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Always On/Dynamic ISDN (AO/DI) is an on-demand service that is designed to optimize the use of an existing Integrated Services Digital Network (ISDN) signaling 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, the Bandwidth Allocation Control Protocol (BACP) and the Bandwidth Allocation Protocol (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. Since 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 are disconnected after use. Figure 124 shows the AO/DI environment and how ISDN and X.25 resources are implemented.
AO/DI provides the following benefits:
AO/DI functionality is based on the following technology modules:
PPP over X.25 is accomplished through the following process:
1. The X25 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 125 shows the virtual interface creation process.
Multilink offers load balancing, packet fragmentation, and the bandwidth allocation functionality that is key to AO/DI structure. The Multilink PPP bundle process is achieved through the following process:
1. The ppp multilink bap command initiates Multilink PPP and, subsequently, BAP. The virtual access interface that is created above the X.25 virtual circuit (over the D channel) becomes the first member link of the Multilink PPP bundle.
2. The ppp multilink idle-link command works in conjunction with the dialer load-threshold command in order to add additional B channels as needed to boost traffic throughput. When an additional 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, it disengages "receive only" mode. The X.25 SVC stays active. Figure 126 shows the MLP bundle sequence.
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.
Basic Rate Interface (BRI)---An ISDN interface composed of two B channels and one D channel for circuit-switched communication of voice, video, and data.
bearer channel (B Channel)---A channel that carries data on the ISDN interface.
cloning---A virtual interface created and configured by applying a specific virtual template interface. The template is the source of the generic user information and router-dependent information. The result of cloning is a virtual access interface configured with all the commands in the template.
customer premise equipment (CPE)---Terminating equipment installed at customer sites.
dialer interface---A logical interface that includes settings needed to make calls in order to reach specific remote destinations (subnetworks).
E.164---ITU-T recommendation for international telecommunication numbering, especially in ISDN, BISDN, and SMDS.
Integrated Services Digital Network (ISDN)---Communication protocols offered by telephone companies that permit telephone networks to carry data, voice, and other source traffic.
Multilink PPP (MLP)---A protocol that provides the capability of splitting and recombining packets to a single end system across a logical pipe (also called a bundle) formed by multiple links. Multilink PPP provides bandwidth on demand and reduces transmission latency across WAN links.
Primary Rate Interface (PRI)---An ISDN interface to primary rate access. Primary rate access consists of a single 64-kpbs D channel plus 23 (T1) or 30 (E1) B channels.
signaling channel (D Channel)---A channel used to carry control signals on the ISDN interface.
virtual access interface---A unique virtual interface that is created dynamically and exists temporarily. Virtual access interfaces can be created and configured differently by different applications, such as virtual profiles and virtual private dialup networks.
virtual circuit---A logical circuit created to ensure reliable communication between two network devices. A virtual circuit can be either permanent (a PVC) or switched (an SVC).
X.121---ITU-T standard describing an addressing scheme used in X.25 networks. X.121 addresses are sometimes called International Data Numbers (IDNs).
X.25---An ITU-T standard that defines how connections between DTE and DCE are maintained for remote terminal access and computer communications in public data networks (PDNs). X.25 specifies LAPD, a data-link layer protocol, and PLP, a network layer protocol.
AO/DI supports PPP encapsulation on switched X.25 virtual circuits (VCs) only.
The X.25 encapsulation (per RFC 1356), PPP, BAP, and BACP 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 peers. It is recommended you configure PPP multilink with the BAP option due to the differences between the ISDN (E.164) and X.25 (X.121) numbering formats.
In order to implement AO/DI you must configure the AO/DI client and server for PPP, incorporating BAP and X.25 module commands. This involves configuring the BRI or PRI interfaces with the appropriate X.25 commands and the dialer interfaces with the necessary PPP or BAP commands.
The following tasks are required before you can configure specific commands required to run AO/DI:
Use the following commands in interface configuration mode, to configure PPP and BAP under the dialer interface on the AO/DI client:
| Command | Purpose |
|---|---|
Enable PPP BACP bandwidth allocation negotiation. | |
Enable PPP on the interface. | |
Enable DDR on the interface. | |
Set the dialer load threshold. | |
dialer-group group-number | Control access to this interface by adding it to a dialer access group. |
Enable the interface to initiate additional links upon peer request (optional). | |
Enable the interface to initiate additional links. | |
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or dialer string dial-string [:isdn-subaddress] | Enable a serial interface or an ISDN interface to initiate and receive calls, to or from, remote sites. Specify 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. In order to configure the interface for the X.25 parameters, use the following commands in interface configuration mode:
| Command | Purpose |
|---|---|
x25 address address | Configure the X.25 address. |
x25 htc circuit-number | Set the highest two-way circuit number. For X.25 the default is 1024. |
x25 win packets | Set the default virtual circuit receive window size. The default is 2 packets.1 |
x25 wout packets | Set the default virtual circuit transmit window size. The default is 2 packets.1 |
| 1The default input and output window sizes are typically defined by your network administrator. Cisco IOS- configured window sizes must be set to match that of the network. |
For details and usage guidelines for X.25 configuration parameters, refer to Cisco IOS Release 11.3 Wide-Area Networking Configuration Guide and Command Reference.
Use the following commands in interface configuration mode to configure PPP and BAP under the dialer interface on the AO/DI server:
| Command | Purpose |
|---|---|
Enable PPP BACP bandwidth allocation negotiation. | |
Enable PPP on the interface. | |
Enable DDR on the interface. | |
Set the dialer load threshold. | |
dialer-group group-number | Control access to this interface by adding it to a dialer access group. |
Enable the interface to accept additional links upon peer request. | |
Enable the interface to initiate additional links (optional). |
BAP configuration commands are optional. For information on how to configure BACP/BAP refer to the Cisco IOS Release 11.3 Dial Solutions Configuration Guide.
The AO/DI server BRI, PRI, or serial interface must be configured for the X.25 parameters necessary to run PPP over X.25. Use the following commands in interface configuration mode to configure the interface for X.25 parameters:
| Command | Purpose |
|---|---|
x25 address address | Configure the X.25 address. |
x25 htc circuit-number | Set the highest two-way circuit number. For X.25 the default is 1024. |
x25 win packets | Set the default virtual circuit receive window size. The default is 2 packets.1 |
x25 wout packets | Set the default virtual circuit transmit window size. The default is 2 packets.1 |
| 1The default input and output window sizes are typically defined by your network administrator. Cisco IOS-configured window sizes must be set to match that of the network. |
For details and usage guidelines for X.25 configuration parameters, refer to Cisco IOS Release 11.3 Wide-Area Networking Configuration Guide and Command reference.
Once the AO/DI client and server is configured with the necessary PPP, BAP, and X.25 commands, you can configure the routers to perform AO/DI. In order to configure AO/DI, perform the following tasks:
To configure AO/DI, you must complete the tasks in the following section. The last task, to define local number peer characteristics, is optional.
To enable an interface to run the AO/DI client, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
Enable 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 virtual circuit (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 | Enable the interface to initiate a PPP session over the X.25 protocol and remote end mapping. |
Once Multilink PPP is enabled and the primary traffic load is reached (based on the dialer load-threshold value), the MLP bundle will add additional member links (B channels). 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 |
|---|---|
Configure 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 10 digits. A subscriber number format contains 7 digits. To assign a prefix to the phone number that is to be returned, use the optional command in interface configuration mode:
| Command | Purpose |
|---|---|
ppp bap number prefix prefix-number | Specify a primary telephone number prefix for a peer to call for PPP BACP negotiation (optional). |
The AO/DI server will receive calls from the remote end interface running AO/DI client and likewise, must be configured to initiate a PPP session over X.25, allow interface cloning, and be capable of adding additional 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 following command to configure the AO/DI server:
Configure the x25 map command with the calling client's X.121 address. This 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 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 interface cloning-interface no-outgoing | Enable the interface to initiate a PPP session over the X.25 protocol and remote end mapping. |
Once Multilink PPP is enabled and the primary traffic load is reached (based on the dialer load-threshold value), the MLP bundle will add additional member links (B channels). 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 |
|---|---|
Configure 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 10 digits. A subscriber number format contains 7 digits. To assign a prefix to the phone number that is to be returned, use the optional command in interface configuration mode:
| Command | Purpose |
|---|---|
ppp bap number {format national | subscriber} | Specify that the primary telephone number for a peer to call is in either a national or subscriber number format (optional). |
This section provides the following configuration examples:
The following example shows BRI interface 0 configured with the necessary 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 command x25 aodi, x25 map ppp with the no-outgoing option, and ppp bap number format, 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: Mon May 3 11:29:17 PDT 1999
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