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Network Design and Operations

In a network where switches are connected with redundant paths, what role does the Spanning Tree Protocol (STP) play?

  1. AIt logically disables some ports so that the paths form a loop-free tree, preventing broadcast frames from circulating endlessly
  2. BIt spreads frames evenly over several links between switches to increase bandwidth
  3. CIt relays frames between different VLANs so that the VLANs can communicate with each other
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Answer: A. It logically disables some ports so that the paths form a loop-free tree, preventing broadcast frames from circulating endlessly

If switches are connected in a loop, broadcasts and frames to unknown destinations keep circulating and multiplying around the loop (a broadcast storm), and the network stops working. This happens because Ethernet frames have no mechanism like the IP TTL that counts hops and discards the frame. STP has the switches exchange messages called BPDUs and blocks some ports so that the active paths form a tree rooted at the root bridge. If a link in use fails, it moves a blocked port to the forwarding state to switch to another path.

  • A. Correct. It keeps the redundant paths but logically breaks the loop.
  • B. That is the role of link aggregation. STP prevents loops by leaving some redundant links unused.
  • C. Traffic between VLANs needs routing by a router or L3 switch.
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More Network Design and Operations questions

  1. In STP, the bridge IDs of three switches are known: SW1 has priority 32768 and MAC address 00-00-5E-00-53-01, SW2 has priority 61440 and MAC address 00-00-5E-00-53-05, and SW3 has priority 4096 and MAC address 00-00-5E-00-53-0A. Which switch is elected as the root bridge?
  2. Three switches, SW1, SW2 and SW3, are connected in a triangle with one link between each pair. The STP root bridge is SW1, and SW2 has a lower bridge ID than SW3. All links have the same path cost. Which port is blocked (does not forward frames)?
  3. With default timers, classic STP (IEEE 802.1D) takes about 30 to 50 seconds after a failure before a previously blocked port starts forwarding. Which mechanism lets RSTP switch over faster?
  4. Four 1 Gbps links between two switches are bundled into one logical link with link aggregation. Frames are distributed with the common method that chooses the link for each combination of source and destination addresses and port numbers (each flow). Which is closest to the maximum throughput a single TCP connection can achieve?
  5. A router is to be connected with a single link to an L2 switch that has VLAN 10 (192.168.10.0/24) and VLAN 20 (192.168.20.0/24), so that the two VLANs can communicate with each other. Which configuration is appropriate for this setup?

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