port designation differences. RSTP has alternate and backup port designations, which are absent from the STP environment. Ports not participating in spanning tree are called edge ports. Edge ports can be statically configured by the PortFast parameter. The edge port immediately becomes a non-edge port if a BPDU is heard on the port. Non-edge ports participate in the spanning tree algorithm and only non-edge ports generate topology changes (TCs) on the network when transitioning to forwarding state. TCs are not generated for any other RSTP states. In legacy STP, TCNs were generated for any active port that was not configured for PortFast. RSTP speeds the recalculation of the spanning tree when the Layer 2 network topology changes. It redefines STP port roles and states, and the BPDUs. RSTP is proactive and therefore negates the need for the 802.1D delay timers. RSTP (802.1w) supersedes 802.1D, while still retaining backward compatibility. Much of the 802.1D terminology remains, and most parameters are unchanged. In addition, 802.1w is capable of reverting back to 802.1D to interoperate with legacy switches on a per-port basis. The RSTP BPDU format is the same as the IEEE 802.1D BPDU format, except that the Version field is set to 2 to indicate RSTP, and the Flags field makes use of all 8 bits. In a switched domain, there can be only one forwarding path toward a single reference point; this is the root bridge. The RSTP spanning tree algorithm (STA) elects a root bridge in exactly the same way as 802.1D elects a root. However, there are critical differences that make RSTP the preferred protocol for preventing Layer 2 loops in a switched network environment. Many of the differences stem from the Cisco-proprietary enhancements, which are transparent and integrated into the protocol at a low level. These enhancements, such as BPDUs carrying and sending information about port roles only to neighbor switches, require no additional configuration, and generally perform better than the Cisco-proprietary 802.1D enhancements. Because the RSTP and Cisco-proprietary enhancements are functionally similar, features such as UplinkFast and BackboneFast are not compatible with RSTP.
Content 3.2 Implementing RSTP 3.2.2 Describing RSTP Port States RSTP provides rapid convergence following a failure or during reestablishing a switch, switch port, or link. An RSTP topology change causes a transition in the appropriate switch ports to the forwarding state through either explicit handshakes or a proposal and agreement process and synchronization. With RSTP, the role of a port is separated from the state of a port. For example, a designated port could be in the discarding state temporarily, even though its final state is to be forwarding. RSTP port states correspond to the three basic operations of a switch port: discarding, learning, and forwarding. Figure describes the characteristics of RSTP port states. In all port states, a port accepts and processes BPDU frames. Figure compares STP and RSTP port states.
Content 3.2 Implementing RSTP 3.2.3 Describing RSTP Port Roles The port role defines the ultimate purpose of a switch port and the way it handles data frames. Port roles and port states are able to transition independently of each other. Figure depicts the port roles used by RSTP Figure defines port roles. Establishing additional port roles allows RSTP to define a standby switch port before a failure or topology change. The alternative port moves to the forwarding state if there is a failure on the designated port for the segment.
Content 3.2 Implementing RSTP 3.2.4 Explaining Edge Ports An RSTP edge port is a switch port that is never intended to be connected to another switch device. It immediately transitions to the forwarding state when enabled. The edge port concept is well known to Cisco spanning tree users, because it corresponds to the PortFast feature in which all ports directly connected to end stations anticipate that no switch device will be connected to them. The PortFast ports immediately transition to the STP forwarding state, thereby skipping the time-consuming listening and learning stages. Neither edge ports nor PortFast-enabled ports generate topology changes when the port transitions to a disabled or enabled status. Unlike PortFast, an edge port that receives a BPDU loses its edge port status immediately and becomes a normal spanning tree port. When a switch with an edge port receives a BPDU, it generates a TCN. Cisco’s RSTP implementation maintains the PortFast keyword for edge port configuration, thus making an overall network transition to RSTP more seamless. Configuring an edge port to be attached to another switch can have negative implications for RSTP when it is in sync state.
Content 3.2 Implementing RSTP 3.2.5 Describing RSTP Link Types Each port participating in RSTP is categorized with a link type. The link type can predetermine the active role that the port plays as it stands by for immediate transition to a forwarding state if certain parameters are met. These parameters are different for edge ports and non-edge ports. Non-edge ports are categorized into two link types. The link type is automatically determined but can be overwritten. Edge ports, the equivalent of PortFast-enabled ports, and point-to-point links are candidates for rapid transition to a forwarding state. Before the link type can be considered for the purpose of expedient port transition, RSTP must determine the port role. and Root ports do not use the link type parameter. Root ports are able to make a rapid transition to the forwarding state as soon as the port is in sync. In addition, alternate and backup ports do not use the link type parameter in most cases. Designated ports make the most use of the link type parameter. Rapid transition to the forwarding state for the designated port occurs only if the link type parameter indicates a point-to-point link.
Content 3.2 Implementing RSTP 3.2.6 Examining the RSTP BPDU RSTP (802.1w) uses type 2, version 2 BPDUs, so an RSTP bridge can communicate with 802.1D on any shared link or with any switch running 802.1D. RSTP sends BPDUs and populates the flag byte in a slightly different manner than 802.1D: RSTP uses the flag byte of version 2 BPDU as shown in the Figure . The Flag field in the STP BPDU packet contained a TCN and TCA. In RSTP, the Flag field, which is 1 byte long, has been modified to accommodate port designations and proposal/agreement between adjacent switches. BPDUs are sent every 2 seconds. Unlike in legacy STP, each switch generates its own BPDUs regardless if it hears BPDUs from the root. In legacy STP, BPDUs were only generated by the root and propagated throughout the spanning tree domain. As a result, when a switch did not receive a configuration BPDU, it did not know where the failure occurred. In RSTP mode, the switch needs to worry only about its immediate neighbors. Hence, BPDUs also serve as keepalive mechanisms between adjacent switches. If the switch does not hear three consecutive BPDUs from its downstream neighbor, it transitions appropriate ports to facilitate network convergence.
Content 3.2