router connected to internet through serial 0 port WAN services. Fast Ethernets with different network numbers are also connected to the router."/>FIGURE 1.15 A router in an internetwork. Each router LAN interface is a broadcast domain. Routers break up broadcast domains by default and provide WAN services.
Here are some router characteristics that you should never forget:
■ Routers, by default, will not forward any broadcast or multicast packets.
■ Routers use the logical address in a Network layer header to determine the next-hop router to forward the packet to.
■ Routers can use access lists, created by an administrator, to control security based on the types of packets allowed to enter or exit an interface.
■ Routers can provide layer 2 bridging functions if needed and can simultaneously route through the same interface.
■ Layer 3 devices – in this case, routers – provide connections between virtual LANs (VLANs).
■ Routers can provide quality of service (QoS) for specific types of network traffic.
The Data Link Layer
The Data Link layer provides for the physical transmission of data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and will translate messages from the Network layer into bits for the Physical layer to transmit.
The Data Link layer formats the messages, each called a data frame, and adds a customized header containing the hardware destination and source address. This added information forms a sort of capsule that surrounds the original message in much the same way that engines, navigational devices, and other tools were attached to the lunar modules of the Apollo project. These various pieces of equipment were useful only during certain stages of space flight and were stripped off the module and discarded when their designated stage was completed. The process of data traveling through networks is similar.
Figure 1.16 shows the Data Link layer with the Ethernet and IEEE specifications. When you check it out, notice that the IEEE 802.2 standard is used in conjunction with and adds functionality to the other IEEE standards. (You’ll read more about the important IEEE 802 standards used with the Cisco objectives in Chapter 2, “Ethernet Networking and Data Encapsulation.”)
FIGURE 1.16 Data Link layer
It’s important for you to understand that routers, which work at the Network layer, don’t care at all about where a particular host is located. They’re only concerned about where networks are located and the best way to reach them – including remote ones. Routers are totally obsessive when it comes to networks, which in this case is a good thing! It’s the Data Link layer that’s responsible for the actual unique identification of each device that resides on a local network.
For a host to send packets to individual hosts on a local network as well as transmit packets between routers, the Data Link layer uses hardware addressing. Each time a packet is sent between routers, it’s framed with control information at the Data Link layer, but that information is stripped off at the receiving router and only the original packet is left completely intact. This framing of the packet continues for each hop until the packet is finally delivered to the correct receiving host. It’s really important to understand that the packet itself is never altered along the route; it’s only encapsulated with the type of control information required for it to be properly passed on to the different media types.
The IEEE Ethernet Data Link layer has two sublayers:
Media Access Control (MAC) Defines how packets are placed on the media. Contention for media access is “first come/first served” access where everyone shares the same bandwidth – hence the name. Physical addressing is defined here as well as logical topologies. What’s a logical topology? It’s the signal path through a physical topology. Line discipline, error notification (but not correction), the ordered delivery of frames, and optional flow control can also be used at this sublayer.
Logical Link Control (LLC) Responsible for identifying Network layer protocols and then encapsulating them. An LLC header tells the Data Link layer what to do with a packet once a frame is received. It works like this: a host receives a frame and looks in the LLC header to find out where the packet is destined – for instance, the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits.
The switches and bridges I talked about near the beginning of the chapter both work at the Data Link layer and filter the network using hardware (MAC) addresses. I’ll talk about these next.
Switches and Bridges at the Data Link Layer
Layer 2 switching is considered hardware-based bridging because it uses specialized hardware called an application-specific integrated circuit (ASIC). ASICs can run up to high gigabit speeds with very low latency rates.
Bridges and switches read each frame as it passes through the network. The layer 2 device then puts the source hardware address in a filter table and keeps track of which port the frame was received on. This information (logged in the bridge’s or switch’s filter table) is what helps the machine determine the location of the specific sending device. Figure 1.17 shows a switch in an internetwork and how John is sending packets to the Internet and Sally doesn’t hear his frames because she is in a different collision domain. The destination frame goes directly to the default gateway router, and Sally doesn’t see John’s traffic, much to her relief.
FIGURE 1.17 A switch in an internetwork
The real estate business is all about location, location, location, and it’s the same way for both layer 2 and layer 3 devices. Though both need to be able to negotiate the network, it’s crucial to remember that they’re concerned with very different parts of it. Primarily, layer 3 machines (such as routers) need to locate specific networks, whereas layer 2 machines (switches and bridges) need to eventually locate specific devices. So, networks are to routers as individual devices are to switches and bridges. And routing tables that “map” the internetwork are for routers as filter tables that “map” individual devices are for switches and bridges.
After a filter table is built on the layer 2 device, it will forward frames only to the segment where the destination hardware address is located. If the destination device is on the same segment as the frame, the layer 2 device
