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

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Understanding MSTP for EX Series Switches

Copyright © 2010, Juniper Networks, Inc.
An alternate port, which provides an alternate root port.
A backup port, which provides an alternate designated port.
Port assignments change through messages exchanged throughout the domain. An
RSTP device generates configuration messages once per every hello time interval. If an
RSTP device does not receive a configuration message from its neighbor after an interval
of three hello times, it determines that the connection with the neighbor is lost. When a
root port or a designated port fails on a device, the device generates a configuration
message with the proposal bit set. Once its neighbor device receives this message, it
verifies that this configuration message is better than the one saved for that port and
then it starts a synchronizing operation to ensure that all of its ports are in sync with the
new information.
Similar waves of proposal agreement handshake messages propagate toward the leaves
of the network, quickly restoring the connectivity after a topology change (in a
well-designed network that uses RSTP, network convergence can take as little as 0.5
seconds). If a device does not receive an agreement to a proposal message it has sent,
it returns to the original IEEE 802.D convention.
RSTP was originally defined in the IEEE 802.1w draft specification and later incorporated
into the IEEE 802.1D-2004 specification.
VSTP and RSTP can be configured concurrently. You can selectively configure up to 253
VLANs using VSTP; the remaining VLANs will be configured using RSTP. VSTP and RSTP
are the only spanning-tree protocols that can be configured concurrently on the switch.
Understanding STP for EX Series Switches on page 1519
Understanding MSTP for EX Series Switches on page 1521
Understanding VSTP for EX Series Switches on page 1525
Understanding Layer 2 Protocol Tunneling on EX Series Switches on page 1299
Example: Configuring Faster Convergence and Improving Network Stability with RSTP
on EX Series Switches on page 1527
Although RSTP provides faster convergence time than STP, it still does not solve a
problem inherent in STP: All VLANs within a LAN must share the same spanning tree. To
solve this problem, Juniper Networks EX Series Ethernet Switches use Multiple Spanning
Tree Protocol (MSTP) to create a loop-free topology in networks with multiple
spanning-tree regions.
An MSTP region allows a group of bridges to be modeled as a single bridge. An MSTP
region contains multiple spanning tree instances (MSTIs). MSTIs provide different paths
for different VLANs. This functionality facilitates better load sharing across redundant
links.
MSTP region can support up to 64 MSTIs and each instance can support anywhere from
1 through 4094 vlans.
Chapter 64: Spanning-Tree Protocols—Overview
1521