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Bridging, Switching, and Spanning Tree 155
Comparison of LAN Segmentation Using Bridges, Switches, and
Routers
Cisco expects CCNAs to have command of the tradeoffs involved when designing campus
LANs. One key consideration is to understand the different behavior when separating, or
segmenting, LAN segments with some switching device. All the concepts related to LAN
segmentation using bridges, switches, and routers are included in other parts of this chapter or
in other chapters. However, the comparisons made in this section are important and would be
missed if this book simply covered the technical content when covering bridging, switching,
and routing.
The basic workings of a bridge and a switch have already been covered in this chapter. For
review, routing logic is covered first, followed by a comparison of segmentation using the three
different device options.
Routing is covered more fully in other chapters. For comparison, the same example flow of a
client connecting to a Web server is shown, this time with a router separating two Ethernet
segments. This same example is shown for bridging and switching earlier in this chapter. Figure
4-13 illustrates a couple of key features of routing.
The flows in the figure match the numbers in this list, which explains the meaning and
implications of the flows in the figure:
Step 1
The PC is preconfigured with the IP address of the DNS. The PC
notices that the IP address is on a different subnet, so the PC wants
to forward the packet to its default router. However, the PC does
not know its default router's MAC address yet, so it must use ARP
to find that router's MAC address.
Step 2
The router replies to the ARP request with its MAC address,
0200.4444.4444.
Step 3
The PC requests name resolution for the Web server by sending a
packet with the destination IP address of the DNS. The destination
MAC address in the frame sent by the PC is the router's E0 MAC
address. The router receives the frame, extracts the packet, and
forwards it.
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. The router has no
need to forward the ARP broadcast.
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.
ch04.fm Page 155 Monday, March 20, 2000 5:02 PM