location of devices on the network, and determines the best way to move data. This means that it’s up to the Network layer to transport traffic between devices that aren’t locally attached. Routers, which are layer 3 devices, are specified at this layer and provide the routing services within an internetwork.
Here’s how that works: first, when a packet is received on a router interface, the destination IP address is checked. If the packet isn’t destined for that particular router, it will look up the destination network address in the routing table. Once the router chooses an exit interface, the packet will be sent to that interface to be framed and sent out on the local network. If the router can’t find an entry for the packet’s destination network in the routing table, the router drops the packet.
Data and route update packets are the two types of packets used at the Network layer:
Data Packets These are used to transport user data through the internetwork. Protocols used to support data traffic are called routed protocols, and IP and IPv6 are key examples. I’ll cover IP addressing in Chapter 3, “Introduction to TCP/IP,” and Chapter 4, “Easy Subnetting,” and I’ll cover IPv6 in Chapter 14, “Internet Protocol Version 6 (IPv6).”
Route Update Packets These packets are used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route update packets are called routing protocols; the most critical ones for CCNA are RIPv2, EIGRP, and OSPF. Route update packets are used to help build and maintain routing tables.
Figure 1.14 shows an example of a routing table. The routing table each router keeps and refers to includes the following information:
FIGURE 1.14 Routing table used in a router
Network Addresses Protocol-specific network addresses. A router must maintain a routing table for individual routing protocols because each routed protocol keeps track of a network with a different addressing scheme. For example, the routing tables for IP and IPv6 are completely different, so the router keeps a table for each one. Think of it as a street sign in each of the different languages spoken by the American, Spanish, and French people living on a street; the street sign would read Cat/Gato/Chat.
Interface The exit interface a packet will take when destined for a specific network.
Metric The distance to the remote network. Different routing protocols use different ways of computing this distance. I’m going to cover routing protocols thoroughly in Chapter 9, “IP Routing.” For now, know that some routing protocols like the Routing Information Protocol, or RIP, use hop count, which refers to the number of routers a packet passes through en route to a remote network. Others use bandwidth, delay of the line, or even tick count (1/18 of a second) to determine the best path for data to get to a given destination.
And as I mentioned earlier, routers break up broadcast domains, which means that by default, broadcasts aren’t forwarded through a router. Do you remember why this is a good thing? Routers also break up collision domains, but you can also do that using layer 2 (Data Link layer) switches. Because each interface in a router represents a separate network, it must be assigned unique network identification numbers, and each host on the network connected to that router must use the same network number. Figure 1.15 shows how a router works in an internetwork.
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