Day 31. Spanning Tree Protocols

Figure 31-1 Redundant Switched Topology

Without STP, redundancy in the switched network could introduce the following issues:

Broadcast storms: Each switch floods broadcasts endlessly, called a broadcast storm.

Multiple frame transmission: Multiple copies of unicast frames may be delivered to the destination, causing unrecoverable errors.

MAC database instability: Instability in the content of the MAC address table results from copies of the same frame being received on different ports of the switch.

The algorithm used by STP chooses the interfaces that should be placed into a forwarding state. For any interfaces not chosen to be in a forwarding state, STP places the interfaces in blocking state.

Switches exchange STP configuration messages every 2 seconds by default using a multicast frame called the bridge protocol data unit (BPDU). One of the pieces of information included in the BPDU is the bridge ID (BID).

As shown in Figure 31-2, the BID is unique to each switch and is composed of a priority value (2 bytes) and the bridge MAC address (6 bytes).

Figure 31-2 Bridge ID

The default priority is 32,768. The root bridge is the bridge with the lowest BID. Therefore, if the default priority value is not changed, the switch with the lowest MAC address becomes root.

Step 1 Elect a root bridge (switch with lowest BID). There can be only one root bridge per network. All ports on the root bridge are forwarding ports.

Step 2 Elect a root port for each nonroot switch, based on lowest root path cost. Each nonroot switch has one root port. The root port is the port through which the nonroot bridge has its best path to the root bridge.

Step 3 Elect a designated port for each segment, based on the lowest root path cost. Each link will have one designated port.

Step 4 The root ports and designated ports transition to the forwarding state, and the other ports stay in the blocking state.

Table 31-1 summarizes the reasons STP places a port in forwarding or blocking state.

Table 31-1 STP: Reasons for Forwarding or Blocking

Port bandwidth is used to determine the cost to reach the root bridge. Table 31-2 lists the default port costs defined by IEEE, which had to be revised with the advent of 10-Gbps ports.

Table 31-2 Default IEEE Port Costs

STP uses the four states shown in Figure 31-3 as a port transitions from blocking to forwarding.

Figure 31-3 Spanning Tree Port States

A fifth state, disabled, occurs either when a network administrator manually disables the port or a security violation disables the port.

STP: The original specification of STP, defined in 802.1D, provides a loop-free topology in a network with redundant links. STP is sometimes referred to as Common Spanning Tree (CST) because it assumes one spanning tree instance for the entire bridged network, regardless of the number of VLANs.

PVST+: Per-VLAN Spanning Tree Plus is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network.

RSTP: Rapid STP, or IEEE 802.1w, is an evolution of STP that provides faster convergence than STP. However, RSTP still only provides for a single instance of STP.

Rapid PVST+: Cisco enhancement of RSTP that uses PVST+. Rapid PVST+ provides a separate instance of 802.1w per VLAN.

Multiple Spanning Tree Protocol: MSTP is an IEEE standard inspired by the earlier Cisco proprietary Multiple Instance STP (MISTP) implementation. MSTP maps multiple VLANs into the same spanning tree instance. The Cisco implementation of MSTP is MST, which provides up to 16 instances of RSTP and combines many VLANs with the same physical and logical topology into a common RSTP instance.

Part of your switch administration skill set is the ability to decide which type of STP to implement. Table 31-3 summarizes the features of each STP flavor.

Table 31-3 Features of STP Varieties