STP Puzzle #1, Answer Part 3
This post wraps up the discussion of STP Puzzle #1 with a brief look at finding the designated ports (DPs) on each link between switches. As usual, check out the original post to make sense of this one. Note that answer part 1 looked at the reasons why S4 becomes root, and answer part 2 looked at how to determine which ports each non-root switch uses as their root ports (RPs).
Step 3: Find the Designated Port (DP) on Each Switch-Switch Link
The designated port (DP) on a link is the switch that, during STP convergence, sends the best (least cost) Hello onto a link. If a tie occurs, the first tiebreaker looks at the Bridge ID (BID) of the two switches on the link, with the best (lowest) BID winning. If that comparison results in a tie, the switch is hearing its own Hellos. Dealing with that topology is interesting, but unrelated to today’s discussion.
As usual with the steps in the STP puzzle, you can quickly identify some DPs. Focusing on the links between switches, you can look for these cases:
- The root’s local ports always win the DP election, so mark those as DP.
- If either port on the link is an RP, the other port on the link is the DP.
- If the earlier rules do not tell you which switch acts as DP, then apply the normal STP rules to determine the DP: pick the switch with the lowest root cost, or if it’s a tie, pick the switch with the numerically-lower BID.
Today’s post walks through these steps.
Analysis: First Two (Easy) Rules
Figure 4 shows a reminder of the root switch and the known root ports. Note that the earlier analysis determined the RP on S1 and S3, but not on S2.
Figure 4: Root and Root Port Information, STP Puzzle #1
First, look at the root, and mark both its links (F0/1 and F0/2) as DPs. Easy enough.
Next, look at each RP, find the other end of the link, and mark it as DP. In this case, only S1’s F0/3 is newly identified as a DP. Again, pretty simple if you took good notes so far. Figure 5 shows the results to this point, with a DP identified on three of the five links in the topology.
Figure 5: Root, Root Ports, and Known Designated Ports, STP Puzzle #1
Analysis: Last (General) Rule
At this point, the DP has not yet been identified for two links: the S1-S2 link and the S3-S2 link, both shown as blue links in Figure 5. Because neither link has a known RP, and S4 (the root) is not connected to these links, neither of the two earlier easy methods to find the DP work.
To find the DP on each link, simply compare the root cost of the two competing switches. In this case, it’s a short comparison: S2’s root cost cannot be determined from the information in this puzzle. As a result, you cannot find the DP on these two remaining links, because S2 connects to both links.
Conclusions, STP Puzzle #1
Figure 5 sums up the answers for STP puzzle #1. The figure shows all the known information about the root, RPs, DPs, and Root Costs (RC). In this case, the answer does not list any blocking ports, because the information is incomplete.