In 2001, the IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w. RSTP provides significantly faster spanning tree convergence after a topology change, introducing new convergence behaviors and bridge port roles to do this. RSTP was designed to be backwards-compatible with standard STP.
While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times (default: 3 times 2 seconds) or within a few milliseconds of a physical link failure. The Hello time is an important and configurable time interval that is used by RSTP for several purposes; its default value is 2 seconds.
Standard IEEE 802.1D-2004 incorporates RSTP and obsoletes the original STP standard.
Rapid Spanning Tree Operation
RSTP adds new bridge port roles in order to speed convergence following a link failure. The number of states a port can be in has been reduced to three instead of STP's original five.
RSTP bridge port roles:
- Root - A forwarding port that is the best port from non-root bridge to root bridge
- Designated - A forwarding port for every LAN segment
- Alternate - An alternate path to the root bridge. This path is different from using the root port
- Backup - A backup/redundant path to a segment where another bridge port already connects
- Disabled - Not strictly part of STP, a network administrator can manually disable a port
RSTP switch port states:
- Discarding - No user data is sent over the port
- Learning - The port is not forwarding frames yet, but is populating its MAC-address-table
- Forwarding - The port is fully operational
RSTP operational details:
- Detection of root switch failure is done in 3 hello times, which is 6 seconds if the default hello times have not been changed.
- Ports may be configured as edge ports if they are attached to a LAN that has no other bridges attached. These edge ports transition directly to the forwarding state. RSTP still continues to monitor the port for BPDUs in case a bridge is connected. RSTP can also be configured to automatically detect edge ports. As soon as the bridge detects a BPDU coming to an edge port, the port becomes a non-edge port.
- RSTP calls the connection between two or more switches as a "link-type" connection. A port that operates in full-duplex mode is assumed to be point-to-point link, whereas a half-duplex port (through a hub) is considered a shared port by default. This automatic link type setting can be overridden by explicit configuration. RSTP improves convergence on point-to-point links by reducing the Max-Age time to 3 times Hello interval, removing the STP listening state, and exchanging a handshake between two switches to quickly transition the port to forwarding state. RSTP does not do anything differently from STP on shared links.
- Unlike in STP, RSTP will respond to BPDUs sent from the direction of the root bridge. An RSTP bridge will "propose" its spanning tree information to its designated ports. If another RSTP bridge receives this information and determines this is the superior root information, it sets all its other ports to discarding. The bridge may send an "agreement" to the first bridge confirming its superior spanning tree information. The first bridge, upon receiving this agreement, knows it can rapidly transition that port to the forwarding state bypassing the traditional listening/learning state transition. This essentially creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbors to determine if it can make a rapid transition. This is one of the major elements that allows RSTP to achieve faster convergence times than STP.
- As discussed in the port role details above, RSTP maintains backup details regarding the discarding status of ports. This avoids timeouts if the current forwarding ports were to fail or BPDUs were not received on the root port in a certain interval.
- RSTP will revert to legacy STP on an interface if a legacy version of an STP BPDU is detected on that port.
Per-VLAN Spanning Tree and Per-VLAN Spanning Tree Plus
In Ethernet switched environments where multiple Virtual LANs exist, it is often desirable to create multiple spanning trees so that traffic from different VLANs uses different links. Cisco's proprietary versions of Spanning Tree Protocol, Per-VLAN Spanning Tree (PVST) and Per-VLAN Spanning Tree Plus (PVST+), create a separate spanning tree for each VLAN. Both PVST and PVST+ protocols are Cisco proprietary protocols, and few switches from other vendors support them. They use a different multicast address: 01:00:0C:CC:CC:CD. Some devices from Force10 Networks, Alcatel-Lucent, Extreme Networks, Avaya, and BLADE Network Technologies support PVST+. Extreme Networks does so with two limitations: Lack of support on ports where the VLAN is untagged/native, and also on the VLAN with ID 1. PVST works only with ISL (Cisco's proprietary protocol for VLAN encapsulation) due to its embedded Spanning Tree ID. This is the default protocol on Cisco switches that support ISL. Due to high penetration of the IEEE 802.1Q VLAN trunking standard and PVST's dependence on ISL, Cisco defined an additional PVST+ standard that is compatible with 802.1Q encapsulation. This became the default protocol for Cisco switches when Cisco discontinued and removed ISL support from its switches. PVST+ can tunnel across an MSTP Region.
Rapid Per-VLAN Spanning Tree
This is Cisco's proprietary version of Rapid Spanning Tree Protocol. It creates a spanning tree for each VLAN, just like PVST. Cisco refers to this as Rapid Per-VLAN Spanning Tree (RPVST).
VLAN Spanning Tree Protocol
In Juniper Networks environment, if compatibility to Cisco's proprietary PVST protocol is required, VLAN Spanning Tree Protocol (VSTP) can be configured. VSTP maintains a separate spanning-tree instance for each VLAN configured in the switch. The VSTP protocol is only supported by the EX and MX Series from Juniper Networks. There are two restrictions to the compatibility of VSTP:
- VSTP supports only 253 different spanning-tree topologies. If there are more than 253 VLANs, it is recommended to configure RSTP in addition to VSTP, and VLANs beyond 253 will be handled by RSTP.
- MVRP does not support VSTP. If this protocol is in use, VLAN membership for trunk interfaces must be statically configured .
By default, VSTP uses the RSTP protocol as its core spanning-tree protocol, but usage of STP can be forced if the network includes old bridges .
For more information about configuring VSTP on Juniper Networks switches, see the official documentation Understanding VSTP.
