ccna with security

ccna with security.First create a spanning tree. A spanning tree is actually an upside down tree with the root at the top - referred to as the root bridge or root switch in STP. Branches (Ethernet physical connections) that extend outward from the root switch connect to other switches, and branches from those switches connect to other switches, and so on.See the physical topology of the network shown in Figure 13.3, which demonstrates a spanning tree. when STP is run, a logical tree structure similar to the one shown in Figure 13.4 is created. As you can see from Figure 13.4, Switch A is the root switch and is located at the top of the spanning tree. Below it are two branches that are connected to Switch B and Switch C. Both of these switches are connected to Switch E, forming a loop. Switch B is also connected to Switch D. At this point, STP has not yet finished running and the loop has not yet been eliminated.When STP is running, it will determine which ports on the four switches (Switch A, Switch B, Switch C, and Switch E) should be logically disabled to eliminate the loop. This ensures that there is only one path available between any two devices in the network.In fact, the first step in STP is to elect the root switch, which is done using BPDUs. As mentioned earlier, when a switch advertises a BPDU, it includes its own switch ID in it. the switch ID is used to elect the root switch - the switch with the lowest switch ID is elected as the root switch. The switch ID consists of two parts.. The priority of the switch (2 bytes long): on Cisco switches the default is 32768 O. MAC address of the switch (6 bytes long) Figure 13.3 Topological Root Switch with Layer 2 Physical LoopsFigure 13.4 Layer 2 STP Logical TopologyOn Cisco switches, the default priority is 32768,which is defined by IEEE 802.1d. If all switches are Cisco switches and the default priority is not modified, the switch with the lowest MAC address will be elected as the root switch. The switch's priority value can be modified to manipulate the election process; to make a particular switch the root switch, assign it a priority value lower than 32768. By sharing BPDUs, the switch will determine which switch has the smallest switch ID and thus elect it as the root switch. Note that the election process is done almost simultaneously on all switches, and the election result is the same on each switch. In other words, the switch with the smallest switch ID does not tell the other switches that its ID is the smallest, leaving all other switches to act as non-root switches.On Catalyst switches that implement VLANs (discussed in Chapter 12), each VLAN has a separate switch ID and STP instance. Each VLAN has a separate root switch (the root switch can be the same or different for all VLANs) 0 In each VLAN, STP is run to eliminate loops in that VLAN. Cisco calls this concept Per VLAN STP (PVST), which is discussed in the "PVST+" section.Don't forget that the switch with the lowest switch (bridge) ID will be elected as the root switch.