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The Catalyst 3900 is a switch, not a concentrator, so Catalyst 3900s can be connected to form large networks. This chapter describes the methods of connecting Catalyst 3900s; in a cascaded topology, using TokenChannels, or as a stack in a back-to-back configuration or using a Catalyst Matrix.
The topology of a cascaded network of Catalyst 3900 switches is important from a performance perspective. A star topology (as shown in Figure 2-1) offers maximum bandwidth and is preferred for network problem determination because no single cable carries all of the traffic. A serial topology reduces capacity and could divide the network in the event of failure.
As in other configurations, the effective capacity between the Catalyst 3900 switches in a cascaded topology is 32 Mbps when the ports on both switches are set up for FDX operation.
Creating parallel paths directly between two Catalyst 3900 switches results in unsatisfactory network operation unless the spanning-tree algorithm or a TokenChannel is configured. Because the spanning-tree algorithm was designed especially to prevent loops in networks that use transparent bridging, the algorithm will block a port of one of the Catalyst 3900 switches in the parallel paths. If the port in the primary path fails, the port that has been blocked will change automatically to the forwarding state, keeping the network working.
You can also interconnect Catalyst 3900s using the stack port at the back to interconnect two switches (in a back-to-back stack) or to interconnect multiple switches with the Catalyst Matrix.
The Catalyst 3900 allows you to configure TokenChannels. TokenChannels consist of two to eight parallel channels between two Catalyst 3900s. These parallel channels provide improved performance between Catalyst 3900s.
A single TokenChannel can consist of a combination of HDX and FDX connections. For example, a TokenChannel consisting of three connections can have one HDX and two FDX connections. However, both ports in each interconnected pair must be either HDX or FDX. In addition, all ports in a single TokenChannel must belong to the same TrCRF on the Catalyst 3900.
 | Caution You cannot use TokenChannels to interconnect different models of switches. For example, you cannot use a TokenChannel to interconnect a Catalyst 2600 and a Catalyst 3900. Likewise, you cannot use a TokenChannel to interconnect a Catalyst 3900 and a non-Cisco switch. |
A stack of Catalyst 3900 switches is not just a connection of several switches. A stack of Catalyst 3900 switches combine to form a "virtual" single switch.
A Catalyst stack is configured in one of the following two ways:
Two Catalyst 3900 switches can be connected to form a stack by using only a stack port cable and an interface card (Catalyst stack port module) plugged into the back of each Catalyst 3900. This creates a direct connection between the two Catalyst 3900 switches, which is referred to as a back-to-back stack. As an alternative, you can use an 8-port Catalyst Matrix switch to create a stack of up to eight Catalyst 3900 switches. The ProStack port operates in FDX mode at speeds of 140Mbps. It switches packets at wire speeds with low forwarding latency. A proprietary 4 byte header is used to allow the members of the stack to function as one operational system.
When you power-on a Catalyst 3900, it runs through a set of self-diagnostics. Immediately after the diagnostics are completed, the Catalyst 3900 runs through a stack discovery mode. This discovery mode is used to sense whether the switch is cabled to another Catalyst 3900. If it is determined during the discovery mode that the Catalyst 3900 is connected to other switches, the switches are automatically combined to form a stack. At the end of the discovery mode, if it is determined that the Catalyst 3900 is not connected to another switch, it will operate as a stand alone switch.
Advantages of the stack include the following:
The stack discovery mode runs after power-on diagnostics. If, during this stack discovery mode, a Catalyst 3900 senses that it is connected to another Catalyst 3900 in a back-to-back configuration, the two switches will begin to form a stack.
As soon as the stack discovery mode is completed, two things happen:
After a stack has formed and sets up the interbox parameters, the stack operates the same way whether it is in a back-to-back configuration or is in a multi-unit configuration using the Catalyst Matrix interface.
Using a Catalyst Matrix, a multi-unit stack of up to 8 Catalyst 3900 switches can be created. The following sections describe how this multi-unit stack is formed.
The Catalyst Matrix is an eight port switch matrix interface that connects up to eight Catalyst 3900 switches. The Catalyst 3900 senses if it is connected to a Catalyst Matrix and also senses if there are other Catalyst 3900 switches connected to that Catalyst Matrix. The connected Catalyst 3900 switches and the Catalyst Matrix combine logically to form a stack.
Any combination of up to eight Catalyst 3900 switches can be connected to or disconnected from the Catalyst Matrix while it, or any of the switches, are powered on or powered off. A proprietary shielded cable, 1 meter in length, with 50-pin connectors, is used to connect the Catalyst stack equipment together. The cable has cross-over wiring so either end can connect to the Catalyst Matrix, or to the Catalyst 3900 switches. The cable is plugged directly into a stack port I/O connector on the back of the Catalyst Matrix. The other end is plugged into a Catalyst stack port module interface card that is installed in the rear expansion slot in the Catalyst 3900.
When Catalyst 3900 switches first power up, they run through a set of self-diagnostics. Immediately after the diagnostics are completed, the Catalyst 3900 switches run through a stack discovery mode. During this stack discovery mode, if 2 or more Catalyst 3900 switches are connected to a Catalyst Matrix, the switches will sense the connection and combine logically to create a stack configuration.
As soon as the stack discovery mode is completed, two things happen:
When a stack is formed, certain configuration information within all of the different Catalyst 3900 switches must combine to form a common configuration (interbox parameters). The stack operates as a single entity when all of the Catalyst 3900 switches in that stack use the same interbox parameters.
The following is a list of these shared interbox parameters. The parameters in this list are accessed through the console configuration menus. The console menus are described in "Configuring the Catalyst 3900" chapter.
For a stack to operate as a single entity, the interbox parameters must be the same in all of the switches in a stack. Before Catalyst 3900 switches first start to form a stack, there are two possible configuration setups between the Catalyst 3900 switches. The first type of configuration setup is that each one of the different Catalyst 3900 switches is preconfigured with the same parameters. When all of the switches are preconfigured with the same information, the procedure for stack forming is described in the previous sections (back-to-back or multi-unit stack).
If the parameters are different in any of Catalyst 3900 switches trying to form a stack, the following procedures:
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