Juniper Networks EX2500 - Manual (Page 53)

Per VLAN Rapid Spanning Tree „ 39
Chapter 3: Spanning Tree Protocol
Per VLAN Rapid Spanning Tree
Per VLAN Rapid Spanning Tree Plus Protocol (PVRST+) enhances the RSTP
protocol by adding the ability to have multiple Spanning Tree Groups (STGs).
PVRST+ is based on IEEE 802.1w Rapid Spanning Tree Protocol.
In PVRST mode, the EX2500 switch supports a maximum of 128 Spanning Tree
Groups (STGs). Multiple STGs provide multiple data paths, which can be used for
load balancing and redundancy.
To enable load balancing between two EX2500 switches using multiple STGs,
configure each path with a different VLAN and then assign each VLAN to a separate
STG. Each STG is independent. Each STG sends its own Bridge Protocol Data Units
(BPDUs), and each STG must be configured independently.
The STG, or bridge group, forms a loop-free topology that includes one or more
virtual LANs (VLANs). The switch supports 128 STGs running simultaneously. The
default STG 1 may contain multiple VLANs. STGs 2 through 128 each may contain
only one VLAN.
Default Spanning Tree Configuration
In the default configuration, a single STG (STG 1) includes all non-management ports
on the switch. This is called the default STG. Although ports can be added to or
deleted from the default STG, the default STG cannot be deleted from the system.
All other STGs, except the default STG 1, are empty and you must assign a VLAN to
the STG. However, you cannot assign ports directly to an STG. Instead, you add
ports to a VLAN and add the VLAN to the STG. Each STG is enabled by default, and
assigned an ID number from 2 to 128.
By default, the spanning tree on the management port is turned off.
Why Do We Need Multiple Spanning Trees?
The following examples describe why we need multiple spanning trees.
In Figure 7, VLAN 1 and VLAN 2 pass traffic between switch 1 and switch 2. If you
have a single Spanning Tree Group, the switches see an apparent physical loop, and
one VLAN might become blocked, affecting connectivity, even though no logical
loop exists. VLAN 2 traffic is blocked unnecessarily.
Figure 7: Two VLANs on One Spanning Tree Group
Switch 1
VLAN 1, STG 1
Switch 2
VLAN 2, STG 1
X
VLAN 2 traffic blocked by STG 1