Multiple Frame Transmissions
In a redundant topology, multiple copies of the same frame can arrive at the intended host,
potentially causing problems with the receiving protocol. Most protocols are not designed
to recognize or cope with duplicate transmissions. In general, protocols that use a sequencenumbering
mechanism like TCP assume that many transmissions have failed and that the
sequence number has recycled. Other protocols attempt to hand the duplicate transmission
to the appropriate upper-layer protocol (ULP), with unpredictable results.
Example: Multiple Transmissions
Figure 2-19 illustrates how multiple transmissions can occur.
Server/Host X Router Y
Broadcast
Segment 1
Segment 2
Broadcast
Switch A Switch B
48 Chapter 2: Medium-Sized Switched Network Construction
Figure 2-19 Multiple Frame Transmissions
The following describes how multiple copies of the same frame can arrive at the intended
host:
1. When host X sends a unicast frame to Router Y, one copy is received over the direct
Ethernet connection, segment 1. At more or less the same time, switch A receives a
copy of the frame and puts it into its buffers.
2. If switch A examines the destination address ?¬?eld in the frame and ?¬?nds no entry in the
MAC address table for router Y, switch A ?¬‚oods the frame on all ports except the
originating port.
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