with the lowest cost are eligible to be placed in forwarding mode. All other ports that are receiving BPDUs continue in blocking mode. If the path cost and sender BID are equal, as with parallel links between two switches, the switch uses the port ID. In this case, the port with the lowest port ID forwards data frames, and all other ports continue to block data frames. Each bridge advertises the spanning tree path cost in the BPDU. This spanning tree path cost is the cumulative cost of all the links from the root bridge to the switch sending the BPDU. The receiving switch uses this cost to determine the best path to the root bridge. The lowest cost is considered to be the best path. Port cost values per link are shown in the table in the Revised IEEE Spec column. The lower values are associated with higher bandwidth and, therefore, are the more desirable paths. This revised specification uses a nonlinear scale with port cost values. In the previous IEEE specification, the cost value was calculated based on Gigabit Ethernet being the maximum Ethernet bandwidth, with an associated value of 1, from which all other values were derived in a linear manner. In Figure , switch Y receives a BPDU from the root bridge (switch X) on its switch port on the Fast Ethernet segment, and another BPDU on its switch port on the Ethernet segment. The root path cost in both cases is zero. The local path cost on the Fast Ethernet switch port is 19, while the local path cost on the Ethernet switch port is 100. As a result, the switch port on the Fast Ethernet segment has the lowest path cost to the root bridge and is elected as the root port for switch Y. STP selects one designated port per segment to forward traffic. Other switch ports on the segment typically become nondesignated ports and continue blocking, or they could be a root port and continue forwarding, as shown in Figures - . The nondesignated ports receive BPDUs but block data traffic and do not forward data traffic to prevent loops. The switch port on the segment with the lowest path cost to the root bridge is elected as the designated port. If multiple switch ports on a switch have the same path cost and are connecting to the same neighbor switch, the switch port with the lowest sender port ID becomes the designated port. Because ports on the root bridge all have a root path cost of zero, all ports on the root bridge are designated ports. Figure depicts a scenario with switches running STP and exchanging information. This exchange yields the following results: The active topology is the final set of communication paths that are created by switch ports forwarding frames. After the active topology has been established, the switched network must reconfigure the active topology using Topology Change Notifications (TCNs) if a link failure occurs. A TCN BPDU is generated when a bridge discovers a change in topology, usually because of a link failure, bridge failure, or a port transitioning to forwarding state. The TCN BPDU is set to 0x80 in the Type field and is forwarded on the root port toward the root bridge. The upstream bridge acknowledges the BPDU with a Topology Change Acknowledgment (TCA). In the Flag field, the least significant bit is for the TCN, and the most significant bit is for the TCA. The bridge sends this message to its designated bridge, which is the closest neighbor to the root of a particular bridge (or the root, if it is directly connected). The designated bridge acknowledges the topology change back to the sending neighbor and sends the message to its designated bridge. This process repeats until the root bridge gets the message. This is how the root learns about the topology changes in the network. When a topology change occurs the root sends messages throughout the tree so that the content addressable memory (CAM) tables can adjust and provide a new path for the end host devices.
Content 3.1 Describing STP 3.1.7 Explaining Enhancements to STP The 802.1D STP standard was developed long before VLANs were introduced and has some limitations that the Cisco proprietary PVST addresses. PVST allows separate instances of spanning tree and includes Cisco proprietary features, such as PortFast and UplinkFast, which provide much faster convergence. The 802.1Q standard has defined standards-based technologies for handling VLANs. To reduce the complexity of this standard, the 802.1 committee specified only a single instance of spanning tree for all VLANs. Not only does this provide a considerably less flexible approach than Cisco’s PVST, but it also creates an interoperability problem. To address both these issues, Cisco introduced PVST+ in version 4.1 on the Cisco Catalyst 5000 Series (all Cisco Catalyst 4000 and 6000 series switches support PVST+). PVST+ allows the two schemes to interoperate in a seamless and transparent manner in almost all topologies and configurations. There are both advantages and disadvantages to using a single spanning tree. On the upside, it allows switches to be simpler in design and place a lighter load on the CPU. On the downside, a single spanning tree precludes load balancing and can lead to incomplete connectivity in certain VLANs (the single STP VLAN might select a link that is not included in other VLANs). Given these tradeoffs, most network designers have concluded that the downsides of having one spanning tree outweigh the benefits. Two new IEEE standards, RSTP (802.1w) and MSTP (802.1s), improve on the original 802.1D STP standard and provide similar functionality to the Cisco proprietary features. Rapid Spanning Tree Protocol (RSTP) provides much faster convergence, while Multiple Spanning Tree Protocol (MSTP) allows for multiple instances of spanning tree. Per VLAN Rapid Spanning Tree (PVRST) allows RSTP to be implemented, giving faster convergence, while still using the Cisco proprietary PVST. Spanning tree PortFast causes an interface configured as a Layer 2 access port to transition from the blocking to forwarding state immediately, bypassing the listening and learning states. You can use PortFast on Layer 2 access ports that are connected to a single workstation or a server. If an interface configured with PortFast receives a BPDU, spanning tree can put the port into the blocking state by using a feature called BPDU guard. CAUTION: Because the purpose of PortFast is to minimize the time that access ports must wait for spanning tree to converge, it should be used only on access ports. If you enable PortFast on a port connecting to another switch, you risk creating a spanning tree loop. Figure lists the commands used to implement and verify PortFast on an interface. Figure describes the commands. The documents listed in Figure are available on the IEEE Web site. Web Links IEEE www.ieee.org
Content 3.2 Implementing RSTP 3.2.1 Describing the Rapid Spanning Tree Protocol The immediate consideration with STP is convergence time. Depending on the type of failure, it takes anywhere from 30 to 50 seconds to converge the network. RSTP helps with convergence issues that plague legacy STP. RSTP has additional features similar to UplinkFast and BackboneFast that offer better recovery at Layer 2.RSTP is based on the IEEE 802.1w standard. Numerous differences exist between RSTP and STP. RSTP requires a full-duplex point-to-point connection between adjacent switches to achieve fast convergence. Half duplex generally denotes a shared medium in which multiple hosts share the same wire; a point-to-point connection cannot reside in this environment. As a result, RSTP cannot achieve fast convergence in half-duplex mode. STP and RSTP also have