(Ethernet Bridges, i.e. switches) Consider the above extended LAN, where squares

Question

(Ethernet Bridges, i.e. switches)

Consider the above extended LAN, where squares are bridges (also known as switches) and circles are hosts.

Assume we start in a state where all bridges have no knowledge of any hosts.

a) Assume first that X sends an ARP REQUEST message looking for Y. After this completes, what has each bridge learned?

b) Assume then that Y sends an ARP REPLY message to X. After this completes, what has each bridge learned?

c) Assume that after (a) and (b) above complete, Z sends an ARP REQUEST looking for Y. After this completes, what has each bridge learned?

Explanation / Answer

Solution:

a) Assume first that X sends an ARP REQUEST message looking for Y. After this completes, what has each bridge learned?

Answer:

- Host X sends an ARP Request to Bridge F.

- Bridge F doesn't have any MAC entries in its ARP cache. F will broacast the ARP request and add MAC adress and of Host X in its table.

- SImilarly, Other Bridges doesn't have MAC entry of Host X. So, it just broadcasts the ARP request of X and adds the MAC entry of X in its table.

So, Each Bridge learns MAC address of Host X with respective Port number.

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b) Assume then that Y sends an ARP REPLY message to X. After this completes, what has each bridge learned?

Answer:

- When Y sends an ARP Reply message. The message travels through Bridges D, E and F as host Y know the MAC address of host X from ARP request which host X had sent.

So, Bridges D, E and F learns the MAC address of host Y as well when it forwards the ARP Reply message to host X.

Other Bridges A, B and C will not learn any new entries.

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c) Assume that after (a) and (b) above complete, Z sends an ARP REQUEST looking for Y. After this completes, what has each bridge learned?

Answer:

Now, when Z sends an ARP REQUEST looking for Y.

Z sends an ARP request to B, which does not know the address of host Y. So, it will broadcast the ARP Request of Z to Bridges A, E and D.

Now, Bridges A, B, D and E has learnt the MAC address of Host Z.

- Now, When D receives the ARP request it does know the address of host Y. So, it will forward the request to host Y.

and Bridge E will also forward the request to Bridge D as it also knows the address of host Y. But the Message will be dropped by Bridge D as it is found duplicate.

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