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Bridging, Switching, and Spanning Tree 147

Step 3

The PC requests name resolution by the DNS for the Web server's
name.

Step 4

The DNS returns the IP address of the Web server to the PC.

Step 5

The PC does not know the Web server's MAC address, but it does
know its IP address, so the PC sends an ARP broadcast to learn the
MAC address of the Web server.

Step 6

The Web server replies to the ARP, stating that its MAC address is
0200.3333.3333.

Step 7

The PC can now send frames directly to the Web server.

Now consider the same protocol flow, but with the DNS on a separate segment and a transparent
bridge separating the segments, as shown in Figure 4-7. The computers act no differently,
sending the same frames and packets. The transparent bridge forwards all broadcasts, all unicast
destination frames not in its bridge table, and multicasts.

Figure 4-7 illustrates several important ideas related to segmentation. The ARP requests in
Steps 1 and 5 are forwarded by the bridge because they are broadcasts. Likewise, requests from
the client to and from the DNS are forwarded. However, the rest of the frames from the client
to the Web server and back are not forwarded by the bridge because the bridge knows that both
MAC addresses (client and Web server MACs) are on the same Ethernet as its E0 interface.
Also, because there is no redundant path through other bridges, there is no need to use the
Spanning-Tree Protocol to block interfaces and limit the flow of frames.

Some characterizations of transparent bridge behavior, as compared to a single segment with
no bridges, are listed here:

Broadcasts and multicast frames are forwarded by a bridge.

Transparent bridges perform switching of frames using Layer 2 headers and Layer 2 logic
and are Layer 3 protocol-independent. This means that installation is simple because no
Layer 3 address group planning or address changes are necessary. For example, because
the bridge retains a single broadcast domain, all devices on all segments attached to the
bridge look like a single subnet.

Store-and-forward operation is typical in transparent bridging devices. Because an entire
frame is received before being forwarded, additional latency is introduced (as compared
to a single LAN segment).

The transparent bridge must perform processing on the frame, which also can increase
latency (as compared to a single LAN segment).

ch04.fm Page 147 Monday, March 20, 2000 5:02 PM