Dell PowerConnect J-8216 Software Guide for JUNOS version 10.3 - Page 1349

Chapter 64: Spanning-Tree Protocols-Overview • 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. Related • Understanding STP for J-EX Series Switches on page 1275 Documentation • Understanding MSTP for J-EX Series Switches on page 1277 • Understanding VSTP for J-EX Series Switches on page 1281 • Understanding Layer 2 Protocol Tunneling on J-EX Series Switches on page 1056 • Example: Configuring Faster Convergence and Improving Network Stability with RSTP on J-EX Series Switches on page 1283 Understanding MSTP for J-EX Series Switches 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, J-EX Series 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. 1277