Frame Types and Network Numbers

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NWLink uses frame types and network numbers to communicate with other computers on the same segment and to provide correct packet routing.

In order for a computer running Windows 2000 to connect to computers running IPX/SPX, a frame type and network number must be configured for each computer. This frame type and network number must be identical to that used on the local network segment. The NWLink Auto Detect feature detects the frame type and external network number (also known as the network number) and is the recommended option for configuring these fields.

The Auto Detect feature works as follows:

  1. When the NWLink protocol is initialized, it sends a RIPX request using a specific frame type. The RIPX request is a broadcast request for the local network. If a response is not received, NWLink sends additional requests.

  2. When a response is received, the frame type for NWLink is set to the frame type of the response, and the IPX network number is set to the value of the Source Network number in the IPX header of the RIP response.

  3. If the computer has multiple network adapters attached to different networks, such as Token Ring, FDDI, and Ethernet, you can run Auto Detect for each adapter.

  4. If there are multiple RIPX responses, containing multiple network numbers, Auto Detect uses a counting algorithm to determine the most likely network number.

  5. If there is no RIPX response to any request, NWLink sets the frame type to Ethernet 802.2 (for Ethernet network adapters) and the network number to 0.

Occasionally, a misconfigured host causes Auto Detect to select an inappropriate frame type and network number combination for the adapter. This is usually caused by an incorrect manual setting on a computer on the network. Because Auto Detect uses the response of the RIPX request, if a computer replies with an incorrect frame type and network number, Auto Detect detects this incorrect configuration and uses it. Auto Detect might also select an incorrect network number if multiple computers responded. It uses a heuristic algorithm to determine the most likely network number but does not validate the packet information it receives.

If the Auto Detect feature selects an inappropriate frame type for a particular adapter, you can manually reset an NWLink frame type or network number for that given adapter. To manually determine the frame type and external network number set on a computer running Windows 2000, execute the IPXROUTE CONFIG command at the command prompt. This is the only way to view IPX information on Windows 2000 servers. Figure 12.6 illustrates the resulting screen.

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Figure 12.6 Frame Type Displayed in the Command-Line Interface

To manually determine the frame type, internal network number, and external network number set on a server running NetWare, type CONFIG on the NetWare server console or inspect the Autoexec.ncf file. Figure 12.7 shows the Autoexec.ncf file.

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Figure 12.7 Frame Type Displayed in the NetWare Autoexec.ncf File

You can change the frame type and network number on a Windows 2000 computer. You must be a member of the Administrator group to do so.

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Caution

In most cases, you do not need to change the frame type and network number because Auto Detect should correctly detect them. If you choose an incorrect setting, your client cannot connect to resources using IPX.

To change the internal network number

  1. In Control Panel, double-click Network and Dial-up Connections .

  2. Right-click a connection, and then click Properties .

  3. On the General tab, click NWLink IPX/SPX/NetBIOS Compatible Transport Protocol , and then click Properties .

  4. Type a unique value in the Internal network number box, and then click OK .

To change the frame type and external network number

  1. In Control Panel, double-click Network and Dial-up Connections .

  2. Right-click a connection, and then click Properties .

  3. On the General tab, click NWLink IPX/SPX/NetBIOS Compatible Transport Protocol , and then click Properties .

  4. Select Manual frame type detection , and then click Add .

  5. In the Manual Frame Detection dialog box, in Frame type , click a frame type.

  6. In Network number , type the appropriate network number, and then click Add .

  7. Repeat steps 4-6 for each frame type you want to include, and then click OK .

Both Windows 2000 and NetWare support frame types for Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) topologies.

Table 12.2 lists the frame types that Windows 2000 supports.

Table   12.2 Supported Frame Types

Network Type

Supported Frame Types

Ethernet

Ethernet II, 802.2, 802.3, 802.2 Subnet Access Protocol (SNAP)

Token Ring

802.5 and 802.5 SNAP

FDDI

802.2, 802.3, and SNAP

Frame types are based upon the Institute of Electrical and Electronics Engineers, Inc., (IEEE) standards and define packet formats used by various topologies.

Figure 12.8 shows the IEEE defined frame type packet structure.

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Figure 12.8 NWLink and Supported IPX Frame Types

Ethernet II

The Ethernet II frame includes a 2-byte Type field that immediately follows the source address. The Type field contains a unique value, called an EtherType , which identifies the receiving computer for which the upper-layer protocol should handle the contents of the Data field.

Ethernet 802.2

Several frame types are based on the IEEE 802.3 standards, which define operations for the physical layer and the media access control (MAC) sublayer. NetWare combines the IEEE 802.3 standard for the physical layer and the IEEE 802.2 standard for operations of the Logical Link Control (LLC) sublayer and refers to the combination of these standards as the Ethernet 802.2 frame type.

Ethernet II and Ethernet 802.2 frames can run on the same network. Because the value in the Ethernet II Type field always equals a number greater than 1,500 and the value in the Ethernet 802.2 Length field always equals a number 1,500 or less, network hardware and software can distinguish between these two frame types; therefore, both can co-exist on the same network.

Subnet Access Protocol

The Subnet Access Protocol (SNAP) frame type is derived from the Ethernet 802.2 frame type. The SNAP portion of the frame is carried in the Data field. This frame type was designed to give individual vendors the ability to assign their own unique values for protocols running on their hardware.

Ethernet 802.3

Like the 802.2 frame, the 802.3 frame differs from the Ethernet II frame by using a Length field in the place of the Type field in Ethernet II. It is often called "802.3 raw" because it does not use the 802.2 LLC header in the Data field.

Token Ring 802.5

The Token Ring protocol is composed of two distinct frame subtypes, the MAC frames and non-MAC frames.

The MAC frames carry management information for the Token Ring network. They carry signaling information and are not passed on by bridges and routers. They reside only on the LAN ring where the packets originate.

The non-MAC frames carry data for Token Ring networks. Non-MAC frames carry data between computer stations on the ring and are passed on by bridges and routers.

The content of fields early in a Token Ring frame can redefine the composition of fields later in the frame.

Network Numbers

NWLink can be configured with two distinctly different types of network numbers for routing purposes: the external network number and the internal network number. NWLink uses an IPX external network number to identify the computer's local network segment within the routed environment. NWLink also uses a virtual network number, known as an internal network number, to identify a logical network inside the computer. The logical network inside the computer is used to provide optimal routing to services running on the computer.

In Windows 2000 you can change both the external network number and internal network number in Control Panel. For information about procedures on manually setting network numbers, see "NWLink Auto Detect," earlier in this chapter.

For more information about network numbers see "IPX Routing" in this book.

External Network Number

The external network number is associated with the physical network adapter and the local area network segment (a segment is analogous to a TCP/IP subnet). The external network number identifies the computer's local segment and is used to facilitate communications within the routed environment. Packets transmitted across the network contain the computer's external network number or, if an internal number exists, the internal network number, along with an appended MAC address to identify the individual computer. All computers on the same network segment that use a given frame type must have the same external network number, and that number must be unique for each network segment.

If the external network number on this segment changes, if you replace a network adapter, or if you connect the computer to a different segment, you might not be able to communicate over IPX. If you lose IPX connectivity, you need to reset the external network number and then, if prompted, reboot the computer.

To determine the external network number and frame type that have been set on a computer running Windows 2000, execute the IPXROUTE CONFIG command in the command-line interface.

To determine the external network number and frame type that have been set on a server running NetWare, enter CONFIG in the command-line interface of the NetWare server or inspect the Autoexec.ncf file.

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Note

It is possible to run multiple IPX networks over the same physical network segment if those networks use different frame types.

Internal Network Number

The internal network number is used for internal routing purposes. Internal network numbers are used by service-supporting hosts; that is, hosts running services that SAP-based clients need to access. Service-supporting hosts use internal network numbers to help clients determine the optimal route for transmitting packets to services running on the service-supporting hosts. If only external network numbers are used, different routers might send a client host multiple routes with the same route metrics, when only one route is actually optimal. However, if you specify an internal network number, you create a virtual network inside the service-supporting host, and packets are always forwarded to the services running on the service-supporting host using an optimal path. For more information about how the internal network number is used to provide optimal routing, see "IPX Routing" in this book.

By default, an internal network number is not specified. When NWLink is installed, the Auto Detect feature does not detect the internal network number automatically. Instead, NWLink sets the internal network number of 00000000 on the computer's network adapter by default. When set to 00000000, the internal network number is not used.

This default behavior works when NWLink is bound to a single network adapter or to multiple network adapters. However, you must have an internal network number when your computer is acting as a server running Windows 2000 for an application that uses NetWare SAP, such as SQL Server, Microsoft Exchange, or SNA Server.

Depending on your configuration, you might already have an internal network number. File and Print Services for NetWare, Routing and Remote Access service, and the SAP agent automatically install a unique internal network number during its setup procedures. If you have previously installed File and Print Services for NetWare, Routing and Remote Access service, or the SAP agent, you might already have a unique internal network number set for your computer's network adapter.

The default value of zero (00000000) for the internal network number is not seen when you enter the IPXROUTE CONFIG command on the command prompt. However, if you have configured such an internal network number, you can later view it by using ipxroute config , or in a trace of a RIP or a SAP broadcast from your computer.