Port forwarding tables¶
Port forwarding tables are used in tree-shaped networks to automatically build the forwarding tables that allow the network nodes to forward packets towards their destinations.
- Consider the tree-shaped network shown in the figure below. Assume that the port-forwarding tables are as shown in the figure.
Which of the following affirmations are true ?
If C sends a packet towards A, R3 will immediately forward it on its North interfaceIf B sends a packet towards C, R3 will immediately forward it on its West interfaceIf A sends a packet towards C, R1 will send it to both its East and South interfaces.If A sends a packet towards B, R1 will send it to both its East and South interfaces.If A sends a packet towards B, R1 will send it only to its East interface.
If A sends a packet towards C, R1 will send it only to its South interface.
If R1 receives from R3 a packet whose destination is A, it sends it on both its West and East interfaces.
- Consider the tree-shaped network shown in the figure below. Assume that A has sent a packet towards B but B has not yet replied.
Upon reception of this packet, the port forwarding table of R3 will be updated as :
Dest. OutPort A N B E Upon reception of this packet, the port forwarding table of R1 will be updated as :
Dest. OutPort A W B S Upon reception of this packet, the port forwarding table of R2 will be updated as :
Dest. OutPort A W B W The port forwarding table of R2 will never be updated with information about destination B.Upon reception of this packet, the port forwarding table of R3 will be updated as :
Dest. OutPort A N B N
![\tikzstyle{arrow} = [thick,->,>=stealth]
\tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
\tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
\tikzset{ftable/.style={rectangle, dashed, draw} }
\node[host] (A) {A};
\node[router, right=of A] (R1) { R1 };
\node[ftable, above=of R1] (FR1) { \begin{tabular}{l|l}
Dest. & OutPort \\
\hline
A & W \\
\end{tabular}};
\node[router,right=of R1] (R2) {R2};
\node[ftable, right=of R2] (FR2) { \begin{tabular}{l|l}
Dest. & OutPort \\
\hline
A & W \\
\end{tabular}\\};
\node[router,below=of R1] (R3) {R3};
\node[ftable, below=of R3] (FR3) { \begin{tabular}{l|l}
Dest. & OutPort \\
\hline
A & N \\
\end{tabular}\\};
\node[host, right=of R3] (B) {B};
\node[host, left=of R3] (C) {C};
\path[draw,thick]
(A) edge (R1)
(R1) edge (R2)
(R1) edge (R3)
(R3) edge (B)
(C) edge (R3);
\draw[arrow, dashed] (FR1) -- (R1);
\draw[arrow, dashed] (FR2) -- (R2);
\draw[arrow, dashed] (FR3) -- (R3);](_images/tikz-73d239243fd8e25746492d11065fb56621832b3e.png)
![\tikzstyle{arrow} = [thick,->,>=stealth]
\tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
\tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
\tikzset{ftable/.style={rectangle, dashed, draw} }
\node[host] (A) {A};
\node[router, right=of A] (R1) { R1 };
\node[ftable, above=of R1] (FR1) { \begin{tabular}{l|l}
Dest. & OutPort \\
\hline
A & W \\
\end{tabular}};
\node[router,right=of R1] (R2) {R2};
\node[ftable, right=of R2] (FR2) { \begin{tabular}{l|l}
Dest. & OutPort \\
\hline
A & W \\
\end{tabular}\\};
\node[router,below=of R1] (R3) {R3};
\node[ftable, below=of R3] (FR3) { \begin{tabular}{l|l}
Dest. & OutPort \\
\hline
A & N \\
\end{tabular}\\};
\node[host, below=of R2] (B) {B};
\path[draw,thick]
(A) edge (R1)
(R1) edge (R2)
(R2) edge (R3)
(R2) edge (B);
\draw[arrow, dashed] (FR1) -- (R1);
\draw[arrow, dashed] (FR2) -- (R2);
\draw[arrow, dashed] (FR3) -- (R3);
`B` sends a reply to `A`. Which of the following affirmations are true ?](_images/tikz-52857e22b3bb13a5414fc9b56e68e50c4d020b9f.png)