Advantages of Routers

Because routers operate at a higher layer than the network layer and use logical addressing, they provide many advantages over bridges and switches, including:

■ Logical addressing at layer-3 allows you to build hierarchical networks that scale to very large sizes. This is discussed in Chapter 12.
■ They contain broadcasts and multicasts. When a broadcast or multicast is received on an interface, it is not forwarded to
another interface, by default. Routers are used to solve broadcast problems. (Actually, routers also create separate bandwidth and collision domains, but bridges and switches provide a cheaper solution.)
■ Routers can typically find a better path to a destination than bridges, since routing protocols support a rich metric structure.
■ Routers allow you to connected different media types together, like Ethernet and Token Ring or FDDI and PPP, without any conversion issues.
■ Routers can switch packets on the same interface using VLANs. (VLANs are discussed in Chapter 8.)
■ Routers have advanced features that allow you to implement Quality of Service using queuing or traffic shaping, filtering traffic using access lists, or protecting traffic using encryption. (Access lists are discussed in Chapter 13.)

By using logical addresses, routers can create a hierarchical network that supports thousands of devices. Bridges and switches, on the other hand, do not support hierarchical addressing: MAC addresses support a flat addressing space. In other words, you can’t typically change MAC addresses to fit a specific network layout. Also, since routers use logical addresses, it is much easier to implement policy decisions, such as traffic filtering or quality service, since the decisions are made on logical, more easily handled addresses than the physical addresses that bridges and switches use. For example, since logical addresses support a network component, you could filter an entire network number. To accomplish this with a bridge, you would have to filter each individual device’s MAC address within the network segment.

Another problem with layer-2 devices is that they don’t operate very well when connecting different media types, Ethernet and Token Ring, for instance. At layer 2, this process is called translational bridging. There are many reasons why layer-2 devices have issues translating media types, but the main reason is that since both topologies are layer-2, the bridge has to translate the layer-2 information from the different media types. This is very process-intensive and can create many problems.

For example, Ethernet supports frame sizes up to 1,500 bytes, while Token Ring supports frame sizes up to 16KB in size for 16Mbps speeds. Therefore, if a large Token Ring frame had to be sent to an Ethernet segment, the bridge would have to fragment the information into multiple Ethernet frames. There might also be a speed difference between the media types: Ethernet supports 10Mb while Token Ring supports 4Mbps, 16Mbps, and 100Mbps, and this difference could cause congestion problems on a bridge or switch.

Also, the translation process between frame types is not always easy. For example, some media types order their bits from low-to-high, while others order them high-to-low, which can create translation issues. Fortunately, routers provide a clean solution to this translation process. Routers don’t actually translate between different frame or media types; instead, they strip off the layer-2 frame, make a routing decision on the layer-3 packet, and then encapsulate the layer-3 packet in the correct layer-2 frame type for the interface the packet needs to exit. This process is described more thoroughly later in this chapter, in the section “Transferring Information Between Computers.”
Another advantage routers have over layer-2 devices is that they contain broadcast problems. When a router receives a broadcast, it processes that broadcast, but by default, it will not forward the broadcast out any of its other ports. This is different from bridges and switches, which flood broadcast traffic. If broadcasts are affecting the bandwidth and performance of your network, you should break up your network into multiple broadcast domains and use a router to route between the different domains. Each broadcast domain in a network needs a unique layer-3 network number.