Overview of Dial Networking

Remote access or dial networking refers to setting up one or more network access servers (NAS) to allow on-demand service connectivity. Dial networking is often used for connection to the small office/home office (SOHO), remote office/branch office (ROBO), and individual remote users. In the United States, many companies regard telecommuting (SOHO and ROBO) as a way to solve problems with space and save on rental, office furniture, and parking costs. Telecommuting also can help attract new employees, particularly if it saves them hours of commuting or provides flex hours. It can assist an organization in conforming to the Clean Air Act and make employees more productive.

In Europe, companies are looking for solutions that allow central offices to connect to remote sites. Dial-on-Demand Routing (DDR) allows you to quickly enable a WAN connection through the use of analog telephone lines (POTS). Also, DDR can save money because the line is used on an as-needed basis, whereas a leased line is paid for even when the line is not in use.

Analog modems have been (and still are) widely used to enable users to gain remote access to the network. As users needed additional bandwidth, they started looking at ISDN. ISDN provides additional bandwidth (up to 128 KB) without requiring a leased line. Today, with the advent of higher-speed analog modems (33.6 KB and higher), analog modems once again can provide the necessary connectivity over serial lines needed by business executives, salespeople, and technical support engineers, depending on their application use and needs.

This chapter addresses network management with respect to the needs of the remote access network manager/administrator. For managing events, the administrator needs information about the following:

• Modems errors and status (faulty modems)

• Equipment T1/E1 controller status and signaling

• Lines errors, signaling, and status

For performance management, the administrator needs information about the following:

• Modem utilization

• Line utilization

• Data transferred by user

• Most-frequently-accessed locations

Also, this chapter will look at the two basic types of calls in dial networking: sent and received, as follows:

• Circuit-Switched Digital Calls Circuit usually ISDN 56 Kbps or 64 Kbps data calls that use the point-to-point protocol (PPP). An ISDN router that functions as an access server or a terminal adapter connected to a client workstation initiates these calls. Individual synchronous serial DS0s (B channels) are used to transport circuit switched digital calls across WANs. These calls do not transmit across standard plain old telephone service (POTS) lines.

• Analog Modem Calls Analog modem calls that travel through traditional telephone lines and ISDN lines. Regardless of the media used, these calls are initiated by a modem and terminate on another modem at the remote end.

Access Through Virtual Terminal (VTY) Lines

A third type of call is the asynchronous character stream call, which enters the router or access server through VTY lines and virtual asynchronous interfaces (VTY-async). These virtual lines and interfaces terminate incoming character streams that do not have a physical connection to the access server or router (such as a physical serial interface).

For example, if you begin a PPP session over an asynchronous character stream, a VTY-async interface is created to support the call. Generally, you autocommand on the presence of characters, usually a Telnet, possibly to a paired async port on another terminal server; or use a tunnel and then protocol translate back.

The following types of calls are terminated on a virtual asynchronous interface: Telnet, LAT, V.120, TN3270, and PAD calls. I have not found a way to manage these calls for either fault or performance. You can get a TTY trap on termination of a character-triggered Telnet and the traps could let you manage repeated failures, but not one failure and then the inability to re-establish Telnet.

An overall dial example may be an AS5300 that receives and routes both incoming digital and analog calls that come in through E1/T1 PRI interfaces. Analog calls originated by modem users pass through to the asynchronous interfaces, lines, and modems. Digital calls originated by remote ISDN users pass through to the E1/T1 controllers, ISDN dialer interface, loopback interface, and D-channel serial interfaces.

In later sections, both MIB variables and Cisco IOS show commands are described or illustrated for data gathering. Syslog and AAA logs generally will not be discussed.

Access Physical Resources

To connect to the NAS, some type of connection at the server must be configured. A channelized T1 (CT1) or E1 (CE1) is an analog line that was originally intended to support analog voice calls, but has evolved to support analog data. Digital calls (ISDN or ADSL) do not transmit over CT1 or CE1 lines. One of the key differences between the CT1/CE1 and non-channelized lines is that the channelized lines do not have D-channel signaling. The T1 and E1 controller that is provided on the Cisco NAS can have its available channels allocated in several ways:

• All channels can be configured to support ISDN PRI (T1-PRI (23B+D) or E1-PRI (30B+D)).

• All channels can be configured to support robbed-bit signaling on a T1, if you are not running ISDN PRI. This enables a Cisco AS5x00 modem to receive and transmit analog calls using in-band signaling.

• All channels can be configured to support channel-associated signaling (CAS) on an E1, if you are not running ISDN PRI. This enables a Cisco AS5x00 modem to receive and transmit analog calls using out-of-band (OOB) signaling.

• All channels can be configured in a single channel group.

• Channels can be mixed and matched to support ISDN PRI and channel grouping.

• Channels can be mixed and matched to support ISDN PRI, robbed-bit signaling, and channel grouping across the same T1 line.

Other physical access resources are to use the ISDN BRI (2B+D), physical terminal (TTY) lines for asynchronous interfaces or modems, synchronous serial ports for point-to-point leased-line, or dial-up communications.

The different configuration options above are covered in the Cisco documentation online (www.cisco.com). Also, the Cisco Press book Cisco IOS Dial Solutions contains different configuration examples.

Access Logical Resources: Logical Constructs and Logical Interfaces

A logical construct is an intangible device in an NAS that stores data or configuration information for physical interfaces. Therefore, you could say that a logical construct stores the core protocol characteristics to assign to physical interfaces. It is important to note that no data packets are forwarded to a logical construct. This means that there is no actual network management available for the logical construct. Cisco uses two types of logical constructs in its access servers and routers:

• Group Asynchronous Interface Stores and projects specific protocol characteristics to a specified range of asynchronous interfaces (see Example 17-1).

• Virtual Template Interfaces Stores and projects protocol configuration information for temporary virtual access interfaces (triggered by multilink or virtual private dial-up network (VPDN) session events) and protocol translation sessions (see Example 17-2).

Example 17-1 Sample IOS configuration for a group-asynchronous interface.

 interface Group-Async1  ip unnumbered Loopback0 <- loopback for IP Address  encapsulation ppp <- encapsulation type  async mode interactive <- setting the interactive mode  peer default ip address pool dialin_pool  no cdp enable <- turning off Cisco Discovery Protocol  ppp authentication chap pap dialin <-setting authentication  group-range 1 48 <- assigning the channels to the group 

Example 17-2 Sample IOS configuration for a virtual template interface.

 ip unnumbered Loopback0 <- loopback for IP Address no ip mroute-cache <- turn off multicast route cache peer default ip address pool dialin_pool ppp authentication chap pap dialin <- setting authentication ppp multilink <-enable multilink PPP (MPPP) 

A logical interface is a device on an NAS that does receive and transmit data packets. Also, the logical interface controls physical interfaces. Cisco IOS provides three logical interfaces used for dial access:

• Dialer interfaces

• Virtual access interfaces

• Virtual asynchronous interfaces (VTY-async)

A dialer interface is a parent interface that stores and projects protocol configuration information that is common to all D-channel members of a dialer rotary group. Data packets pass through dialer interfaces, which in turn initiate dialing for inbound calls. Without a dialer interface configuration, each D channel would have to be configured separately. If a dialer interface engages in a multilink session, a dialer interface is in control of a virtual access interface, which in turn controls the D channel for a PRI (for example, s0:23).

A virtual access interface is a temporary interface created to terminate incoming PPP streams that do not have physical connections. Virtual access interfaces are cloned from virtual interface templates and are not directly configurable by a user. After the tunnels or multilink sessions are ended, the dynamically created virtual access interface disappears. Thus, you would not see this type of interface through an SNMP query and would not be able to get the interface utilization statistics.

A VTY-async is created on demand to support calls that enter the router through a non-physical interface. They are not user-configurable. For example, asynchronous character stream calls terminate on non-physical interfaces. These types of calls include inbound Telnet, LAT, and PPP over character-oriented protocols (such as V.120 or X.25), and Packet Assembler Disassembler (PAD) calls. A virtual asynchronous interface is also used to terminate L2F/L2TP (Layer 2 Forwarding/Layer 2 Tunneling) tunnels, which are often travelling companions with multilink sessions. A virtual asynchronous line is used to access a virtual asynchronous interface.

Key-Managed Modem Benefits

There are two important capabilities that managed modems provide the network administrator. First, they give the network administrator the ability to remotely control a modem; being physically located near the access servers is not necessary. Second, managed modems provide access to everything that is happening in the modem.

Through the management features available with managed modems, a network administrator can access the modem to capture statistics. This type of support enables the administrator to understand and solve problems that might take days with unmanaged modems.

Another advantage involves the management of the NAS versus the remote devices. Gathering the data that is locally aggregated on the NAS is better than polling all the remote devices. In other words, in a hub and spoke environment, collect from the hub and not from all the spokes. This fact probably seems obvious for remote access users who are dialing in with a modem over a POTS line there really is no other way to manage the access.

However, it is also true in the case of ISDN used for either SOHO or direct user connectivity. Again, the NAS is an aggregation point for data and can help reduce the amount of network management overhead by polling it rather than a remote device. It is still important to enable traps on the remote devices, and possibly include the use of RMON events and alarms to threshold data from remote devices.

In the preceding sections, you saw various types of interfaces for Cisco devices, the intended use of each kind of interface, and whether packets flow through the interface. This information affects whether or not you can obtain data and manage the interface.