Content Overview This module introduces the fundamentals of Spanning Tree Protocol (STP) in a switched network. It explains how the root bridge and its backup are elected, and also covers features for enhancing STP performance, such as Rapid STP (RSTP) and Multiple STP (MSTP). In addition, you will learn how EtherChannel is configured and how it interoperates with STP. The module provides guidelines on improving STP resiliency when network faults occur.

Content 3.1 Describing STP 3.1.1 Describing Transparent Bridges Switches have replaced bridges as the network device for implementing transparent bridging in modern networks. The basic functionality of a switch is identical to that of a transparent bridge on a per-VLAN basis. To understand STP, it is helpful to look at the behavior of a transparent bridge without spanning tree. A transparent bridge has these characteristics:

It must not modify the frames that are forwarded.
It learns addresses by “listening” on a port for the source address of a device. When a source MAC address is read in frames coming into a specific port, the bridge assumes that the frames destined for that MAC address can be sent out of that port. The bridge then builds a table that records which source addresses are seen on which port. A bridge is always listening and learning MAC addresses in this manner.
It must forward all broadcasts out of all ports, except for the port that initially received the broadcast.
If a destination address is unknown to the bridge, it forwards the frame out of all ports, except for the port that initially received the frame. This is called unicast flooding.

Transparent bridging must be transparent to the devices on the network. End stations require no configuration. The existence of the bridging protocol operation is not directly visible to the end stations. As with traditional shared Ethernet, transparent bridges inherently lack the capability to provide redundancy. STP provides a mechanism in the Ethernet transparent bridge environment to discover the Layer 2 topology dynamically and to ensure that there is only one path through the network. Without STP, there is no way to make a transparent bridge environment redundant. STP also protects a network against accidental miscablings because it prevents unwanted bridging loops. Note: The spanning tree algorithm is implemented in other media types, such as Token Ring. STP has a different purpose and function in Token Ring than in Ethernet, because bridging loops can be desirable in Token Ring.

Content 3.1 Describing STP 3.1.2 Identifying Traffic Loops A bridge loop occurs when there is no Layer 2 mechanism, such as time-to-live, to manage the redundant paths and stop the frame from circulating endlessly. Station A has two potential paths to station B via the two intermediate bridges. Figure describes what happens when station A sends frames to station B if there are no provisions to deal with redundant paths enabled.

Content 3.1 Describing STP 3.1.3 Explaining a Loop Free Network In a loop free network, the network cannot create Layer 2 broadcast storms or flooded unicast storms. A loop free network can be achieved manually by shutting down or disconnecting all redundant links between bridges. However, this leaves no redundancy in the network and requires manual intervention in the event of a link failure. STP resolves this problem: If there are alternative links to a destination on a switch, only one link is used to forward data. The switch ports associated with the alternative paths remain aware of the network topology and forward frames over an alternative link if a failure occurs on a primary link. The spanning tree algorithm (STA) runs on each switch to activate or block redundant links. To find the redundant links, the STA chooses a reference point in the network and determines if there are redundant paths to that reference point. If the STA finds a redundant path, it chooses which path forwards frames and which paths are blocked. This effectively severs the redundant links within the network until they are needed when the primary link toward the reference point fails. Spanning tree standards often refer to a “bridge,” but it is likely that all the devices exchanging spanning tree information are Layer 2 switches.

Content 3.1 Describing STP 3.1.4 Describing the 802.1D Spanning Tree Protocol With 802.1D STP, switches reconfigure the paths over which they forward frames, thereby creating a loop free path when there are redundant switch paths through the network. This is accomplished by forwarding traffic over specific ports and by blocking traffic from being forwarded out of other ports. STP prevents loops by using the following mechanisms:

STP communicates Layer 2 information between adjacent switches by exchanging bridge protocol data unit (BPDU) messages.
A single root bridge is chosen to serve as the reference point from which a loop free topology is built for all switches exchanging BPDUs.
Each switch, except for the root bridge, selects a root port that provides the best path to the root bridge.
In a triangular design similar to the one in Figure , on the link between the two nonroot switch ports, a port on one switch becomes a designated port, and the port on the other switch is in a blocking state and does not forward frames. This effectively breaks any loop. Typically, the designated port is on the switch with the best path to the root bridge.

STP sends BPDUs out of every port of the bridge. The information provided in a BPDU includes the following:

Root ID: The lowest bridge ID (BID) in the topology
Cost of path: Cost of all links from the transmitting switch to the root bridge
BID: BID of the transmitting switch
Port ID: Transmitting switch port ID
STP timer values: Maximum age, hello time, forward delay

BPDUs contain the required information for STP configuration. The Type field for the BPDU message is 0x00, and it uses the multicast MAC address 01-80-C2-00-00-00.

Content 3.1 Describing STP 3.1.5 Describing the Root Bridge STP uses a root bridge, root ports, and designated ports to establish a loop free path through the network. The first step in creating a loop free spanning tree is to select a root bridge to be the reference point that all switches use to establish forwarding paths. The STP topology is converged after a root bridge has been selected, and each bridge has selected its root port, designated bridge, and the participating ports. STP uses BPDUs as it transitions port states to achieve convergence. Spanning tree elects a root bridge in each broadcast domain on the LAN. Path calculation through the network is based on the root bridge. The bridge is selected using the bridge ID (BID), which consists of a 2-byte Priority field plus a 6-byte MAC address. In spanning tree, lower BID values are preferred. The Priority field value helps determine which bridge is going to be the root and can be manually altered. In a default configuration, the Priority field is set at 32768. When the default Priority field is the same for all bridges, selecting the root bridge is based on the lowest MAC address. The root bridge maintains the stability of the forwarding paths between all switches for a single STP instance. A spanning tree instance is when all switches exchanging BPDUs and participating in spanning tree negotiation are associated with a single root. If this is done for all VLANs, it is called a Common Spanning Tree (CST) instance. There is also a Per VLAN Spanning Tree (PVST) implementation that provides one instance, and therefore one root bridge, for each VLAN. The BID and root ID are each 8-byte fields carried in a BPDU. These values are used to complete the root bridge election process. A switch identifies the root bridge by