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Why do we need extended bridge ID?
The extended system ID value is added to the bridge priority value in the BID to identify the priority and VLAN of the BPDU frame. When two switches are configured with the same priority and have the same extended system ID, the switch having the MAC address with the lowest hexadecimal value will have the lower BID.
What affects STP root port election?
Root Port selection is based on the port having lowest cost to the Root Bridge (CAT1). When this BPDU receive by any other switch it will add its own port cost (according to the above mentioned value). So if BPDU receive by a Fast Ethernet port (100 Mbps) it will calculate path cost to root as 19 (0+19).
What is extended system ID?
The Extended System ID is a value of 1 to 4095 corresponding to the respective VLAN participating in STP. The Bridge Priority increments in blocks of 4096 to allow the System ID Extension to squeeze in between each increment.
How is the root bridge elected?
The root bridge is selected by manually configuring its bridge priority to a low value. 32768 is the default value out of a range from 0 to 61440. If all switches in a single spanning tree have the same bridge priority, the switch with the lowest MAC address will become the root bridge.
When does a switch become a root bridge?
As all the switches have default priority therefore there is a tie on the basis of priority. Now, the switch with the lowest Mac address will become a root bridge. Here, switch A will become the root bridge as it has the lowest Mac address. Therefore, the ports of switch A will be in forwarding state i.e designated port.
What’s the difference between root ID and bridge ID?
The bridge ID is the mac-address of the switch you are on. The root ID is the mac-address of the switch that is the root bridge for that vlan.
Which is the designated port for the root bridge?
Thus port Gi0/1 of Switch B will be selected as the Designated Port for the segment between Switch B and Switch C as Switch B advertised lowest Bridge ID on to the Segment. All ports of the Root Bridge are Designated Ports and any port that is not a Root Port or a Designated Port becomes the Blocking Port and is blocked to prevent loops.
Which is the root bridge in spanning tree?
Bridge Priority – It is priority, which is assigned to every switch, 32768 by default. Root Bridge – The root bridge is the bridge with lowest Bridge I’d. All the decisions like which ports are the root ports (the port with best path to the root bridge) are made from the perspective of root bridge.