A consultant has been tasked to assess a client’s network. The client reports frequent network outages.
Upon viewing the spanning tree configuration, the consultant notices that an old and law performing
edge switch on the network has been elected to be the root bridge. Which of the following explains this
scenario?
A.
The switch also serves as the DHCP server
B.
The switch has the lowest MAC address
C.
The switch has spanning tree loop protection enabled
D.
The switch has the fastest uplink port
If a series of switches are using STP, then they have to elect a root bridge. Ideally the root bridge is in the center of the group (since all traffic ends up traveling through it). In this case, the root bridge is on the edge. This usually occurs because the STP election process looks for the lowest priority, lowest MAC address, and lowest port number when making the determination. In this case, I am certain that Answer B is the correct one.
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In STP all switches send BPDUs (Bridge Protocol Data Unit) which contain a priority and the BID (Bridge ID).
The BID is 8 bytes long. 6 bytes is used for the MAC address of the bridge. 12 bits is used to indicate the VLAN, this is called extended system ID. 4 bits are used to
set the priority. Lower priority means it is preferred compared to a higher. The priority is set in multiples of 4096.
If there is a tie in priority then the lowest MAC address will determine which bridge becomes the root.
B is the answer
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