Brocade Communications Systems Brocate Ethernet Access Switch 6910 - Manual (Page 810)

760 Brocade 6910 Ethernet Access Switch Configuration Guide
53-1002581-01
Overview
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FIGURE 112 STP Root Ports and Designated Ports
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge
Protocol Data Units) transmitted from the Root Bridge. If a bridge does not get a Hello BPDU after a
predefined interval (Maximum Age), the bridge assumes that the link to the Root Bridge is down.
This bridge will then initiate negotiations with other bridges to reconfigure the network to
reestablish a valid network topology.
RSTP – RSTP is designed as a general replacement for the slower, legacy STP. RSTP is also
incorporated into MSTP. RSTP achieves much faster reconfiguration (i.e., around 1 to 3 seconds,
compared to 30 seconds or more for STP) by reducing the number of state changes before active
ports start learning, predefining an alternate route that can be used when a node or port fails, and
retaining the forwarding database for ports insensitive to changes in the tree structure when
reconfiguration occurs.
MSTP – When using STP or RSTP, it may be difficult to maintain a stable path between all VLAN
members. Frequent changes in the tree structure can easily isolate some of the group members.
MSTP (which is based on RSTP for fast convergence) is designed to support independent spanning
trees based on VLAN groups. Using multiple spanning trees can provide multiple forwarding paths
and enable load balancing. One or more VLANs can be grouped into a Multiple Spanning Tree
Instance (MSTI). MSTP builds a separate Multiple Spanning Tree (MST) for each instance to
maintain connectivity among each of the assigned VLAN groups. MSTP then builds a Internal
Spanning Tree (IST) for the Region containing all commonly configured MSTP bridges.
FIGURE 113 MSTP Region, Internal Spanning Tree, Multiple Spanning Tree
An MST Region consists of a group of interconnected bridges that have the same MST
Configuration Identifiers (including the Region Name, Revision Level and Configuration Digest –
see
“Configuring Multiple Spanning Trees” on page 774). An MST Region may contain multiple
MSTP Instances. An Internal Spanning Tree (IST) is used to connect all the MSTP switches within an
MST region. A Common Spanning Tree (CST) interconnects all adjacent MST Regions, and acts as a
virtual bridge node for communications with STP or RSTP nodes in the global network.
x
Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
x
Region R
IST
(for this Region) MST 1
MST 2