TCP/IP Illustrated Episode 16
BPDU Structure
To determine the links in the spanning tree, STP uses BPDUs that adhere to the format shown in Figure 3-15.
The format shown in Figure 3-15 applies to both the original STP as well as the newer RSTP (see Section 3.4.1.6). BPDUs are always sent to the group address 01:80:C2:00:00:00 (see Chapter 9 for details of link-layer group and Internet multicast addressing) and are not forwarded through a bridge without modification. In the figure, the DST, SRC, and L/T (Length/Type) fields are part of the conventional Ethernet (802.3) header of the frame carrying the example BPDU. The 3-byte LLC/ SNAP header is defined by 802.1 and for BPDUs is set to the constant 0x424203. Not all BPDUs are encapsulated using LLC/SNAP, but this is a common option.
Building the Spanning Tree
The first job of STP is to elect the root bridge. The root bridge is discovered as the bridge in the network (or VLAN) with the smallest identifier (priority combined with MAC address). When a bridge initializes, it assumes itself to be the root bridge and sends configuration BPDUs with the Root ID field matching its own bridge ID, but if it detects a bridge with a smaller ID, it ceases sending its own frames and instead adopts the frame it received containing the smaller ID to be the basis for further BPDUs it sends. The port where the BPDU with the smaller root ID was received is then marked as the root port (i.e., the port on the path to the root bridge). The remaining ports are placed in either blocked or forwarding states.
Topology Changes
The next important job of STP is to handle topology changes. Although we could conceivably use the basic database aging mechanism described earlier to adapt to changing topologies, this is a poor approach because the aging timers can take a long time (5 minutes) to delete incorrect entries. Instead, STP incorporates a way to detect topology changes and inform the network about them quickly. In STP, a topology change occurs when a port has entered the blocking or forwarding states. When a bridge detects a connectivity change (e.g., a link goes down), the bridge notifies its parent bridges on the tree to the root by sending topology change notification (TCN) BPDUs out of its root port. The next bridge on the tree to the root acknowledges the TCN BPDUs to the notifying bridge and also forwards them on toward the root. Once informed of the topology change, the root bridge sets the TC bit field in subsequent periodic configuration messages. Such messages are relayed by every bridge in the network and are received by ports in either the blocking or forwarding states. The setting of this bit field allows bridges to reduce their aging time to that of the forward delay timer, on the order of seconds instead of the 5 minutes normally recommended for the aging time. This allows database entries that may now be incorrect to be purged and relearned more quickly, yet it also allows stations that are actively communicating to not have their entries deleted erroneously.
Rapid Spanning Tree Protocol (RSTP) (Formerly 802.1w)
One of the perceived problems with conventional STP is that a change in topology is detected only by the failure to receive a BPDU in a certain amount of time. If the timeout is large, the convergence time (time to reestablish data flow along the spanning tree) could be larger than desired. The IEEE 802.1w standard (now part of [802.1D-2004]) specifies enhancements to the conventional STP and adopts the new name Rapid Spanning Tree Protocol (RSTP). The main improvement in RSTP over STP is to monitor the status of each port and upon indication of failure to immediately trigger a topology change indication. In addition, RSTP uses all 6 bits in the Flag field of the BPDU format to support agreements between bridges that avoid some of the need for timers to initiate protocol operations. It reduces the normal STP five port states to three (discarding, learning, and forwarding, as indicated by the state names in parentheses in Figure 3-14). The discarding state in RSTP absorbs the disabled, blocking, and listening states in conventional STP. RSTP also creates a new port role called an alternate port, which acts as an immediate backup should a root port cease to operate.
RSTP uses only one type of BPDU, so there are no special topology change BPDUs.
802.1ak: Multiple Registration Protocol (MRP)
The Multiple Registration Protocol (MRP) provides a general method for registering attributes among stations in a bridged LAN environment. [802.1ak-2007] defines two particular “applications” of MRP called MVRP (for registering VLANs) and MMRP (for registering group MAC addresses). MRP replaces the earlier GARP framework; MVRP and MMRP replace the older GVRP and GMRP protocols, respectively. All were originally defined by 802.1q.