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Home > Archive > CCNA > March 2004 > about ICMP msg
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| worrywarm 2004-02-28, 2:27 pm |
| PC1 - (e0)R1(s1) - (S0)R2(S1) - (S0)R3(e0)-PC2
PC1 is trying to communicate with PC2. The e0 interface on R3 is down.
Which of the following are true? (Choose two)
A. R3 will use ICMP to inform PC1 that PC2 cannot be reached.
B. R3 will use ICMP to inform R2 that PC2 cannot be reached.
C. R3 will use ICMP to inform PC1, R1, and R2 that PC2 cannot be reached.
D. R3 will send a Destination Unreachable message type.
E. R3 will send a Router Selection message type.
F. R3 will send a Source Quench message type.
Answer: B, D
my question is why we chose B instead of A? the layer 3 packet being fowarded from R2 to R3 has a source IP address of PC1, destination IP address of PC2. Therefore, R3 should notify PC1 that PC2 is unreachable.
?????? anyone can explain??
THanks a lot!
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| Yankee 2004-02-29, 8:56 am |
| Let me try...
The layer 3 address of the destination is as you say PC1, but all R3 knows is that PC1 is located somewhere out its S0. Because S0 is a point to point there is no data link address (layer 2), R3 knows to just dump the message out S0 and R2 will get it, but to be totally technical R2 is R3's destination for that message. R2 will then do the same thing to get it to R1 who will finally get it to PC1.
The question is trying to get you to understand that there are hop by hop layer two destinations along the way to the layer 3 final destination.
I'm sure somebody else can say this better than I did,
Yankee | |
| worrywarm 2004-02-29, 11:10 am |
| I think I understand what you are trying to tell me, but one thing I'm confused about is that peer-to-peer layer are through encapsulation of frames,packets...In this case, what are forwarded between the serial ports of two routers? layer 3 packets. Although the packet is sent back to the R1, but the content of the packet would not show anything of R1 but the PC1...I'm not sure of what I'm talking about, lol...
IF I still cann't understand, I may have to memorize this situation
THank you, yankee! | |
| dmaftei 2004-02-29, 12:48 pm |
| quote:
Originally posted by worrywarm
my question is why we chose B instead of A? the layer 3 packet being fowarded from R2 to R3 has a source IP address of PC1, destination IP address of PC2. Therefore, R3 should notify PC1 that PC2 is unreachable.
Your reasoning is correct. ICMP "destination unreachable" is sent back to the IP source of the undelivarable datagram. | |
| worrywarm 2004-02-29, 1:25 pm |
| thank you! dmaftei
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| Yankee 2004-03-01, 5:06 am |
| I liked my answer :P
Yankee |
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