Spanning Trees
| The assumption made so far in this chapter is that all the switches are running one instance of STP for the whole switched network. This is sometimes referred to as a Common Spanning Tree (CST). IEEE 802.1Q on non-Cisco switches uses CST to remove loops. Cisco, on the other hand, has two proprietary forms of STP implementations: Per-VLAN Spanning Tree or Shared Spanning Tree (PVST) and PVST+. By default, PVST is used on ISL trunks and 802.1Q trunks between Cisco switches, where a separate instance of STP is run for each VLAN. That means for each VLAN you have, you'll have a separate STP algorithm and database, a separate root switch and BPDUs for each VLAN. PVST+ is used in mixed trunk environments in which you have both ISL and 802.1Q trunks. PVST+ allows CST BPDUs to be correctly incorporated into Cisco's native PVST and vice versa.
CSTWith CST, only one instance of STP is running for all the VLANs. STP will run in the default management VLAN, which is typically VLAN 1. Because only one instance of STP exists, one root switch is elected and all loops are removed. CST has two advantages compared to PVST:
Figure 4.4 shows an example of CST, with Switch 1 being the root bridge for the whole network (including both VLANs 1 and 2) and X representing blocking links. Figure 4.4. CST example.
There is a downside to CST, however. For one, it will likely create suboptimal paths in your switched network. This can be seen in Figure 4.4 with VLAN 2. For VLAN 2, off of Switch 5, to get to the users off of Switch 8, it has to go through an extra switch: Switch 4. And it is even worse if either of these groups wants to access the same VLAN off of Switch 9. The other downside of CST is that as your network grows, convergence problems become worse, and STP eventually runs out of steam. PVSTTo solve the scalability and convergence problems of CST, Cisco's PVST uses a separate instance of STP per VLAN. That means for each VLAN, you'll have a root, port costs, path costs, and priorities and all these can be different per VLAN. To ensure unique bridge IDs for each VLAN, Cisco switches have a pool of MAC addresses to choose from. For some switches, this pool can include up to 1,024 MAC addresses. Actually, it's recommended that you tune STP per VLAN to create the most optimal paths for each VLAN. The size of each STP topology is reduced because only switches that connect a VLAN together are included, thereby decreasing convergence time and increasing scalability. PVST is also more stable because links connected to switches not connected to a specific VLAN are not included in the STP topology. Given this capability with PVST, VLAN 2's topology might look like that shown in Figure 4.5, where Switch 8 is the root. In this example, notice that not every switch has a path back to the root, such as Switch 4. Switch 4 and the switches behind it do not have any ports associated with VLAN 2. One nice feature of PVST is that if VLAN 2 is configured on any of these other switches, STP will rerun and include a path to the new addition. Figure 4.5. PVST example.
The downside of PVST is that the switch will be multicasting BPDUs on each VLAN and must have a topology database for each VLAN, thus creating a lot of additional overhead. Plus, to make your network optimal, you'll have to examine your network closely and make the appropriate STP configuration changes for each VLAN, which is a time-consuming process. PVST+PVST+ is a Cisco extension to its PVST protocol. PVST+ allows the incorporation of both IEEE's 802.1Q CST and Cisco's PVST in a switched network. One nice feature of PVST+ is that you do not have to configure anything on your switches to use it it works automatically. It detects CST and PVST and makes the appropriate changes or adjustments. The following are some of the enhancements built into PVST+:
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