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150 Chapter 4: Bridges/Switches and LAN Design
The following list provides some additional insights relating to the steps shown in Figure 4-8:
Step 1
The PC is preconfigured with the IP address of the DNS. The PC
notices that the DNS IP address is in the same subnet as its own IP
address; therefore, the PC sends an ARP broadcast hoping to learn
the DNS's MAC address.
Step 2
The DNS replies to the ARP request with its MAC address,
0200.2222.2222.
Step 3
The PC requests name resolution for the Web server by sending a
packet with the destination IP address of the DNS.
Step 4
The DNS returns the IP address of the Web server to the PC in the
DNS reply.
Step 5
The PC does not know the Web server's MAC address, so it sends
an ARP broadcast to learn the MAC address. Because it is a MAC
broadcast, the switch forwards the frame on all ports.
Step 6
The Web server replies to the ARP, stating that its MAC address is
0200.3333.3333.
Step 7
The PC can now connect to the Web server.
The two ARP broadcasts (Steps 1 and 5) are sent out all switch ports because switches and
bridges do not perform the broadcast firewall function that a router performs. After the
switching table (often called the address table) is built, the switch forwards unicasts only out of
the appropriate ports. In other words, frames sent from the client to the Web server, and vice
versa (which are unicasts), are never sent out port E2.
The switch network has created three separate Ethernet segments, as compared to the
transparent bridge network in Figure 4-7, which creates two LAN segments. Each segment is
called a collision domain because frames sent by any device on that segment could collide with
other frames on the segment. Switches can be used to create many collision domains.
Another feature of switches is that they forward broadcasts and multicasts on all ports.
However, they reduce the impact of collisions because devices on separate switch ports are on
separate Ethernet segments (which are separate collision domains). This behavior of switches
resulted in the creation of the terms collision domain and broadcast domain. Figure 4-9 shows
a network with six collision domains--six sets of interface cards for which CSMA/CD logic is
used to share the LAN segment.
Each collision domain is separated by either a transparent bridge, a switch, or a router. The
figure suggests that the segments on either side of the bridge could be 10Base2, 10Base5, or any
shared hub. The segment between the router and switch, and between the switch and the PCs
on the right, can be a single cable, as shown. In either case, if a bridge (transparent bridge or
switch) or routing function separates devices, the devices are in separate collision domains.
ch04.fm Page 150 Monday, March 20, 2000 5:02 PM