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  3. A network administrator needs to force a high-performance switch that is located in the MDF to become the root bridge for a redu

A network administrator needs to force a high-performance switch that is located in the MDF to become the root bridge for a redu

admin2009-05-19  71
问题 A network administrator needs to force a high-performance switch that is located in the MDF to become the root bridge for a redundant path switched network. What can be done to ensure that this switch assumes the role of the Root Bridge?
A. Configure the switch so that it has a lower priority than other switches in the network.
B. Assign the switch a higher MAC address than the other switches in the network have.
C. Configure the switch for full-duplex operation and configure the other switches for half-duplex operation.
D. Connect the switch directly to the MDF router, which will force the switch to assume the role of root bridge.
E. Establish a direct link from the switch to all other switches in the network.
F. None of the above
选项
答案A
解析 Explanation:
For all switches in a network to agree on a loop-free topology, a common frame of reference must exist. This reference point is called the Root Bridge. The Root Bridge is chosen by an election process among all connected switches. Each switch has a unique Bridge ID (also known as the bridge priority) that it uses to identify itself to other switches.

The Bridge ID is an 8-byte value. 2 bytes of the Bridge ID is used for a Bridge Priority field, which is the priority or weight of a switch in relation to all other switches. The other 6 bytes of the Bridge ID is used for the MAC Address field, which can come from the Supervisor module, the backplane, or a pool of 1024 addresses that are assigned to every Supervisor or backplane depending on the switch model. This address is hardcoded, unique, and cannot be changed.  

The election process begins with every switch sending out BPDUs with a Root Bridge ID equal to its own Bridge ID as well as a Sender Bridge ID. The latter is used to identify the source of the BPDU message. Received BPDU messages are analyzed for a lower Root Bridge ID value.

If the BPDU message has a Root Bridge ID (priority) of the lower value than the switch’s own Root Bridge ID, it replaces its own Root Bridge ID with the Root Bridge ID announced in the BPDU. If two Bridge Priority values are equal, then the lower MAC address takes preference.
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