Routing & Switching Strategies
A large number of decisions are made in order to get a single packet across a network. These decisions are made at each and every device, including routers, switches, and hosts.
Switching: Forwarding & Filtering Traffic
There are many types of switching: packet, circuit, multilayer, virtual circuit, wide area network (WAN), and local area network (LAN). Circuiting and virtual circuit switching almost always refer to WAN or telephone technologies. Packet switching usually concerns a router or a WAN switch.
It is highly probable that a network will be a mixture of Ethernet and 802.11 nodes. These nodes will run the Internet Protocol at Layer 3 of the Transmission Control Protocol/Internet Protocol (TCP/IP) networking model. The applications will be designed for TCP or the User Datagram Protocol (UDP).
TCP/IP Model
| Application | FTP, telnet, email, games, printing, HTTP |
| Transport | Transmission Control Protocol (TCP), User Datagram Protocol (UDP) |
| Internet (Internetwork) | Internet Protocol (IP), ICMP, IGMP |
| Link layer (Network) | Ethernet, 802.11 |
| Physical | Ethernet, 802.11 |
Switches operate at Layer 2 of the TCP/IP model or the Open System Interconnection (OSI) model. Switches forward data packets on the same network using MAC addresses. At Layer 2, the data-link layer, packets are known as Ethernet frames. Switches know where to forward Ethernet frames by keeping a table of MAC addresses.
Switches also:
- Process the Cyclical Redundancy Check (CRC)
- Filter out traffic that should not be forwarded, such as local unicast frames
- Prevent the forwarding of collisions
- Prevent the forwarding of frames with errors
Switches provide a collection of features that are part of most medium and large networks:
- Virtual local area networks (VLANs)
- Simple network management protocol (SNMP)
- Remote management
- Statistics collection
- Port mirroring
- Security such as 802.1X port-based authentication
Routing: Finding Paths
Routing is typically divided into two components: host and router. Routers handle traffic flowing between networks, but hosts make many decisions long before the packets hit the network. Most routing protocols used to find pathways to destinations are router-based, however.
Routers operate at Layer 3, the internetwork or network layer of the TCP/IP model. A router’s main function is to forward traffic to destination networks via the destination address in an IP packet. Routers know where to send data packets by maintaining a table of IP addresses.
Routers also resolve MAC addresses by using the Address Resolution Protocol (ARP). At the network layer, data is known as a packet, and Layer 2 (data-link layer) frames and MAC addresses do not exist beyond the router. This means that an Ethernet frame is destroyed once it hits a router.
When operating in a network, a router can act as the default gateway for hosts, as in most home networks. A router can be installed as an intermediate hop between other routers without any direct connectivity to hosts. Routers can also perform other tasks such as network address translation, managing access control lists, terminating virtual private networks, or quality of service.
