What is the Rapid Spanning Tree Protocol (802.1w) and how do I configure it?
Product and Software: This article applies to all Aruba controllers and ArubaOS version 3.4 and later.
The Rapid Spanning Tree Protocol (802.1w) is an evolution of the spanning tree protocol (802.1d). 802.1d reconvergence times are no longer acceptable in present day networks. Rapid Spanning Tree Protocol (RSTP) implements new port states, port roles, and the concept of edge ports and point-to-point links. RSTP is backward compatible with STP.
RSTP Port States
IEEE 802.1d Port States
IEEE 802.1w Port States
RSTP Port Roles
· Root Port: A port on each bridge that provides lowest cost path to root bridge
· Designated Port: Port on LAN segment that provides lowest cost path to root bridge. If more than one bridge is on same LAN segment, each bridge listens to BPDUs from other and select the port that provides lowest cost path to root.
· Alternate Port: Port that offers an alternate path to root bridge than one provided by root port. A port is ‘alternate port’ if:
· it is not root or designated port
· if bridge is not designated bridge for attached LAN segment
· Backup Port: Port on designated bridge if bridge has two or more connections to same LAN segment and port is neither root port nor designated port.
Port states are shown here using three switches sw1 (root bridge), sw2, and sw3 (designated bridge).
Link /Port Types
RSTP defines two types of ports:
• point-to-point ports: These are the links between bridges. By default, all ports are point-to-point ports.
• edge ports: These are the ports the connect end hosts.
RSTP uses same multicast address as STP to transmit BPDUs. It makes use of unused fields in 802.1d BPDU to accommodate additional information, such as port role.
Protocol identifier is all 0s, but version is set to 2. BPDU type filed is used to indicate RSTP BPDU.
BPDU Type is set to:
· 0000 0000 for STP
· 1000 0000 for TCN
· 0000 0010 for RSTP
The Flags field is used to transmit state machine information in addition to port role.
The flags are:
· Bit 7 = Topology Change Ack
· Bit 6 = Agreement
· Bit 5 = Forwarding
· Bit 4 = Learning
· Bit 3 & 2;
· 00 = unknown
· 01= Alternate / Backup
· 10= Root
· 11= Designated
· Bit 1 = Proposal
· Bit 0 = Topology Change
RST BPDU Example with Proposal Flag Set
IEEE 802.3 Ethernet
Spanning Tree Protocol
Protocol Identifier: Spanning Tree Protocol (0x0000)
Protocol Version Identifier: Rapid Spanning Tree (2)
BPDU Type: Rapid/Multiple Spanning Tree (0x02)
BPDU flags: 0x3e (Forwarding, Learning, Port Role: Designated, Proposal)
0... .... = Topology Change Acknowledgment: No
.0.. .... = Agreement: No
..1. .... = Forwarding: Yes
...1 .... = Learning: Yes
.... 11.. = Port Role: Designated (3)
.... ..1. = Proposal: Yes
.... ...0 = Topology Change: No
Root Identifier: 1000 / 00:0b:86:60:16:1c
Root Path Cost: 4
Bridge Identifier: 8192 / 00:0b:86:61:10:48
Port identifier: 0x8002
Message Age: 1
Max Age: 20
Hello Time: 2
Forward Delay: 15
Version 1 Length: 0
RSTP and Backward Compatibility
RSTP is compatible with 802.1d. RSTP defaults to 802.1d on a port that receives 802.1d BPDU. Other ports will operate in RSTP mode.
RSTP is the default mode of operation. In 3.4 there is no option to run an 802.1d STP. To disable spanning tree use 'no spanning-tree' command in global configuration mode. Disabling spanning-tree is not recommended as network becomes vulnerable to layer 2 loops.
Using the WebUI:
To disable spanning tree or change timers, navigate to Network > Controller > System Settings. After making changes, click Apply to save the configuration.
(MC3200) #show spanning-tree
Spanning Tree is executing the IEEE compatible Rapid Spanning Tree protocol
Bridge Identifier has priority 32768, address 00:0b:86:61:3a:b4
Configured hello time 2, max age 20, forward delay 15
We are the root of the spanning tree
Topology change flag is not set , detected flag not set , changes 167
Times: hold 1, topology change 35 hello 2, max age 20, forward delay 15
Timers: hello 0, notification 0
Last topology change: 0 days, 0 hours, 1 min, 34 secs
All ports by default are configured are point-to-point ports. Any port can be configured in edge mode by using the “portfast” option.
(SDM3-332x) #configure t
Enter Configuration commands, one per line. End with CNTL/Z
(MC3200) (config) #interface gigabitethernet 1/0
(MC3200) (config-if)#spanning-tree portfast
Using the WebUI:
To change the spanning tree configuration for a specific port, navigate to Configuration > Network > Port. Under Port Selection, select the port and make the appropriate changes for “Spanning Tree”. To configure a port as an edge port, check the “portfast” option. Click Apply to save the configuration.
(MC3200) #show spanning-tree interface gigabitethernet 1/0
Interface GE 1/0 (port 1) in Spanning tree is FORWARDING
Port path cost 19, Port priority 128 Role DESIGNATED
PortFast DISABLED P-to-P ENABLED
Designated root has priority 32768 address 00:0b:86:61:3a:b4
Designated bridge has priority 32768 address 00:0b:86:61:3a:b4
Designated port is 1, path cost 0
Timers: message age 0, forward delay 0, hold 0
Counts: BPDUs received 363, sent 721074