Owner's Manual

520 | Chapter 18. STP
NETGEAR 8800 User Manual
Overview
STP is a bridge-based mechanism for providing fault tolerance on networks. STP allows you
to implement parallel paths for network traffic and to ensure that redundant paths are:
• Disabled when the main paths are operational.
• Enabled if the main path fails.
Compatibility Between IEEE 802.1D-1998 and IEEE
802.1D-2004 STP Bridges
The IEEE 802.1D-2004 compliant bridges interoperate with the IEEE 802.1D-1998 compliant
bridges. To ensure seamless operation of your STP network, read this section before you
configure STP.
Differences in behavior between the two standards include the:
• Default port path cost
• Bridge priority
• Port priority
• Edge port behavior
This section describes the bridge behavior differences in more detail.
Default Port Path Cost
The 802.1D-2004 standard modified the default port path cost value to allow for higher link
speeds. A higher link speed can create a situation whereby an 802.1D-1998 compliant bridge
could become the more favorable transit path.
For example, in Figure 32, bridge A is the root bridge running the new 802.1D-2004
standard, bridges B and C are running the old 802.1D-1998 standard, and bridges D, E, and
F are running the new 802.1D-2004 standard. In addition, all ports are 100 Mbps links. The
ports on bridges B and C have a default path cost of 19, and the ports on bridge A, D, E, and
F have a default path cost of 200,000.