Understanding Stp For Ex Series Switches - Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual

CHAPTER 70
Spanning-Tree Protocols—Overview

Understanding STP for EX Series Switches

Copyright © 2010, Juniper Networks, Inc.
Understanding STP for EX Series Switches on page 1751
Understanding RSTP for EX Series Switches on page 1753
Understanding MSTP for EX Series Switches on page 1755
Understanding BPDU Protection for STP, RSTP, and MSTP on EX Series
Switches on page 1756
Understanding Loop Protection for STP, RSTP, VSTP, and MSTP on EX Series
Switches on page 1758
Understanding Root Protection for STP, RSTP, VSTP, and MSTP on EX Series
Switches on page 1759
Understanding VSTP for EX Series Switches on page 1760
Juniper Networks EX Series Ethernet Switches provide Layer 2 loop prevention through
Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP), Multiple Spanning
Tree Protocol (MSTP), and VLAN Spanning Tree Protocol (VSTP). The default factory
configuration for EX Series switches uses RSTP. If your network includes 802.1D 1998
bridges, you can explicitly configure STP. Note that when doing so, the EX Series switches
use the IEEE 802.1D 2004 specification, force version 0, which is an RSTP configuration
that is compatible with basic STP. If you use VLANs, we recommend that you enable
MSTP unless your network requires the device compatibility provided by VSTP.
STP is a protocol designed to intelligently avoid loops in a network by creating a tree
topology (spanning tree) of the entire bridged network with only one available path
between the tree root and a leaf. The tree root is a switch within the network elected by
the STA (spanning tree algorithm) to use in computing the shortest path (lowest root
path cost) between bridges throughout the network and the root bridge. Frames travel
through the network to their destination–a leaf such as an end-user PC–along branches.
A tree branch is a network segment, or link, between bridges.
STP uses frames called Bridge Protocol Data Units (BPDUs) to create and maintain the
spanning tree. A BPDU frame is a message sent from one switch to another to
communicate information about itself, such as its bridge ID, root path costs, and port
MAC addresses. The initial exchange of BPDUs between switches determines the root
bridge. Simultaneously, BPDUs are used to communicate the cost of each link between
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