We are in Scaling Networks labs Let's begin with the topology and switches 2960 Labels This is the addressing table S1 on vlan 1 and ip 192.168.1.1 subnet mask 24 or 255.255.255.0 255.255.255.0 is decimal and 24 is CIDR and so on cabling Initialize and reload the switches, they are already These configurations I don't wanna do it, because are made in other labs for this lab is important the configuration of the IP's on Switch 2 IP address is ready on three switches Vlan 1 is created by default and exists Verify connectivity on three switches this is the Switch 1, 192.168.1.1 ping to Switch 2, 192.168.1.2 after a second attempt all is successful finally to Switch 3 The connectivity is verified between three switches Determine the Root Bridge before switches were called bridges Every spanning tree instance has a switch designated as the root bridge The switch with the lowest identifier (BID) becomes the root bridge the BID is made up of a bridge priority value the priority value can range from 0 to 65535 in increments of 4096 all switches have a value of 32768 by default then this three switches has a priority of 32768 by default deactivate all ports on the switches the gigabit interfaces in this switch are g1/1 and g1/2 and no g0/1 and g0/2 all LEDs in S1 area red Switch 2 Switch 3 configure connected ports as trunks 1,2,3 and 4 ports are used on the three switches Only activate ports fa0/2 and fa0/4 on all switches Let's see the spanning tree information something very important is all switches have a value of 32768 by default that is the BID if all switches have the same value we need the mac address values all switches have a base mac address and is different in all switches if all switches have the same priority the BID is given by the mac address mac is represented by hexadecimal numbers the lowest mac address will be the root bridge the BID is given by the following formula The Bridge ID Priority (BID) is calculated by the priority value and the system ID (the vlan) I'm using vlan 1 the priority (32768) + vlan (1) = BID (32769) this value is given on the three switches, then the root bridge is selected based on the MAC address the switch with the lowest mac address will be the root bridge use show spanning-tree command on the three switches the results of the manual may differ from the results of laboratory or Packet Tracer these are the root values and these are the values of my own local switch local switch has a BID of 32769 (32768 + 1) this is the mac address 00E0.8F3C.5E11 the root has the same priority but a lower mac address 0007.ECD9.3164 because 0007 is lower than 00E0 in hexadecimal Switch 2 the local bridge has a BID 32769 and a mac address of 0007.ECD9.3164 the root has the same BID and the mac is the same too, ie this switch is the root bridge (Switch 2) finally Switch 3 these area the values of the root bridge (Switch 2) and the mac address of the Switch 3 is 0010.1135.C89A that is higher than the root bridge mac address (Switch 2) let's see the table remember that we have only activated ports fa0/2 and fa0/4 on the switches ignore the red ports take a look on the green ports and orange the root is the Switch 2, to know what is the root in a network is ver simple, look for the switch with two active ports on green and the ports on the other side of the link are green too Switch 2 is the root it has two activated ports on green and ports on the other side of these links are green too fa0/2 is green and the port on the other side of the link is green too fa0/4 is green and the port on the other side of the link is green too take a look on Swtich 3 Switch 3 has two ports on green, fa0/2 port has a green port on the other side of the link but fa0/4 port has an orange port on the other side of the link then isn't the root Swithc 2 is the root and its local ports are designated ports ports on the other side of the link are root ports fa0/4 and fa0/2 on Switch 2 are designated ports fa0/2 on Switch 1 is a root port and fa0/2 on Switch 3 is a root port finally fa0/4 on Switch 3 is a designated port and fa0/4 on Switch 1 is the alternate port and is orange this port is no shutdown but traffic will not flow over this port, to avoid loops if Switch 1 needs to communicate with Switch 3 will use only the green ports a ping message from S1 to S3 will pass through S1, S2 and S3 a ping message from S3 to S1 will pass through S3, S2 and S1 if the alternate port does not exist, a broadcast message would spin endlessly by the three switches alternate function port is to be the end of the network to prevent loops these two ports are designated ports and ports in the other side of the links are root ports this port is designated and this port is alternate S2 is the root bridge it had the lowest bridge ID (Priority value + extended system ID (VLAN) + MAC address of switch) S3 f0/2 and S1 f0/2 are the root ports fa0/2 S2, fa0/4 S2, and fa0/4 S3 are designated ports fa0/4 S1 is the alternate port The spanning tree algorithm (STA) uses the root bridge as the reference point and then determines which
ports to block based on path cost If path costs are equal it then compares BIDs. Lower numbers are preferred. In the output above, the link between S1 and S3 has the highest cost to the root bridge The path cost through both switches is the same, so STA selected the path through the switch with the lower BID, and blocked the port (F0/4) on the switch with the higher BID (S1) Everything just explained, is in this section Change port cost is not supported by Packet Tracer the cost of port fa0/2 on Switch 2 can be modified to 18 against the default value of 19 Packet Tracer doesn't support this command if you change the port cost, the roles of the ports can be changed too STP looks at path cost first. The port with the lower path cost will always be preferred over a port with a higher path cost remove port cost changes Packet Tracer doesn't support this command too Observe STP port selection based on port priority Activate ports fa0/1 and fa0/3 on all switches wait a moment for the convergence of the network with redudant links there is another process of STP about 30 seconds use show spanning-tree command it's so simple, like the network without redundant links This is the mac base the cost on all ports is 19 but priority number the priority on all ports is the same (128) but the lowest priority is this (128.1) because is the fa0/1 fa0/2 has a priority of 128.2 and so on the theory says, first use the cost, next use the BID and finally use the priority number in this switch all costs are equal all switches have the same BID the only one difference is the priority number that explains why fa0 / 1 is a root port and not an alternate port take a look on Switch 2 Switch 2 is the root because it has all its local ports on green if you use only one vlan if you use multiple vlans, this will not work on the other side of the link, everything can vary Switch 2 is the root bridge this is the root port (S3 fa0/1) and this is the another root port (S2 fa0/1), both are green all ports of Switch 2 (fa0/1 to fa0/4) are designated ports the ports fa0/4 and fa0/3 on Switch 3 are designated ports too Take a look on Switch 2 all its ports are designated Switch 3 These area the designated ports fa0/3 and fa0/4 the fa0/1 is the root port and alternate port is the fa0/2 Why did STP select these ports as the root port The default port value of the ports is 128; therefore, STP used the port number to break the tie It selected the lower port number as the root port, and blocked the higher-numbered port with the redundant path to the root bridge After a root bridge has been selected, what is the first value STP uses to determine port selection? Path cost. It selects the path with the lower accumulated cost If the first value is equal on the two ports, what is the next value that STP uses to determine port selection? BID by selecting the lower value If both values are equal on the two ports, what is the next value that STP uses to determine port selection? An aggregation of the port priority and the port number, the lower value is preferred see you soon, thank you very much
