ccna workbook pdf

ccna workbook pdf.1327PVST+STP does not guarantee that the final loop-free network will be optimal; for example, see Figure 13.5, which is a poorly designed network. In this network, there are two VLANs, and the root switch is switch 8. x indicates that the port is in a blocked state so that all loops are eliminated. If you look at the configuration of VLAN2, you will see that it is definitely not optimal. For example, for a VLAN2 device connected to switch 1 to access a VLAN2 device connected to switch 4, it must go through switch 2, switch 3, switch 6, switch 9, switch 8, and switch 4. In addition, for a VLAN1 device connected to switch 5 or switch 7 to access a VLAN1 device connected to switch 4, it must forward traffic sequentially to switch 8 and Switch 4 0When running only one STP instance, it is referred to as Common Spanning Tree (CST) Cisco also supports PVST+ ( Per-VLAN Spanning Tree Plus ) o When using PVST+, each VLAN has a separate STP instance, root switch, priority setting, and BPDU O In this case, the BPDU contains an additional field that is part of the switch ID (bridge ID), so the switch ID contains three subfields: switch priority, extended system ID, and MAC address. The Extended System ID is an additional subfield that contains the VLAN ID ( VID) of the STP instance.|VLAN 1|VLAN 2Figure 13.5 STP and VLANs With the addition of this field, it is possible to have switches with different priorities in different VLANs and therefore have multiple root switches - one for each VLAN. In PVST+, each VLAN will create its own loop-free topology by default. Of course, as with CST, PVST+ does not guarantee that the loop-free network is optimal, but you can tune the STP in each VLAN to ensure that its traffic pattern is optimal. Another advantage of PVST+ is that when the STP in one VLAN changes, it does not affect the STP instances in other VLANs, which makes the topology more stable. With this in mind, it is highly recommended that you implement VTP pruning to prune the VLANs it is not using from the switch's trunk connection. Pruning is discussed in Chapter 12. the downside of PVST+ is that the overhead is higher because each VLAN has its own STP instance: more BPDUs and STP tables are required per switch. Also. it makes no sense to use PVST+ unless it is optimized for the network. and optimizing PVST+ means that you need to do more work and monitor it.Exam Essentials PVST+ supports the use of one STP instance per VLAN, allowing you to optimize for each VLAN and ensure that the transport path is optimal.CST only supports the use of the same STP instance for all VLANs.13.2.8 Simple STP ExampleTo help you become familiar with how 802.1dSTP works, take a look at an STP example, as shown in Figure 13.6. It is assumed here that none of these switches support fast STP ( RSTP), which will be discussed later in this chapter, but only 802.1dSTP; in addition, it is assumed that there is only one VLANo for eachswitch ports, both labeled with a letter and a number, where the letter is the port identifier and the number is the port overhead. Electing the Root Switch