IE-SW-VL08M Series User’s Manual
Featured Functions
3-34
Rapid Spanning Tree Protocol (RSTP) implements the Spanning Tree Algorithm and Protocol
defined by IEEE Std 802.1w-2001. RSTP provides the following benefits:
y
The topology of a bridged network will be determined much more quickly compared to STP.
y
RSTP is backward compatible with STP, making it relatively easy to deploy. For example:
¾
Defaults to sending 802.1D style BPDUs if packets with this format are received.
¾
STP (802.1D) and RSTP (802.1w) can operate on different ports of the same switch. This
feature is particularly helpful when switch ports are connected to older equipment, such as
legacy switches.
RSTP provides essentially the same functionality as STP. To see how the two systems differ, see the
Differences between RSTP and STP
section in this chapter.
NOTE
The STP protocol is part of the IEEE Std 802.1D, 1998 Edition bridge specification. The following
explanation uses bridge instead of switch.
What is STP?
STP (802.1D) is a bridge-based system that is used to implement parallel paths for network traffic.
STP uses a loop-detection process to:
y
Locate and then disable less efficient paths (i.e., paths that have a lower bandwidth).
y
Enable one of the less efficient paths if the most efficient path fails.
The figure
below shows a network made up of three LANs separated by three bridges. Each segment
uses at most two paths to communicate with the other segments. Since this configuration can give
rise to loops, the network will overload if STP is NOT enabled.
If STP is enabled, it will detect duplicate paths and prevent, or
block
, one of them from forwarding
traffic. In the following example, STP determines that traffic from LAN segment 2 to LAN segment
1 should flow through Bridges C and A as this path has a greater bandwidth and is therefore more
efficient.
Bridge B
Bridge C
LAN 1
LAN 2
LAN 3
Bridge A
