Official PYQ · 2006Q83Data Link Layer: Framing, Error Detection, MAC, Ethernet BridgingMCQ1 mark

GATE CS 2006 Q83 — Data Link Layer: Framing, Error Detection, MAC, Ethernet Bridging

Consider the diagram shown below where a number of LANs are connected by (transparent) bridges. In order to avoid packets looping through circuits in the graph, the bridges organize themselves in a spanning tree. First, the root bridge is identified as the bridge with the least serial number. Next, the root sends out (one or more) data
units to enable the setting up of the spanning tree of shortest paths from the root bridge to each bridge.

Each bridge identifies a port (the root port) through which it will forward frames to the root bridge. Port conflicts are always resolved in favour of the port with the lower index value. When there is a possibility of multiple bridges forwarding to the same LAN (but not through the root port), ties are broken as follows: bridges closest to the root get preference and between such bridges, the one with the lowest serial number is preferred.

Consider the spanning tree $B1, B5, B3, B4, B2$ for the given connection of LANs by bridges, that represents the depth first traversal of the spanning tree of bridges. Let host $H1$ send out a broadcast ping packet. Which of the following options represents the correct forwarding table on $B3$ ?

Choose one optionA

\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\ \hline \text{H1, H2, H3, H4} & \text{3} \\ \hline \text{H5, H6, H9, H10} & \text{1} \\ \hline \text{H7, H8, H11, H12} & \text{2} \\ \hline \end{array}

\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\ \hline \text{H1, H2} & \text{4} \\ \hline \text{H3, H4} & \text{3} \\ \hline \text{H5, H6} & \text{1}\\ \hline \text{H7, H8, H9, H10, H11, H12} & \text{2} \\ \hline \end{array}

\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\ \hline \text{H3, H4} & \text{3} \\ \hline \text{H5, H6, H9, H10} & \text{1} \\ \hline \text{H1, H2} & \text{4} \\ \hline \text{H7, H8, H11, H12} & \text{2} \\ \hline \end{array}

\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\ \hline \text{H1, H2, H3, H4} & \text{3} \\ \hline \text{H5, H7, H9, H10} & \text{1} \\ \hline \text{H7, H8, H11, H12} & \text{4} \\ \hline \end{array}

View worked solution

Answer: A

To determine the correct forwarding table for bridge $B3$ , we analyze the active ports on $B3$ based on the spanning tree topology:

Blocked Ports: In the spanning tree configuration, Port 4 of $B3$ is blocked to prevent loops. Consequently, no hosts will be forwarded through Port 4. This immediately eliminates options B and C, which map hosts to Port 4.
Port 3 (Root Port): Port 3 is the root port of $B3$ $B 3$ facing towards the root bridge $B1$ $B 1$ .
- It directly connects to the LAN containing hosts $H3$ and $H4$ .
- It also forwards traffic towards the root bridge $B1$ , where hosts $H1$ and $H2$ reside.
- Therefore, hosts $H1, H2, H3, H4$ are reached via Port 3.
Port 1: This port connects to the branch containing hosts $H5, H6, H9, H10$ .
Port 2: This port connects to the branch containing hosts $H7, H8, H11, H12$ .

Thus, the correct forwarding table matches Option A.

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Published 10 Oct 2026

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