Refer to the exhibit.
BPDUGuard is enabled on both ports of Switch
A.
Initially, LinkA is connected and forwarding traffic. A new LinkB is then attached between
SwitchA and HubA. Which two statements about the possible result of attaching the second
link are true? (Choose two.)
The switch port attached to LinkB does not transition to up.
A.
Initially, LinkA is connected and forwarding traffic. A new LinkB is then attached between
SwitchA and HubA. Which two statements about the possible result of attaching the second
link are true? (Choose two.)
The switch port attached to LinkB does not transition to up.
B.
One or both of the two switch ports attached to the hub goes into the err-disabled state
when a BPDU is received.
C.
Both switch ports attached to the hub transitions to the blocking state.
D.
A heavy traffic load could cause BPDU transmissions to be blocked and leave a switching
loop.
E.
The switch port attached to LinkA immediately transitions to the blocking state.
Explanation:
we know that there will have only one Designated port for each segment (notice that the two
ports of SwitchA are on the same segment as they are connected to a hub). The other port
will be in Blocking state. But how does SwitchA select its Designated and Blocking port? The
decision process involves the following parameters inside the BPDU:
* Lowest path cost to the Root
* Lowest Sender Bridge ID (BID)
* Lowest Port ID
In this case, both interfaces of SwitchA have the same “path cost to the root” and “sender
bridge ID” so the third parameter “lowest port ID” will be used. Suppose two interfaces of
SwitchA are fa0/1 & fa0/2 then SwitchA will select fa0/1 as its Designated port (because
fa0/1 is inferior to fa0/2) -> B is correct.
Suppose the port on LinkA (named portA) is in forwarding state and the port on LinkB
(named portB) is in blocking state. In blocking state, port B still listens to the BPDUs. If the
traffic passing through LinkA is too heavy and the BPDUs can not reach portB, portB will
move to listening state (after 20 seconds for STP) then learning state (after 15 seconds) and
forwarding state (after 15 seconds). At this time, both portA & portB are in forwarding state
so a switching loop will occur -> D is correct.