Regions, Legacy Stp And Rstp Switches, And The Common Spanning Tree (Cst); Tree (Cst); Mstp Operation With 802.1Q Vlans - HP procurve 8100fl series Management And Configuration Manual

Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
9-10
All MSTP switches (as well as STP and RSTP switches) in a network use
BPDUs (Bridge Protocol Data Units) to exchange information from which to
build multiple, active topologies in the individual instances within a region
and between regions. From this information:
■ The MSTP switches in each LAN segment determine a designated bridge
and designated port or trunk for the segment.
The MSTP switches belonging to a particular instance determine the root
■
bridge and root port or trunk for the instance.
■ For the IST instance within a region, the MSTP switches linking that
region to other regions (or to STP or RSTP switches) determine the IST
root bridge and IST root port or trunk for the region. (For any Multiple
Spanning-Tree instance—MSTI—in a region, the regional root may be a
different switch that is not necessarily connected to another region.)
The MSTP switches block redundant links within each LAN segment,
■
across all instances, and between regions, to prevent any traffic loops.
As a result, each individual instance (spanning tree) within a region
determines its regional root bridge, designated bridges, and designated ports
or trunks.
Regions, Legacy STP and RSTP Switches, and the Common
Spanning Tree (CST)
The IST instance and any MST instances in a region exist only within that
region. Where a link crosses a boundary between regions (or between a region
and a legacy STP or RSTP switch), traffic is forwarded or blocked as
determined by the Common Spanning Tree (CST). The CST ensures that there
is only one active path between any two regions, or between a region and a
switch running STP and RSTP. (Refer to figure 9-2 on page 9-5.)

MSTP Operation with 802.1Q VLANs

As indicated in the preceding sections, within a given MST instance, a single
spanning tree is configured for all VLANs included in that instance. This means
that if redundant physical links exist in separate VLANs within the same
instance, MSTP blocks all but one of those links. However, you can prevent
the bandwidth loss caused by blocked redundant links for different VLANs in
an instance by using a port trunk. The following example shows how you can
use a port trunk with 802.1Q (tagged) VLANs and MSTP without unnecessarily
blocking any links or losing any bandwidth.