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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 $B3$?

  1. $\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\\hline  \text{H1, H2, H3, H4} & \text{3}   \\\hline \text{H5, H6, H9, H10} & \text{1} \\\hline \text{H7, H8, H11, H12} & \text{2}  \\\hline \end{array}$
  2. $\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\\hline  \text{H1, H2} & \text{4} \\\hline   \text{H3, H4} & \text{3} \\\hline  \text{H5, H6} & \text{1}\\\hline \text{H7, H8, H9, H10, H11, H12} & \text{2}  \\\hline \end{array}$
  3. $\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\\hline  \text{H3, H4} & \text{3} \\\hline   \text{H5, H6, H9, H10} & \text{1} \\\hline  \text{H1, H2} & \text{4}  \\\hline  \text{H7, H8, H11, H12} & \text{2}  \\\hline \end{array}$
  4. $\begin{array}{|l|l|}\hline \textbf{Hosts} & \textbf{Port} \\\hline  \text{H1, H2, H3, H4} & \text{3} \\\hline   \text{H5, H7, H9, H10} & \text{1} \\\hline  \text{H7, H8, H11, H12} & \text{4}  \\\hline \end{array}$
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Ethernet bridging is syllabus of GATE 2021.

Source: https://gatecse.in/gate-2021-syllabus-including-changes/
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This answer contains step-by-step explanation of how to find root ports, designated ports & blocked ports. https://gateoverflow.in/1855/gate-cse-2006-question-82

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1 Answer

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Option is A  see this as we go with options, option A match only with this picture.

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anyone who hav confusion wat r these ports ..

A bridge device has two (or more) ports. The one that is connected on the side where the STP root resides is called 'root port'. A port not facing the root but forwarding traffic (while lowest cost) from another segment is called 'designated port'.

Blocked or Non-Designated Port (NDP) - It is a port that is in a blocking state in the STP topology.

Those ports will be calculated after creating the spanning tree of this diagram ..

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it is how this packet sent by H1 reaches other parts of the LAN and on the way through which ports.
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Also, port 4 is a blocked port for the Bridge B3.In all options B,C,and D, 4 is being used which is incorrect.
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Answer:

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