Juniper Networks EX2500 - Manual (Page 51)

Rapid Spanning Tree Protocol „ 37
Chapter 3: Spanning Tree Protocol
„ When you remove a port from a VLAN that belongs to an STG, that port is
removed from the STG. However, if that port belongs to another VLAN in the
same STG, the port remains in the STG.
As an example, assume that port 1 belongs to VLAN 2, and VLAN 2 belongs to
STG 2. When you remove port 1 from VLAN 2, port 1 is also removed from
STG 2. The port moves to the default VLAN 1.
However, if port 1 belongs to both VLAN 1 and VLAN 2 and both VLANs belong
to STG 1, removing port 1 from VLAN 2 does not remove port 1 from STG 1
because VLAN 1 is still a member of STG 1.
„ An STG cannot be deleted, only disabled. If you disable the STG while it still
contains VLAN members, Spanning Tree will be off on all ports belonging to
that VLAN.
The relationship between port, trunk groups, VLANs, and spanning trees is shown
in Table 9.
Rapid Spanning Tree Protocol
Rapid Spanning Tree Protocol (RSTP) provides rapid convergence of the spanning
tree and provides for fast re-configuration critical for networks carrying
delay-sensitive traffic such as voice and video. RSTP significantly reduces the time
to reconfigure the active topology of the network when changes occur to the
physical topology or its configuration parameters. RSTP reduces the bridged-LAN
topology to a single spanning tree.
RSTP parameters are configured in Spanning Tree Group 1. Spanning Tree
Groups 2 through 128 do not apply to RSTP. There are new STP parameters to
support RSTP, and some values to existing parameters are different.
RSTP is compatible with devices that run 802.1D (1998) Spanning Tree Protocol. If
the switch detects 802.1D (1998) BPDUs, it responds with 802.1D
(1998)-compatible data units. RSTP is not compatible with Per VLAN Spanning Tree
(PVST+) protocol.
Port State Changes
The port state controls the forwarding and learning processes of Spanning Tree. In
RSTP, the port state has been consolidated to the following: discarding, learning,
and forwarding. Table 10 compares the port states between 802.1D (1998)
Spanning Tree and 802.1D (2004) Rapid Spanning Trees.
Table 10: RSTP vs. STP Port States
Operational Status STP Port State RSTP Port State
Enabled Blocking Discarding
Enabled Listening Discarding
Enabled Learning Learning
Enabled Forwarding Forwarding
Disabled Disabled Discarding