Routers inspect layer 3 of the message, which contains the network and station address of the packet's destination. If the packet is destined for the WAN, the router performs a layer 2 conversion by stripping off the LAN packet (Ethernet, Token Ring, FDDI), encapsulating the data into a WAN packet (Frame Relay, X.25, HDLC) and sending it on its merry way.
Because routers inspect layer 3 of the protocol, which has much more information than layer 2, they can determine the subnet (domain) the user is in and the type of message being sent (e-mail, Web access, file transfer, etc.). They can keep human resources traffic from getting to engineering and they can filter traffic based on "who should be doing what" kinds of criteria.
Routers have been used as network backbones for years. First as a series of routers cabled together, then as a collapsed backbone, using a type of router with a high-speed backplane similar to a switch. Standing at the center of the network, they improve performance by providing a central high-speed crossover between LAN segments.
The problem with routers is overhead. The common protocols in use (IPX, IP, etc.) are designed as connectionless systems, which means that the data message transmitted is split into multiple packets, each of which is a fully self-contained entity with source and destination addresses. Routers back up the protocol stack to analyze layer 3 of every packet they receive.
There is a trend toward multilayer switches, which can examine layer 3 and switch at layer 2. They inspect the first packet of the message (layer 3) and forward the remaining packets at layer 2 at higher speed because there is less processing at that layer. Cabletron has perhaps the most comprehensive multilayer approach, because it has implemented this on both LAN switches and ATM switches, while providing solid network management of the entire infrastructure. Ipsilon Networks provide a similar capability, but only for IP traffic over ATM (a bit more on this later).
There will always be a need for routing. The question is whether it is done in a router or in a device that does routing. Also, depending on which vendor you talk to, routers are either on the verge of extinction or thriving like mad. Cisco, being the largest router manufacturer, has a vested interest in high-end, high-priced routers much like IBM has in its mainframes. However, Cisco routers work well and many network managers are thoroughly familiar with them. IBM mainframes work well and many IS managers are thoroughly familiar with them. We could go on and on here.
VIRTUAL LANS AND VIRTUAL ROUTING
Traditionally, in order to improve traffic on a LAN that starts to get overloaded, a bridge or port configuration hub is used to split the LAN into multiple segments. These devices keep LAN packets transmitting within their own segment from wasting bandwidth in other segments. If a packet's destination is outside the segment, it is then bridged or routed to that segment.
LAN segments can be physically broken apart in the wiring closet, but virtual LANs, or VLANs, allow the operation to be performed logically rather than physically. Users are placed into a logical segment based upon their need to communicate with each other. Hence, traffic is contained within the logical segments, and users can be reassigned via software whenever they move to a different location.
Although the ATM Forum is expected to standardize VLANs on ATM switches in the future, there is no standardization of VLANs on LAN switches. Vendors' implementations are proprietary. In addition, although VLANs work at layer 2, they
still require a layer 3 router in order to send a message from one VLAN to another.
The problem with VLANs is that they are difficult to troubleshoot, because the logical segments don't mirror the physical segments. After all, that's the whole idea. They