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When the primary switch fails, the switches will elect a new root bridge. This will set the bridge priority to 28672, which is lower than the default priority but higher than the root primary. To do that, we enter the ‘root secondary’ command. What if the primary root bridge fails? To optimize further, we need to assign the other core switch as the secondary root bridge in case the primary root bridge is not operational. This will set the bridge priority to 24576, which is lower than the default priority. By doing that, we need to manually configure a value of the root bridge or manually assign it as a root bridge by using the ‘root primary’ command. To prevent having a suboptimal network, we need to manually choose a root bridge within the network. That will create a poor performance of the network. All traffic will go and process first on Switch6 before it goes to the destined switch. Let’s assume that Switch6 is the oldest switch in the group. The example spanning tree topology below shows the LAN switches that elect Switch6 as the Root Bridge by using the MAC address election. We should avoid electing a root bridge using the MAC address, which can cause a suboptimal network performance as it will choose the oldest switch with the lowest MAC address in the network. Spanning Tree Priority Root Bridge Optimization Electing an older switch as the root bridge will cause a suboptimal operation on your network. Most of the older switches have a lower value of MAC address and have lower bandwidth and limited CPU/memory as compared to newer switches. The lowest MAC address will be elected as the Root Bridge. If all the spanning tree bridge priority has the same priority value on all the switches, then the MAC address will be the tiebreaker.
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If a switch has the lowest bridge priority value among the switches within the LAN, then it will be elected as the spanning tree root bridge. The root bridge is taking precedence over the MAC address. Each switch has its own bridge ID and has a default priority value of 32768. The switch selects a root bridge by using the switch priority and the MAC address.
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A root bridge is the central point of all switches and will be responsible for forwarding the traffic. The switch assigns a root bridge within the interconnected switches. BPDU contains STP information needed to elect a root switch and detect loops. Switches send out Bridge Protocol Data Unit (BPDU) on all active interfaces. By default, spanning trees are enabled on most interconnected Cisco switches. Only ports which have a device connected and active will show the mac address detail.Spanning Tree Protocol is a Layer 2 loop prevention mechanism that will block one port on the network switch if it detects a loop of broadcast messages within its architecture. With the above command, one can figure out which Mac address is on which port of catalyst switch. Switch# show mac-address-table | include 0009.aabb.06e9 Or if you know the mac address and want to know which port the mac address is coming from, use the following command Switch# show mac-address-table | include Fa0/5 If you know the switch port you can use the following command Only ports which have the device connected and active will show the mac address detail. mac address of the connected device) and port number. The mac address or CAM table shows the Vlan associated with the port, MAC being learned on the port (i.e. With the command, you can figure out which MAC address is on which port. Total Mac Addresses for this criterion: 5