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Why has this switch not been elected the root bridge for VLAN1?

Refer to the exhibit.

Why has this switch not been elected the root bridge for VLAN1?

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A.
It has more than one interface that is connected to the root network segment.

B.
It is running RSTP while the elected root bridge is running 802.1d spanning tree.

C.
It has a higher MAC address than the elected root bridge.

D.
It has a higher bridge ID than the elected root bridge.

Explanation:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a008009482f.shtml
When a switch receives a BPDU, it first compares priority, the lower number wins. If a tie, compare
MAC, the smaller one wins. Here Switch has 32769 priority which is greater than 20481 so switch will
not elect for root bridge. It says the bridge priority for Switch is 32769, andthe root priority is 20481.
Which means that some other switch has the lower priority and won the election for VLAN 1.

9 Comments on “Why has this switch not been elected the root bridge for VLAN1?

  1. Jay Z says:

    The picture doesn’t match the answer, wtf.

    Both bridge priority is 24606.

    Criteria to select a path to the root bridge::

    Lowest root bridge ID – Determines the root bridge
    Lowest cost to the root bridge – Favors the upstream switch with the least cost to root
    Lowest sender bridge ID – Serves as a tie breaker if multiple upstream switches have equal cost to root
    Lowest sender port ID – Serves as a tie breaker if a switch has multiple (non-Etherchannel) links to a single upstream switch




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  2. jay z says:

    The root bridge is the bridge with the lowest bridge ID.

    To compare two bridge IDs, the priority is compared first. If two bridges have equal priority, then the MAC addresses are compared. In the above example, both SwA and SwB have the same bridge ID (32768) so they will compare their MAC addresses. Because SwB has lower MAC address it will become root bridge.




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  3. JAYNONIMOUS says:

    In case anyone is still wondering, yes, the picture does not match the question. As far as I can tell, the rest of the questions are accurate and correct.




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