英文原版文献：
Internetworking Basics
Networks and networking have grown exponentially over the last 15 years—understandably so. They’ve had to evolve at light speed just to keep up with huge increas in basic missioncritical ur needs such as sharing data and printers, as well as more advanced demands such as video conferencing. Unless everyone who needs to share network resources is located in the same office area (an increasingly uncommon situation), the challenge is to connect the sometimes many relevant networks together so all urs can share the networks’ wealth.
It’s also likely that at some point, you’ll have to break up one large network into a number of smaller ones becau ur respon has dwindled to a trickle as the network grew and grew and LAN traffic congestion reached overwhelming proportions. Breaking up a larger network into a number of smaller ones is called network gmentation, and it’s accomplished using routers, switches, and bridges.
Possible caus of LAN traffic congestion are
(1)Too many hosts in a broadcast domain
(2)Broadcast storms
(3)Multicasting
(4)Low bandwidth
(5)Adding hubs for connectivity to the network
(6)A large amount of ARP or IPX traffic (IPX is a Novell routing protocol that is like IP, but really, really chatty)
Routers are ud to connect networks together and route packets of data from one network to another. Cisco became the de facto standard of routers becau of their high-quality router products, great lection, and fantastic rvice. Routers, by default, break up a broadcast domain, which is the t of all devices on a netwo
rk gment that hear all broadcasts nt on that gment. Breaking up a broadcast domain is important becau when a host or rver nds a network broadcast, every device on the network must read and process that broadcast—unless you’ve got a router. When the router’s interface receives this broadcast, it can respond by basically saying“Thanks, but no thanks,” and discard the broadcast without forwarding it on to other networks. Even though routers are known for breaking up broadcast domains by default, it’s important to remember that they break up collision domains as well.
Two advantages of using routers in your network are
(1)They don’t forward broadcasts by default.
(2)They can filter the network bad on layer 3 (Network layer) information (i.e., IP address).
Four router functions in your network can be listed as
(1)Packet switching
(2)Packet filtering
(3)Internetwork communication
(4)Path lection
Remember that routers are really switches, but they’re actually what we call layer 3 switches (we’ll talk about layers later in this chapter). Unlike layer 2 switches that forward or filter frames, routers (layer 3 switches) u logical addressing and provide what is called packet switching. Routers can also provide packet filtering by using access-lists (discusd in Chapter 10), and when routers connect two or more networks together and u logical addressing (IP), this is called an internetwork. Lastly, routers u a routing table (map of the internetwork) to make path lections and to forward packets to remote networks.
Converly, switches aren’t ud to create internetworks, they’re employed to ad
d functionality to an internetwork LAN. The main purpo of a switch is to make a LAN work better—to optimize its performance—providing more bandwidth for the LAN’s urs. And switches don’t forward packets to other networks as routers do. Instead, they only “switch” frames from one port to another within the switched network. Okay, you may be thinking, “Wait a minute, what are frames and packets?” I’ll tell you all about them later in this chapter, I promi!
By default, switches break up collision domains. This is an Ethernet term ud to describe a network scenario wherein one particular device nds a packet on a network gment, forcing every other device on that same gment to pay attention to it. At the same time, a different device tries to transmit, leading to a collision, after which both devices must retransmit, one at a time. Not very efficient! This situation is typically found in a hub environment where each host gment connects to a hub that reprents only one collision domain and only one broadcast domain. By contrast, each and every port on a switch reprents its own collision domain.
Note：Switches create parate collision domains, but a single broadcast domain.
Routers provide a parate broadcast domain for each interface.
The term bridging was introduced before routers and hubs were implemented, so it’s pretty common to hear people referring to bridges as “switches.” That’s becau bridges and switches basically do the same thing—break up collision domains on a LAN. So what this means is that a switch is basically just a multiple-port bridge with more brainpower, right? Well, pretty much, but there are differences. Switches do provide this function, but they do so with greatly enhanced management ability and features. Plus, most of the time, bridges only had two or four ports. Yes, you could get your hands on a bridge with up to 16 ports, but that’s nothing compared to the hundreds available on some switches!
