Understanding Rstp For Ex Series Switches - Juniper JUNOS OS 10.3 - SOFTWARE Manual

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Understanding RSTP for EX Series Switches

1520
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OS for EX Series Ethernet Switches, Release 10.3
Configuration BPDUs: Contain configuration information about the transmitting switch
and its ports, including switch and port MAC addresses, switch priority, port priority,
and port cost.
Topology Change Notification (TCN) BPDUs: When a bridge needs to signal a topology
change, it starts to send TCNs on its root port. The designated bridge receives the TCN,
acknowledges it, and generates another one for its own root port. The process continues
until the TCN reaches the root bridge.
STP uses the information provided by the BPDUs to elect a root bridge, identify root ports
for each switch, identify designated ports for each physical LAN segment, and prune
specific redundant links to create a loop-free tree topology. All leaf devices calculate the
best path to the root device and place their ports in blocking or forwarding states based
on the best path to the root. The resulting tree topology provides a single active Layer 2
data path between any two end stations.
Understanding MSTP for EX Series Switches on page 1521
Understanding RSTP for EX Series Switches on page 1520
Understanding VSTP for EX Series Switches on page 1525
Understanding Layer 2 Protocol Tunneling on EX Series Switches on page 1299
Juniper Networks EX Series Ethernet Switches use Rapid Spanning Tree Protocol (RSTP)
to provide better reconvergence time than that provided by the base Spanning Tree
Protocol (STP). RSTP identifies certain links as point to point. When a point-to-point
link fails, the alternate link can transition to the forwarding state.
Although STP provides basic loop prevention functionality, it does not provide fast network
convergence when there are topology changes. STP's process to determine network
state transitions is slower than RSTP's because it is timer-based. A device must reinitialize
every time a topology change occurs. The device must start in the listening state and
transition to the learning state and eventually to a forwarding or blocking state. When
default values are used for the maximum age (20 seconds) and forward delay (15
seconds), it takes 50 seconds for the device to converge. RSTP converges faster because
it uses a handshake mechanism based on point-to-point links instead of the timer-based
process used by STP.
For networks with VLANs, you can use VLAN Spanning Tree Protocol (VSTP) to run one
or more STP or RSTP instances for each VLAN on which VSTP is enabled. VSTP takes
the paths of each VLAN into account when calculating routes.. VSTP uses RSTP instances
by default.
An RSTP domain running on a switch has the following components:
A root port, which is the "best path" to the root device.
A designated port, which indicates that the switch is the designated bridge for the other
switch connecting to this port.
Copyright © 2010, Juniper Networks, Inc.