ccna without ccent

ccna without ccent.The root switch will be re-elected whenever there is a change in the network topology, such as when the root switch fails or a new switch is added.All other switches in the Layer 2 topology expect to see BPDUs from the root switch within the maximum expiration time (default is 20 seconds),If they do not see a BPDU message from the root switch within that time, they will assume that the root switch has failed and will begin electingto start electing a new root switch.13.2.3 Root PortAfter the root switch is selected, every other switch in the network needs to select out of its own ports a port for going to the root switch, which is called the root port. For some switches (such as Switch D in Figure 13.4), this is very easy-it has only one port available to access the switched topology. However, on other switches (such as Switch B, Switch C, and Switch E in Figure 13.4), there are multiple ports that can be used to go to the root switch. If there are multiple ports available, you need to take a judicious approach to selecting the best one. In STP, there are multiple factors to consider when selecting the root port. It is important to note that the root switch does not have a root port - it does not need a port that travels to itself!Port Overhead and Priority First, there is an overhead assigned to each port, which is called port overhead. The lower the overhead the better the port. Port overhead is the port bandwidthThe inverse indicator of port bandwidth. In the STP implementation of 802.Id, there are two sets of port overheads: an older calculation and a newer calculation, as shown in Table 13.1. Cisco's Catalyst 1900 switches, which are no longer sold by Cisco, use the old 802.1d port overhead values, while Cisco's other Catalyst switches, including those that are still currently sold (such as the 2960 and 6500 switches), use the new port overhead values. Switches always give preference to ports with low overhead. Each port also has a priority-port priority value that defaults to 32. again, the switch always chooses the port with the lower priority value.One of the main reasons for replacing the old method with the new method is that the old method of dividing the port speed by 1000 has an inherent flaw: it assumes that the port speed does not exceed 1 Gbit/s (1000 Mbit/s). Today's Ethernet standards indicate that 10 Gbit/s ports are already present in enterprise networks. When using the old method of calculating port overhead, the overhead is the same for 1 Gbit/s and 10 Gbit/s links.Table 13.1 STP Port Overheads