Enterprise and ISP Designs | Troubleshooting Remote Access Networks (CCIE Professional Development)

From an application point of view, you can break down ISDN into the following main categories. Based on the structure of the core side, you can recognize a separate enterprise design and an ISP design in these categories:

ISPs, more often than enterprises , use the MMP, which is not mandatory for an enterprise solution.
ISPs often use a pool of IP addresses, whereas enterprises often assign an IP subnet from their routable or private address space for remote users based on their needs.
ISPs use NAT/PAT techniques or assign a subnet for an extra charge, and an enterprise, using NAT/PAT, assigns a /32, /30, /29, or even a /28 subnet to the remote user. NAT/PAT is more common on the end user side, performing the NAT/PAT on their customer premises equipment (CPE). ISPs rarely perform NAT/PAT for the remote user on their own routers.
ISPs use one-way authentication so that they can troubleshoot the CPE remotely without configuring the Challenge Handshake Authentication Protocol (CHAP) host name of the core router on the remote user's router. Enterprises use two-way authentication, or more sophisticated authentication schemes.
ISPs do not make outgoing calls, and enterprises do allow outgoing calls.
ISPs can apply a flat rate fee for the service; however, enterprises usually pay usage-based charges with volume discounts and usually the charges are charged by the ISP to the enterprise.

Often, combinations of these categories exist in a design. You might want to review some of the basic design techniques, but before that, review the basic ISDN configuration of the ISDN switch type, SPID(s), and LDN(s). They might seem easy to configure, but the reality is that they can cause serious issues, especially when the service is a new-install. Each of these is discussed in the following sections.

Setting the ISDN Switch Type

The switch types supported in the latest versions of IOS (12.1.x. and later) are outlined in Example 10-1. The most commonly used types in the U.S. and Canada are basic-5ess, basic-dms100, and basic-ni1.

Example 10-1. Determining the ISDN Switch Type

 804-isdn(config)#  isdn switch-type ?  basic-1tr6            1TR6 switch type for Germany basic-5ess            Lucent 5ESS switch type for the US basic-dms100          Northern Telecom DMS-100 switch type for the US basic-net3            NET3 switch type for the UK, Europe, Asia and Australia basic-ni              National ISDN switch type for the US basic-qsig            QSIG switch type basic-ts013           TS013 switch type for Australia (obsolete) ntt                   NTT switch type for Japan vn3                   VN3 and VN4 switch types for France

Besides the listed switches, there is a category of custom design switches for the U.S. and Canada:

 custom-5ess            Lucent 5ESS switch type for the US custom-dms100          Northern Telecom DMS-100 switch type for the US

The custom design switches are point-to-point designs and usually do not require SPID and LDN information. It is common to define only a phone number.

The command and options for setting the switch types for 776 routers are as follows :

  SEt SWitch 5Ess   DMS   NI-1   PERM64   PERM128   PERM2X64

If you don't specify any switch type, the router's default is 5Ess.

In 77x routers the switch type 5Ess applies to all types of 5Ess switches. It is convenient when custom switch types need to be configured because all you need to know is that the switch is 5Ess. This setting is part of the general configuration of the router, which is called the system level in 77x terminology.

For Cisco IOS Software scenarios, the configuration command for switch type National ISDN-1 (NI-1 or N11) is as follows:

 804-isdn(config)#  isdn switch-type basic-ni

You can configure more than one switch type in one router configuration by using Cisco IOS Software Release 12.0 and later. An example of a BRI0 interface configuration is as follows:

 804-isdn(config-if)#  isdn switch-type basic-5ess

The global configuration mode and the interface configuration mode settings work together and follow the logic that if an interface (BRI0, BRI1, BRI2, and so on) is configured for different switch types, it takes precedence over the settings in the global configuration mode. If not, the BRIx interface takes the switch type from the global configuration mode as a default one.

Additionally, the switch type can be different from the profile that the LEC is using or emulating on the switch. You can have DMS100 running NI1 or any other combination. It is common to see the DMS and 5ess switches running in NI mode in North America. When using Cisco 77x routers, it is preferable to configure the router, based on the provider's profile name, coded as NI or 5E, and if it does not work, configure the switch type. In the case of Cisco IOS Software, either set both global and interface configurations to the type of the local exchange carrier's (LEC's) profile (NI1), or set the switch type (DMS100) in the global configuration, and set the LEC's profile type (NI1) under the Basic Rate Interface (BRI).

Setting SPIDs and LDNs

When an LEC installs the local loop (installing the ISDN service), there is always minimum information available to the user, including SPIDs, LDNs, and the switch type. The NI-1 and the DMS100 both require SPIDs. Basic-5ESS is a point-to-multipoint switch, which is capable of providing up to eight SPIDs per service. As previously mentioned, custom-5ESS is point-to-point and does not require SPIDs, and by default, they do not provide a voice capability. When working with LECs, the network professional might see different settings for voice and data, such as the following:

A service might not be provided (voice or data).
Calls might be restricted by rejecting the third call, if two channels are up and running.
All incoming calls might be forwarded to let the router decide how to handle the priority of the call (voice and data).

The variety of settings and the use of different names by providers (for example, GTE has a setting called Umbrella or U setting) is a real challenge for network engineers , and requires maximum attention to the details of the configuration.

The LDN is not a mandatory part of the configuration, unless the service is provisioned for both voice and data.

Example 10-2, Example 10-3, and Example 10-4 provide examples of valid ISDN settings for 77x router images that use Software Version c760-in.r.US 4.4(2). Example 10-5 uses a Cisco IOS Software router, which is provisioned for voice and data.

Example 10-2. A Valid SPID Setting Example for 77x Router for Switch Type NI1

 776-isdn>set switch ni1 776-isdn>set 1 spid 40857647400101 776-isdn>set 1 directorynumber 5764740 776-isdn>set phone1 = 5764740 776-isdn>set 2 spid 40857647410101 776-isdn>set 2 directorynumber 5764741 776-isdn>set phone2 = 5764741

Example 10-3. A Valid SPID Setting Example for 77x Router for Point-to-Point Switch Type 5ESS

 776-isdn>set switch 5ess 776-isdn>set 1 directorynumber 4920672 776-isdn>set phone1 4920672

Example 10-4. A Valid SPID Setting Example for Point-to-Multipoint Switch Type 5Ess

 776-isdn>set switch 5ess 776-isdn>set ppp multilink on 776-isdn>set 1 spid 0173125470 776-isdn>set 2 spid 0173125480 776-isdn>set 1 directorynumber 7312547 776-isdn>set 2 directorynumber 7312548 776-isdn>set phone1 7312547 776-isdn>set phone2 7312548

In the case of Cisco IOS Software Release 12.0 and later, under the BRIx interface, you usually see the configuration in Example 10-5.

Example 10-5. Cisco IOS SPID Settings for Interface BRI0

 804-isdn(config-if)#isdn switch-type basic-ni 804-isdn(config-if)#isdn spid1 53075358360101 7535836 804-isdn(config-if)#isdn spid2 53075358600101 7535860

A relatively new feature of IOS (available in IOS 12.1(6), for example) sets the switch type, and not the SPIDs. A command available under the BRI0 interface is isdn autodetect , which enables the router to request the SPID assignment from the switch. The router reports no LDN, spid1 NOT sent, spid1 NOT valid , but the indicator state equals 8, which is a valid state (see Table 13-1). The router responds with MULTIPLE_FRAME_ESTABLISHED for both terminal endpoint identifiers (TEIs), and the configuration is fully operational.

The user's router needs to be configured to meet the requirements of the core router. The end user requires the following minimum information to configure routers:

Host name.
Password. Password Authentication Protocol (PAP) or CHAP host name password (depends on enterprise's choice for authentication method).
SPIDs, LDNs (optional), and switch type.
DNS server IP address of the ISP and the domain name (optional). These settings are not relevant to ISDN connectivity, but to DHCP server settings in the configuration.
NAT/PAT configuration or the subnet of addresses (optional). Another option here can be if the IP address is dynamically obtained (negotiated).

As mentioned, the IP address of the CPE can be statically or dynamically assigned, can be part of the enterprise's IP range, or can be a NAT/PAT address. Some of the common techniques here are designing a pool of IP addresses, or performing PAT or NAT translation to assign an IP address to the remote user.