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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. A step-by-step breakdown of the functional components is covered in the "Functional Description" section of this document. Figure 1 shows the AO/DI environment and how ISDN and X.25 resources are implemented.
AO/DI provides the following benefits:
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.
This feature is supported on these platforms:
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:
Perform the following tasks in interface configuration mode, to configure PPP and BAP under the dialer interface on the AO/DI client:
| Task | Command |
|---|---|
| Enable PPP BACP bandwidth allocation negotiation. | ppp multilink bap |
| Enable PPP on the interface. | encapsulation ppp |
| Enable DDR on the interface. | dialer in-band |
| Set the dialer load threshold. | dialer load-threshold load |
| Control access to this interface by adding it to a dialer access group. | dialer-group group-number |
| Enable the interface to initiate additional links upon peer request (optional). | ppp bap callback accept |
| Enable the interface to initiate additional links. | ppp bap call request |
| Enable a serial interface or an ISDN interface to initiate and receive calls, to or from, remote sites. or Specify the destination string (telephone number) for calling:
|
dialer map protocol next-hop-address [name hostname] [spc] [speed 56 | 64] [broadcast] [modem-script modem-regexp] system-script system-regexp] dialer string dial-string [:isdn-subaddress] |
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, perform the following tasks in interface configuration mode:
| Task | Command | Default |
|---|---|---|
| Configure the X.25 address. | x25 address address | no default value |
| Set the highest two-way circuit number. | x25 htc circuit-number | 1024--for X.25 |
| Set the default virtual circuit receive window size. | x25 win packets | 21 |
| Set the default virtual circuit transmit window size. | x25 wout packets | 22 |
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.
Perform the following tasks in interface configuration mode to configure PPP and BAP under the dialer interface on the AO/DI server:
| Task | Command |
|---|---|
| Enable PPP BACP bandwidth allocation negotiation. | ppp multilink bap |
| Enable PPP on the interface. | encapsulation ppp |
| Enable DDR on the interface. | dialer in-band |
| Set the dialer load threshold. | dialer load-threshold load |
| Control access to this interface by adding it to a dialer access group. | dialer-group group-number |
| Enable the interface to accept additional links upon peer request. | ppp bap call accept |
| Enable the interface to initiate additional links (optional). | ppp bap callback request |
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. Perform the following tasks in interface configuration mode to configure the interface for X.25 parameters:
| Task | Command | Default |
|---|---|---|
| Configure the X.25 address. | x25 address address | no default value |
| Set the highest two-way circuit number. | x25 htc circuit-number | 1024--for X.25 |
| Set the default virtual circuit receive window size. | x25 win packets | 21 |
| Set the default virtual circuit transmit window size. | x25 wout packets | 22 |
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.
No MIBs are supported by this feature.
This feature supports the following RFCs:
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 2 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 3 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.
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, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Enable the AO/DI client on an interface. | x25 aodi |
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, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Enable the interface to initiate a PPP session over the X.25 protocol and remote end mapping. | x25 map ppp x.121-address interface cloning-interface |
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, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the interface to enter "receive-only" mode so that MLP links are added as needed. | ppp multilink idle-link |
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, perform the optional task in interface configuration mode:
| Task | Command |
|---|---|
| Specify a primary telephone number prefix for a peer to call for PPP BACP negotiation (optional). | ppp bap number prefix prefix-number |
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. Perform the following tasks 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, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Enable the interface to initiate a PPP session over the X.25 protocol and remote end mapping. | x25 map ppp x.121-address interface cloning-interface no-outgoing |
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, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure the interface to enter "receivce-only" mode so that MLP links are added as needed. | ppp multilink idle-link |
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, perform the optional task in interface configuration mode:
| Task | Command |
|---|---|
| Specify that the primary telephone number for a peer to call is in either a national or subscriber number format (optional). | ppp bap number format [national | subscriber] |
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 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
This section documents the commands required to configure AO/DI. The existing ppp bap number and x25 map commands are modified to accept new keywords, which can be used to configure AO/DI. Only the keyword information that applies to AO/DI configurations is provided. Existing keyword options and related commands used with this feature are documented in the Cisco IOS Release 11.3 command references.
To specify a local telephone number that peers can dial to establish a multilink bundle, use the ppp bap number interface configuration command. To remove a previously configured number, use the no form of this command.
ppp bap number {default phone-number | secondary phone-number | prefix prefix-number}| prefix prefix number | Prefix number for the PPP BAP phone number. |
| format national | subscriber | Format for the primary phone number to be dialed should be either national or subscriber where the number of digits assigned to the number is as follows:
· 10-digit number for a national format. · 7-digit number for a subscriber format. |
| default phone-number | Primary (base) phone number for the interface and the number that can be used for incoming dial calls. |
| secondary phone-number | Telephone number for the second B channel. Applies only to BRI interfaces that have a different number for each B channel or to dialer interfaces that are BRIs. |
No base number is provided.
Interface configuration
The ppp bap number command first appeared in Cisco IOS Release 11.3.
The prefix and format keywords first appeared in Cisco IOS Release 11.3 T.
Use this command to supply a local default number to be exchanged between peers in order to establish a multilink bundle.
Use the prefix keyword on the AO/DI client side to specify what will precede any number dialed to a multilink peer. This command is used by the client side for dialing instructions when communicating with the server. For example, the client issues a call request to the server whereby the server issues a call response which includes the dialing number the client should use and the format this number should be in (national or subscriber). The client then dials the number supplied by the server, preceded by any prefix information contained in the ppp bap number prefix command. Figure 4 shows an overview about the information exchange between the client and the server.
Use the format keyword on the AO/DI server side to specify how many digits should be returned by BAP. BAP will return the numbers based on either a national or subscriber format. The value that is returned is preceded by the prefix before dialing occurs. For example, if the format national keywords are configured, then the national format (which is equivalent to 10 digits) is returned by BAP (during BAP negotiation) from the server.
Use the format keyword its used on the server side to specify how many digits should be returned by BAP. BAP will return the numbers based on either a national or subscriber format. The value that is returned is preceded by the prefix before dialing. For example, if the format national keywords are configured, then the national format (which is equivalent to 10 digits) is returned by BAP (during BACP negotiation), from the server to the client.
This command is applicable on both the dialer interface and the individual physical interfaces.
In the following example, the AO/DI client uses a ppp bap prefix value of 9, which indicates that the dialed number of 5551234 will be preceded by a 9. The number that is actually dialed is 95551234. The AO/DI server uses a subscriber format, which indicates that when the client asks the server for the numbers to dial, BAP will return 7-digits.
interface dialer1 ppp bap number prefix 9
interface dialer1 ppp bap number format subscriber ppp bap number default 5555678
In the following example, the AO/DI client uses a ppp bap prefix value of 1, which indicates that the dialed number of 5551234 will be preceded by a 1. The number that is actually dialed is 19195555678 because the server is using a national format, and BAP therefore, returns 10-digits.
interface dialer1 ppp bap number prefix 1
interface dialer1 ppp bap number format national ppp bap number default 9195555678
To configure the PPP Multilink bundle so that the first link member enters into "receive-only" mode when an additional B channel is added, use the ppp multilink idle-link command. To remove this functionality, use the no form of this command.
ppp multilink idle-linkThis command has no keywords or arguments.
Multilink idle-link is not set.
Interface configuration
This command first appeared in Cisco IOS Release 11.3 T.
Use this command to ensure that the primary link in the AO/DI bundle enters "receive-only" mode as soon as the first additional B channel is added to the bundle. Additional member links are added to the Multilink PPP bundle in order to increase traffic throughput and are disconnected once the traffic load is reduced. During this timeframe, the X.25 SVC remains active. The dialer load-threshold value determines the traffic threshold limits for the bundle.
The following example configures the interface (dialer interface 1) to add additional links to the MLP bundle once the traffic load on the primary link is reached:
interface dialer1 ppp multilink idle-link
To enable the AO/DI client on an interface, use the x25 aodi interface configuration command. To remove AO/DI client functionality, use the no form of this command.
x25 aodiThis command has no arguments or keywords.
AO/DI client is not enabled.
Interface configuration
This command first appeared in Cisco IOS Release 11.3 T.
Use this command to enable the AO/DI client on an interface.
The following example enables the AO/DI client on the interface running X.25, using the x25 aodi command:
interface bri0 isdn x25 dchannel isdn x25 static-tei 8 interface bri0:0 x25 aodi x25 address 12135551234 x25 htc 4 x25 win 3 x25 wout 3 x25 map ppp 12135556789 interface dialer 1
To enable a PPP session over the X.25 protocol, use the x25 map ppp interface configuration command. To remove a prior mapping, use the no form of this command.
x25 map ppp x121-address interface cloning-interface no-outgoing| x.121 address | The X.121 address as follows:
· Client side--The calling number. · Server side--The called number. |
| interface | The interface to be used for cloning the configuration. |
| no-outgoing | Ensures that the X.25 map does not originate calls. |
Disabled
Interface configuration
This command first appeared in Cisco IOS Release 11.3 T.
Use x25 map ppp command to allow a PPP session to run over X.25.
The interface keyword refers to the interface that will be used to clone the configuration.
The following example enables the AO/DI client on the interface and configures the D channel (BRI interface 0:0) with the x25 map statement in order to allow PPP sessions over X.25 encapsulation with the configured AO/DI server:
interface BRI0:0 x25 address 16193368208 x25 aodi x25 htc 4 x25 win 3 x25 wout 3 x25 map ppp 16193368209 interface dialer 1
The following example enables the AO/DI server to receive calls from the AO/DI client and configures the D channel (BRI0:0) with the x25 map statement which allows PPP sessions over X.25 encapsulation with the configured AO/DI client:
interface BRI0:0 x25 address 16193368209 x25 htc 4 x25 win 3 x25 wout 3 x25 map ppp 16193368208 interface dialer 1
The debug x25 command is revised to add the aodi keyword, which supports the AO/DI feature. Only the information that applies to debug ppp aodi is provided. Refer to the Cisco IOS Release 11.3 Debug Command Reference for complete debug x25 information.
Use the debug x25 EXEC command with the aodi keyword to display information about an interface running PPP over an X.25 session. To disable debugging output, use the no form of this command.
[no] debug x25 aodi| aodi | Causes the debug x25 command to display AO/DI events and processing information. |
Use this command to debug X.25 connections between a client and server configured for AO/DI.
Use the debug x25 aodi command to display interface PPP events running over an X.25 session.
The examples below show the normal sequence of events for both the client and server sides:
Router# debug x25 aodi PPP-X25: Virtual-Access1: Initiating AODI call request PPP-X25: Bringing UP X.25 AODI VC PPP-X25: AODI Client Call Confirm Event Received PPP-X25: Cloning interface for AODI is Di1 PPP-X25: Queuing AODI Client Map Event PPP-X25: Event:AODI Client Map PPP-X25: Created interface Vi2 for AODI service PPP-X25: Attaching primary link Vi2 to Di1 PPP-X25: Cloning Vi2 for AODI service using Di1 PPP-X25: Vi2: Setting the PPP call direction as OUT PPP-X25: Vi2: Setting vectors for RFC1598 operation on BRI3/0:0 VC 0 PPP-X25: Vi2: Setting the interface default bandwidth to 10 Kbps PPP-X25: Virtual-Access2: Initiating AODI call request PPP-X25: Bringing UP X.25 AODI VC PPP-X25: AODI Client Call Confirm Event Received
Router# debug x25 aodi PPP-X25: AODI Call Request Event Received PPP-X25: Event:AODI Incoming Call Request PPP-X25: Created interface Vi1 for AODI service PPP-X25: Attaching primary link Vi1 to Di1 PPP-X25: Cloning Vi1 for AODI service using Di1 PPP-X25: Vi1: Setting vectors for RFC1598 operation on BRI3/0:0 VC 1 PPP-X25: Vi1: Setting the interface default bandwidth to 10 Kbps PPP-X25: Binding X.25 VC 1 on BRI3/0:0 to Vi1
debug ppp bap
debug ppp bap negotiation
debug ppp multilink
debug ppp negotiation
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