Network Layer Devices
The most common network device found at the Network layer is a router; however, Layer 3 switches may also be implemented to create a WAN. Both routers and Layer 3 switches can carry out these functions:
- Suppress broadcasts or multicasts
- Determine the best path for data transfer (routing)
- Strip down and add to Data Link layer frames
- Implement access lists for packet filtering (permit/deny statements)
- Set up quality of service (QoS) qualifiers to measure network performance
It is important to know that both these devices can be used at the Network layer. However, for the purpose of the CCNA exam, routers are more widely recognized and, therefore, are referred to when discussing Layer 3 functions.
Routers join a minimum of two networks together to create an internetwork or WAN. So far, we have discussed devices that are used at the Physical layer (hubs and repeaters) and the Data Link layer (Layer 2 switches and bridges). Layer 2 switches and bridges create a separate collision domain for each segment of the LAN. Routers and Layer 3 switches create a separate broadcast domain for each segment of a WAN. A broadcast domain is a group of nodes that can receive one another’s broadcast messages. Figure 5.7 demonstrates how a router creates broadcast domains whereas the connected switches create collision domains.
Figure 5.8 demonstrates a simple LAN with one router and two segments. In this network, any traffic that is generated by Matt’s PC has the source MAC and source IP address of that PC. If Matt is sending a frame to the server on the other segment of that WAN, the destination IP address will be that of the server he is trying to reach. Because the server is not on the same segment as Matt, the destination MAC address is that of the router, which is the default gateway. The router takes a look at the frame and at its own routing table. It then decides what interface to use to forward the frame based on the network portion of the IP address. The router attaches its own MAC address as the source MAC address of the frame before sending
the frame to the server.
A routing table on a router contains the following information:
- Network Address
- Interface: Exit interface used to forward packets
- Metric: Distance to reach a remote network
For the exam, you should understand how a packet traverses the network and determine the source and destination IP and MAC address as the packet moves from device to device.
Figure 5.9 exemplifies a WAN with two routers. Each router has a separate routing table to make best path decisions.
Routers provide packet switching between networks and can provide packet filtering based on a network address or application layer port level.
Routers provide internetwork communication, packet switching, and packet filtering.
Two packet types are used at Layer 3:
- Data packets: Transport data across the internetwork and are supported by routed protocols such as IP and IPX.
- Route update packets: Send updates to neighbor routers about all networks connected
to that internetwork and are supported by routing protocols such as RIP, EIGRP, and OSPF.
Specific Cisco router models are reviewed in Chapter 6, “Introduction to Cisco Routers and Switches.” IP configurations are covered in Chapter 8, “Foundation Cisco Configurations.”
Chapter 10, “Introduction to Routing and Routing Protocols,” details routing terminology.
Chapter 11, “Distance Vector Routing Protocols,” reviews RIP and IGRP routing protocols.
Chapter 12, “Link-State and Hybrid Routing Protocols,” discusses OSPF, RIPv2, and EIGRP routing protocols.
Layer 3 Switches
Layer 3 switches are typically called multilayer switches. I already listed the commonalities between routers and Layer 3 switches. There are also a couple of differences worth mentioning. The number one difference between a router and a Layer 3 switch is packet switching throughput. Whereas a router has evolved over the years to process more than one million packets per second (pps), a Layer 3 switch can process millions of pps. That said, Layer 3 switches process more traffic in a shorter time. Whereas routers use microprocessor-based engines, Layer 3 switches use ASIC hardware to perform packet switching. Layer 2 switches use ASIC hardware to forward frames.
The Cisco Catalyst 8500 series switch is an example of a Layer 3 switch. Layer 3 switches are recommended for Campus networks.