User`s manual
NPort S8000 Series Switch Featured Functions
6-17
trigger the bridge to reconfigure the network to account for the change. If you have configured an SNMP trap
destination, when the topology of your network changes, the first bridge to detect the change sends out an
SNMP trap.
Differences between RSTP and STP
RSTP is similar to STP, but includes additional information in the BPDUs that allow each bridge to confirm that
it has taken action to prevent loops from forming when it decides to enable a link to a neighboring bridge.
Adjacent bridges connected via point-to-point links will be able to enable a link without waiting to ensure that
all other bridges in the network have had time to react to the change. The main benefit of RSTP is that the
configuration decision is made locally rather than network-wide, allowing RSTP to carry out automatic
configuration and restore a link faster than STP.
STP Example
The LAN shown in the following figure has three segments, with adjacent segments connected using two
possible links. The various STP factors, such as Cost, Root Port, Designated Bridge Port, and Blocked Port are
shown in the figure.
• Bridge A has been selected as the Root Bridge, since it was determined to have the lowest Bridge Identifier
on the network.
• Since Bridge A is the Root Bridge, it is also the Designated Bridge for LAN segment 1. Port 1 on Bridge A is
selected as the Designated Bridge Port for LAN Segment 1.
• Ports 1 of Bridges B, C, X, and Y are all Root Ports sine they are nearest to the Root Bridge, and therefore
have the most efficient path.
• Bridges B and X offer the same Root Path Cost for LAN segment 2. However, Bridge B was selected as the
Designated Bridge for that segment since it has a lower Bridge Identifier. Port 2 on Bridge B is selected as
the Designated Bridge Port for LAN Segment 2.
• Bridge C is the Designated Bridge for LAN segment 3, because it has the lowest Root Path Cost for LAN
Segment 3:
The route through Bridges C and B costs 200 (C to B=100, B to A=100)
he route through Bridges Y and B costs 300 (Y to B=200, B to A=100)Item 3.3
• The Designated Bridge Port for LAN Segment 3 is Port 2 on Bridge C.
Using STP on a Network with Multiple VLANs
IEEE Std 802.1D, 1998 Edition, does not take into account VLANs when calculating STP information—the
calculations only depend on the physical connections. Consequently, some network configurations will result in
VLANs being subdivided into a number of isolated sections by the STP system. You must ensure that every
VLAN configuration on your network takes into account the expected STP topology and alternative topologies
that may result from link failures.
The following figure shows an example of a network that contains VLANs 1 and 2. The VLANs are connected
using the 802.1Q-tagged link between Switch B and Switch C. By default, this link has a port cost of 100 and
is automatically blocked because the other Switch-to-Switch connections have a port cost of 36 (18+18). This
means that both VLANs are now subdivided—VLAN 1 on Switch units A and B cannot communicate with VLAN
1 on Switch C, and VLAN 2 on Switch units A and C cannot communicate with VLAN 2 on Switch B.