Refer to the exhibit.
BPDUGuard is enabled on both ports of SwitchA. 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.)
A.
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.