Actually, this is not about routers that have Token Ring Interfaces, but routers whose only LAN interface is Token Ring. There are routers that have both Ethernet and Token Ring interfaces, and are good candidates for your lab. All routers should also have serial interfaces.
The concepts described herein will probably be above any newbie hedgeling, but we are going to give it a shot.
IP networks and subnets are added to a network by giving a router interface an IP address from the new IP network/subnet. In effect, that makes the router interface the "door" to the new IP network/subnet. When the interface goes active, or "up, up," the router supporting that interface will broadcast the new IP network/subnet to the entire network. All routers in the network will create a route to the new IP network/subnet based on the rules of the routing protocol that is being used.
A routing table is a map. It contains routes to all of the networks/subnets in your network, and uses the routes as a map to deliver packets to an IP network/subnet. It’s kind of like, “The destination IP address for that packet is in that IP network/subnet. The routing table say that my path to that network/subnet is through that interface.” And the router transmits the packet on the specified interface to a neighboring router, the neighboring router does the same thing, and eventually the packet gets delivered.
The problem is that a network/subnet will not exist in the larger network until the interface is active, or "up, up." When an interface goes active, the router will advertise that the new IP network/subnet is in the network. All physical interfaces, serial, Ethernet or Token Ring, must be electrically activated to go "up, up." With serial interfaces and Ethernet interfaces, it’s easy. Connect the serial interface correctly to a serial interface on another router, and you can get the interface to activate. For an Ethernet interface, connect it to a switch port and the Ethernet interface can be activated.
To use a Token Ring interface, you must connect to the interface to a Token Ring MSAU, another piece of equipment that is no longer used by anyone. Otherwise, the Token Ring interface will not activate, and might as well not exist. So never buy a Token Ring router, right?
Maybe not.
Routers also support a virtual interface called a loopback interface. A loopback interface can be activated without a physical connection. In other words, it is treated by the network as a live interface even thought it does not have a physical connection. It can be given an IP address and the router will advertise the IP network/subnet associated with the IP address. It acts like an Ethernet interface, except it doesn’t actually transmit data.
Which is okay sometimes. Managing routing tables is a major challenge in Cisco networking, whether you are preparing for an exam or running a network. Using a loopback interface allows you to add a new IP network/subnet to the network, and watch the address propagate through the network. A loopback interface is as good as a physical interface for that. Or better, since you can set it up easily. As long as the router has serial interfaces to connect it to the overall network, it will work fine for learning routing protocols and routing table processes. It does not need a LAN interface, so a router that only has a Token Ring LAN interface will work very well for this situation.
Yes, we said that it was okay to get a Token Ring router with four serial ports to serve as your Frame Relay switch. Routers with four serial ports are desirable for this reason, and the price goes up accordingly. If you get a router with Token Ring LAN interface (only) with four serial ports, you can manage the frame relay configurtion by connecting to the console port. Frame relay is a big deal in the Cisco world, both on the CCNA exam as well as in enterprise networks. So any router with four serial ports is good for your lab. Just be a careful shopper. Compare the price to a router with four serial ports AND Ethernet, such as a Cisco 2500 model with those interfaces, or perhaps a Cisco 2600 with those interfaces.
The bottom line: yes, a Token Ring router will work in your lab.
The bottomer line: if you didn’t understand this, don’t get a router that has only a Token Ring LAN interface.
The very bottomest line: If you decide to get a router with a Token Ring interface, don't rely on Token Ring only routers. Make sure you have some routers with Ethernet interfaces.
