Multiple Frame Transmissions, CCNP Coaching Center in Delhi Gurgaon

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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 field in the frame and finds no entry in the
MAC address table for router Y, switch A floods the frame on all ports except the
originating port.
3. When switch B receives a copy of the frame through switch A on segment 2, switch B
also forwards a copy of the frame to segment 1 if it cannot locate an entry in the MAC
address table for Router Y.
4. Router Y receives a copy of the same frame for the second time.
A loop-avoidance mechanism eliminates this problem by preventing one of the four
interfaces from transmitting frames during normal operation, thereby breaking the loop.
MAC Database Instability
MAC database instability results when multiple copies of a frame arrive on different ports
of a switch. This subtopic describes how MAC database instability can arise and explains
what problems can result.
Figure 2-20 illustrates this problem: switch B installs a database entry, mapping the MAC
address of host X to port 1. Sometime later, when the copy of the frame transmitted through
switch A arrives at port 2 of switch B, switch B removes the first entry and installs an entry
that incorrectly maps the MAC address of host X to port 2, which connects to segment 2.
Server/Host X Router Y
Unicast
Segment 1
Segment 2
Unicast
Unicast
Switch A Switch B
Improving Performance with Spanning Tree 49
Figure 2-20 MAC Database Instability
Depending on its internal architecture, the switch in question may or may not cope well
with rapid changes in its MAC database. Again, a loop-avoidance mechanism eliminates
this problem by preventing one of the four interfaces from transmitting frames during
normal operation, thereby breaking the loop.