate bandwidth allocation as required.
AO/DI takes full advantage of existing packet handlers at the central office by using an existing D channel to transport the X.25 traffic. The link associated with the X.25 D channel packet connection is used as the primary link of the multilink bundle. The D channel is a connectionless, packet-oriented link between the customer premise equipment (CPE) and the central office. Because the D channel is always available, it is possible to in turn offer "always available" services. On-demand functionality is achieved by using the B channels to temporarily boost data throughput and by disconnecting them after use. Figure 24 shows the AO/DI environment and how ISDN and X.25 resources are implemented.
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Note On the client side the X.25 switched virtual circuit (SVC) can only be terminated on an ISDN D channel; however, on the server side, the SVC can be terminated on an ISDN BRI using a D channel, a PRI using specific time slots, or a high-speed serial link. |
AO/DI provides the following benefits:
PPP over X.25 is accomplished through the following process:
1. The X.25 map statement on the client side creates a virtual access interface. A virtual access interface is dynamically created and configured by cloning the configuration from a dialer interface (dialer interface 1, for example).
2. The dialer interface goes into "spoofing" mode and stays in this mode until interesting traffic is seen.
3. When interesting traffic is seen, the dialer interface activates the virtual access interface, which creates the X.25 SVC. Once the SVC is established, PPP negotiation begins in order to bring up the line protocol. The client will initiate a call to the remote end server, per the x25 map ppp command.
4. When the AO/DI server receives a call intended for its X.25 map statement, the call is accepted and an event is queued to the X.25 encapsulation manager. The encapsulation manager is an X.25 process that authenticates incoming X.25 calls and AO/DI events, and creates a virtual access interface that clones the configuration from the dialer or BRI interface. Figure 26 shows the virtual interface creation process.
The multilink protocol offers load balancing, packet fragmentation, and the bandwidth allocation functionality that is key to AO/DI structure. The MLP bundle process is achieved through the following process:
1. The ppp multilink bap command initiates MLP and, subsequently, BAP. The virtual access interface that is created above the X.25 VC (over the D channel) becomes the first member link of the MLP bundle.
2. The ppp multilink idle-link command works in conjunction with the dialer load-threshold command in order to add B channels as needed to boost traffic throughput. When a B channel is added, the first member link enters "receive only" mode, allowing the link additions. When the higher throughput is no longer needed, the additional B channels are disconnected and the primary link is the only link in the bundle, the bundle disengages "receive only" mode. The X.25 SVC stays active. Figure 27 shows the MLP bundle sequence.
In previous releases of the Cisco IOS software, when MLP was used in a dialer profile, a virtual access interface was always created as the bundle. It was bound to both the B channel and the dialer profile interfaces after creation and cloning. The dialer profile interface could act as the bundle without help from a virtual access interface. But with recent software enhancements, it is no longer the virtual access interface that is added into the connected group of the dialer profile, but the dialer profile itself. The dialer profile becomes a connected member of its own connected group.
Bandwidth resources are provided by BACP, described in RFC 2125. Once the MLP peers have successfully negotiated BACP, BAP negotiates bandwidth resources in order to support traffic throughput. BAP is a subset of BACP, and it defines the methods and governing rules for adding and removing links from the bundle for MLP. BACP/BAP negotiations are achieved through the following process:
1. Once the MLP session is initiated and BACP is negotiated over the MLP bundle, the AO/DI client issues a BAP call request for additional bandwidth.
2. The AO/DI server responds with the BAP call response, which contains the phone number of the B channel to add. B channels are added, as needed, to support the demand for increased traffic throughput.
3. B channels are disconnected as the traffic load decreases.
To configure X.25 on ISDN using AO/DI, perform the following tasks:
For examples of how to configure X.25 on ISDN using AO/DI in your network, see the section "AO/DI Configuration Examples" at the end of this chapter.
To configure PPP and BAP under the dialer interface on the AO/DI client, use the following commands in interface configuration mode, as required:
| Command | Purpose |
|---|---|
ppp multilink bap | Enables PPP BACP bandwidth allocation negotiation. |
encapsulation ppp | Enables PPP on the interface. |
dialer in-band | Enables dial-on-demand routing (DDR) on the interface. |
dialer load-threshold load | Sets the dialer load threshold. |
dialer-group group-number | Controls access to this interface by adding it to a dialer access group. |
ppp bap callback accept | (Optional) Enables the interface to initiate additional links upon peer request. |
ppp bap call request | Enables the interface to initiate additional links. |
dialer map protocol next-hop-address | Enables a serial interface or an ISDN interface to initiate and receive calls to or from remote sites.
Specifies the destination string (telephone number) for calling:
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The AO/DI client interface must be configured to run PPP over X.25. To configure the interface for the X.25 parameters, use the following commands in interface configuration mode, as required:
| Command | Purpose |
|---|---|
x25 address address | Configures the X.25 address. |
x25 htc circuit-number | Sets the highest two-way circuit number. For X.25 the default is 1024. |
x25 win packets | Sets the default VC receive window size. The default is 2packets.1 |
x25 wout packets | Sets the default VC transmit window size. The default is 2packets.1 |
| 1The default input and output window sizes are typically defined by your network administrator. Cisco IOS-configured window sizes must be sets to match the window size of the network. |
For details and usage guidelines for X.25 configuration parameters, see the Cisco IOS Wide-Area Networking Configuration Guide and Cisco IOS Wide-Area Networking Command Reference publication.
To configure PPP and BAP under the dialer interface on the AO/DI server, use the following commands in interface configuration mode, as required:
| Command | Purpose |
|---|---|
ppp multilink bap | Enables PPP BACP bandwidth allocation negotiation. |
encapsulation ppp | Enables PPP on the interface. |
dialer in-band | Enables DDR on the interface. |
dialer load-threshold load | Sets the dialer load threshold. |
dialer-group group-number | Controls access to this interface by adding it to a dialer access group. |
ppp bap call accept | Enables the interface to accept additional links upon peer request. |
ppp bap callback request | Enables the interface to initiate additional links (optional). |
BAP configuration commands are optional. For information on how to configure BACP/BAP see the chapter "Configuring the Bandwidth Allocation Control Protocol" in this publication.
The AO/DI server BRI, PRI, or serial interface must be configured for the X.25 parameters necessary to run PPP over X.25. To configure the interface for X.25 parameters, use the following commands in interface configuration mode:
| Command | Purpose |
|---|---|
x25 address address | Configures the X.25 address. |
x25 htc circuit-number | Sets the highest two-way circuit number. For X.25 the default is 1024. |
x25 win packets | Sets the default VC receive window size. The default is 2packets.1 |
x25 wout packets | Sets the default VC transmit window size. The default is 2packets.1 |
| 1The default input and output window sizes are typically defined by your network administrator. Cisco IOS-configured window sizes must be sets to match the window size of the network. |
For details and usage guidelines for X.25 configuration parameters, see the Cisco IOS Wide-Area Networking Configuration Guide and Cisco IOS Wide-Area Networking Command Reference publication.
Once the AO/DI client and server is configured with the necessary PPP, BAP, and X.25 commands, configure the routers to perform AO/DI. Perform the tasks in the following sections:
To configure AO/DI, you must complete the tasks in the following section. The last task, to define local number peer characteristics, is optional.
See the section "AO/DI Client Configuration" at the end of this chapter for an example of how to configure the AO/DI client.
To enable an interface to run the AO/DI client, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
x25 aodi | Enables the AO/DI client on an interface. |
You must enable the interface to establish a PPP session over the X.25 protocol. The cloning interface will hold the PPP configuration, which will be cloned by the virtual access interface that is created and attached to the X.25 VC. The cloning interface must also hold the MLP configuration that is needed to run AO/DI.
To add the X.25 map statement that will enable the PPP session over X.25, identify the cloning interface, and configure the interface to initiate AO/DI calls, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
x25 map ppp x121-address interface cloning-interface | Enables the interface to initiate a PPP session over the X.25 protocol and remote end mapping. |
Once MLP is enabled and the primary traffic load is reached (based on the dialerload-threshold value), the MLP bundle will add member links (Bchannels). The addition of another B channel places the first link member into "receive-only" mode and subsequent links are added, as needed.
To configure the dialer interface or BRI interface used for cloning purposes and to place the first link member into receive only mode, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
ppp multilink idle-link | Configures the interface to enter "receive only" mode so that MLP links are added as needed. |
During BACP negotiation between peers, the called party indicates the number to call for BACP. This number may be in either a national or subscriber format. A national format indicates that the phone number returned from the server to the client should contain ten digits. A subscriber number format contains seven digits.
To assign a prefix to the phone number that is to be returned, use the following optional command in interface configuration mode:
| Command | Purpose |
|---|---|
ppp bap number prefix prefix-number | Specifies a primary telephone number prefix for a peer to call for PPP BACP negotiation (optional). |
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NoteThe pppbapnumber prefix command is not typically required on the server side, as the server usually does not initiate calls to the client. This command would only be used on the server in a scenario where both sides are configured to act as both client and server. |
The AO/DI server will receive calls from the remote end interface running AO/DI client and likewise, and must be configured to initiate a PPP session over X.25, allow interface cloning, and be capable of adding links to the MLP bundle. The interface configured for AO/DI server relies on the no-outgoing option for the x25 map command to ensure calls are not originated by the interface. Use the commands in the following sections to configure the AO/DI server:
See the section "AO/DI Server Configuration" at the end of this chapter for an example of how to configure the AO/DI server.
Configure the x25 map command with the X.121 address of the calling client. This task enables the AO/DI server interface to run a PPP over X.25 session with the configured client. The no-outgoing option must be set in order to ensure that calls do not originate from this interface.
To configure an interface for AO/DI server, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
x25 map ppp x121-address interfacecloning-interface no-outgoing | Enables the interface to initiate a PPP session over the X.25 protocol and remote end mapping. |
Once MLP is enabled and the primary traffic load is reached (based on the dialerload-threshold value), the MLP bundle will add member links (Bchannels). The addition of another B channel places the first link member into "receive-only" mode and subsequent links are added, as needed.
To configure the dialer interface or BRI interface used for cloning purposes and to place the first link member into receive only mode, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
ppp multilink idle-link | Configures the interface to enter "receivce only" mode so that MLP links are added as needed. |
During BACP negotiation between peers, the called party indicates the number to call for BACP. This number may be in either a national or subscriber format. A national format indicates that the phone number returned from the server to the client should contain 10digits. A subscriber number format contains 7digits.
To assign a prefix to the phone number that is to be returned, use the following, optional command in interface configuration mode:
| Command | Purpose |
|---|---|
ppp bap number {format national | subscriber} | (Optional) Specifies that the primary telephone number for a peer to call is in either a national or subscriber number format. |
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NoteThe pppbapnumber prefix command is not typically required on the server side, because the server usually does not initiate calls to the client. This command would only be used on the server in a scenario where both sides are configured to act as both client and server. |
This section provides the following configuration examples:
The following example shows BRI interface 0 configured with the PPP, multilink, and X.25 commands necessary for the AO/DI client:
hostname Router_client ! ip address-pool local isdn switch-type basic-5ess x25 routing ! interface Ethernet0 ip address 172.21.71.99 255.255.255.0 ! interface BRI0 isdn switch-type basic-5ess ip address 6.1.1.9 255.0.0.0 encap ppp dialer in-band dialer load-threshold 1 either dialer-group 1 no fair-queue ppp authentication chap ppp multilink bap ppp bap callback accept ppp bap call request ppp bap number prefix 91 ppp multilink idle-link isdn x25 static-tei 23 isdn x25 dchannel dialer rotary-group 1 ! interface BRI0:0 no ip address x25 address 12135551234 x25 aodi x25 htc 4 x25 win 3 x25 wout 3 x25 map ppp 12135556789 interface bri0 ! dialer-list 1 protocol ip permit
The following example shows the configuration for the AO/DI server, which is configured to only receive calls from the AO/DI client. The configuration uses the x25 map ppp command with the no-outgoing option, and the ppp bap number format command, which implements the national format.
hostname Router_server ! ip address-pool local isdn switch-type basic-5ess x25 routing ! interface Ethernet0 ip address 172.21.71.100 255.255.255.0 ! interface BRI0 isdn switch-type basic-5ess ip address 6.1.1.10 255.0.0.0 encap ppp dialer in-band no fair-queue dialer load-threshold 1 either dialer-group 1 ppp authentication pap ppp multilink bap ppp multilink idle-link ppp bap number default 2135550904 ppp bap number format national ppp bap call accept ppp bap timeout pending 20 isdn x25 static-tei 23 isdn x25 dchannel dialer rotary-group 1 ! interface BRI0:0 no ip address x25 address 12135556789 x25 htc 4 x25 win 3 x25 wout 3 x25 map ppp 12135551234 interface bri0 no-outgoing ! dialer-list 1 protocol ip permit
Posted: Tue Jul 18 15:14:44 PDT 2000
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