Limiting Bridge Frame Sizes - 3Com PathBuilder S200 Series Owner's Manual

Limiting Bridge Frame Sizes

Overview
Although there are valid reasons for using larger frame sizes on bridges, there are
limiting factors that must be considered when selecting a maximum frame size.
There are several reasons for limiting the maximum size of the frame, especially
where bridging is done remotely across a WAN:
Conversely, the smaller the frame, the less time it takes to transmit the frame. Since a
smaller frame cannot be forwarded by an intermediary node until it is fully received,
a large frame cannot be forwarded for the time it takes to transmit and receive it over
a given link. On a LAN, this time is less of an issue where the link speed is
approximately 10 Mbps. On a WAN link, it becomes an issue because the
transmission times for large frames become significant.
Example of Frame
Figure 17 shows the effect on transit delay across a network for two cases: in one, an
Sizes end system sends a large packet as a single frame and in the other, the same large
packet is sent as three smaller packets.
Large Packet
Total Transit
Small Packet
Total Transit
Figure 17. How Packet Size Affects Transit Delay
Small packets are forwarded more quickly by intermediate nodes resulting in the end
system receiving several short frames in less time than a long frame. How much
improvement is achieved depends on the transmission times and line speeds
involved. The trade-off in this case is that even though the transit delay is reduced,
the packet-per-second load is increased on all three nodes (and two end systems)
involved. In this case, the factor is at least three if continuous streams of packets are
involved.
Bridging
T0008-16F
Configuring the PathBuilder S200 Series Switch for Bridging Operation
• The larger the frame, the longer it takes to transmit the frame on a WAN link.
• Increasing the frame size also causes a reduction in frame overhead.
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