Refer to the topology shown in the exhibit. Which ports will be STP designated ports if all the links
are operating at the same bandwidth? (Choose three.)
A.
Switch C – Fa0/0
B.
Switch A – Fa0/1
C.
Switch B – Fa0/1
D.
Switch C – Fa0/1
E.
Switch B – Fa0/0
F.
Switch A – Fa0/0
Explanation:
To remove the possibility of bridging loops, STP makes a final computation to identify one
Designated Port on each network segment. Suppose that two or more switches have ports
connected to a single common network segment. If a frame appears on that segment, all the
bridges attempt to forward it to its destination. Recall that this behavior was the basis of a bridging
loop and should be avoided.
Instead, only one of the links on a segment should forward traffic to and from that segment-the
one that is selected as the Designated Port. Switches choose a Designated Port based on the
lowest cumulative Root Path Cost to the Root Bridge . For example, a switch always has an idea
of its own Root Path Cost, which it announces in its own BPDUs. If a neighboring switch on a
shared LAN segment sends a BPDU announcing a lower Root Path Cost, the neighbor must have
the Designated Port. If a switch learns only of higher Root Path Costs from other BPDUs received
on a port, however, it then correctly assumes that its own receiving port is the Designated Port for
the segment.
Notice that the entire STP determination process has served only to identify bridges and ports. All
ports are still active, and bridging loops still might lurk in the network. STP has a set of progressive
states that each port must go through, regardless of the type or identification. These states
actively prevent loops from forming and are described in the next section.
Note:
In each determination process discussed so far, two or more links might have identical Root Path
Costs. This results in a tie condition, unless other factors are considered. All tie-breaking STP
decisions are based on the following sequence of four conditions: Lowest Root Bridge ID Lowest
Root Path Cost to Root Bridge Lowest Sender Bridge ID Lowest Sender Port ID
The three switches have chosen their Designated Ports (DP) for the following reasons: Catalyst A
– Because this switch is the Root Bridge , all its active ports are Designated Ports, by definition. At
the Root Bridge , the Root Path Cost of each port is 0. Catalyst B – Catalyst A port 1/1 is the DP
for the Segment A-B because it has the lowest Root Path Cost (0). Catalyst B port 1/2 is the DP
for segment B-C. The Root Path Cost for each end of this segment is 19, determined from the
incoming BPDU on port 1/1. Because the Root Path Cost is equal on both ports of the segment,
the DP must be chosen by the next criteria-the lowest Sender Bridge ID. When Catalyst B sends a
BPDU to Catalyst C, it has the lowest MAC address in the Bridge ID. Catalyst C also sends a
BPDU to Catalyst B, but its Sender Bridge ID is higher. Therefore, Catalyst B port 1/2 is selected
as the segment’s DP. Catalyst C – Catalyst A port 1/2 is the DP for Segment A-C because it has
the lowest Root Path Cost (0). Catalyst B port 1/2 is the DP for Segment B-C. Therefore, Catalyst
C port 1/2 will be neither a Root Port nor a Designated Port. As discussed in the next section,
any port that is not elected to either position enters the Blocking state. Where blocking occurs,
bridging loops are broken. CCNP BCMSN Exam Certification Guide, 3rd Edition, Cisco Press
