01-03-2016 01:50 AM - edited 03-05-2019 03:02 AM
Hi,
I'm doing a quiz:
Router1#show ip arp
Protocol Address Age(min) Hardware Addr Type Interface
Internet 192.168.20.5 9 0000.0cu7.f892 ARPA FastEthernet0/0
Internet 192.168.60.5 8 0000.0cu7.ac00 ARPA FastEthernet0/1
Internet 192.168.20.1 - 0000.0c63.ae45 ARPA FastEthernet0/0
Internet 192.168.40.5 9 0000.0cu7.4320 ARPA FastEthernet0/2
Internet 192.168.60.1 - 0000.0c63.1300 ARPA FastEthernet0/1
Internet 192.168.40.1 - 0000.0c36.6995 ARPA FastEthernet0/2
My question is:
What will Router1 do when it receives the data frame:
Source MAC Source IP Destination MAC Destination IP
0000.0c07.f892 192.168.20.5 0000.0c63.ae45 192.168.40.5
Solved! Go to Solution.
01-07-2016 03:15 AM
Thanks,
the multiple choice gives the three following answares:
A)Router1 will strip off the source MAC address and replace it with the MAC address 000.0c36.6995
B) Router1 will strip off the destination MAC address and replace it with the MAC address 000.0c07.4320
D) Router 1 will forword the data packet out interface FastEthernet0/2
I have a dubt for A answare because from the output I can see a list with two cases of FastEthernet0/2, so why Router 1 replace the source MAC address with 000.0c36.6995 and not with 0000.0c07.4320 ?
The B is correct because I see the the ARP table associates the destination IP address on the frame whith the hardware address 0000.0c07.4320 and as a consequence the output interface will be FastEthernet 0/2
01-06-2016 03:18 AM
hi giacomo12
router is forward a packet on base of ip add so it will forward packet on 192.168.40.5 (fa0/2)...
01-06-2016 11:36 PM
+1 for Tej
it will forward packet on 192.168.40.5
01-06-2016 04:14 AM
I Agree with you Tej. However I would like to add few more details.
Assuming that this is Class C IP address so Both source and destinations are in different subnets. While forwarding the packet out from Fa0/2, Source and Destination IP remains same. But router will rewrite source MAC and destination MAC.
So destination MAC would be 0000.0cu7.4320 and source MAC would be of Fa0/2.
01-06-2016 08:37 AM
I agree with Abhishek and Tej. The frame is first de-encapuslated. If the source MAC was not in the MAC table, the router will look add the source MAC to the MAC address table. Once the frame is stripped of the source and destination MAC address, the router looks at the IP address in the packet. From the routing table, the router chooses the best route to reach the destination. The ARP table shows us that the 192.168.40.5's best route is out of interface f0/2. After which, the router will re-encapsulate the packet with the source of 0000.0c36.6995 (Default Gateway) and a destination of 0000.0cu7.4320.
It's been a while since I have done encapsulation, but I am fairly certain this is correct.
01-07-2016 03:18 AM
Thanks jhed91166,
why source MAC address 0000.0c36.6995 and not 0000.0c07.4320? Why default gateway? How can I see it on the arp table?
01-07-2016 03:15 AM
Thanks,
the multiple choice gives the three following answares:
A)Router1 will strip off the source MAC address and replace it with the MAC address 000.0c36.6995
B) Router1 will strip off the destination MAC address and replace it with the MAC address 000.0c07.4320
D) Router 1 will forword the data packet out interface FastEthernet0/2
I have a dubt for A answare because from the output I can see a list with two cases of FastEthernet0/2, so why Router 1 replace the source MAC address with 000.0c36.6995 and not with 0000.0c07.4320 ?
The B is correct because I see the the ARP table associates the destination IP address on the frame whith the hardware address 0000.0c07.4320 and as a consequence the output interface will be FastEthernet 0/2
01-07-2016 04:52 AM
Yes B is correct, = its been a while since I have done this chapter. After the L2 information is stripped, the router looks at the packet in the routing table to find the outgoing interface. Once it has the outgoing interface's MAC, it adds it to the packet as the source along with the destination MAC.
I apologize about the confusion.
01-07-2016 07:30 AM
Thanks a lot,
I don't understand why the source MAC address should be 000.0c36.6995 if I don't know the MAC address of FastEthernet0/2 on the Router?
01-07-2016 04:52 AM
Yes B is correct, = its been a while since I have done this chapter. After the L2 information is stripped, the router looks at the packet in the routing table to find the outgoing interface. Once it has the outgoing interface's MAC, it adds it to the packet as the source along with the destination MAC.
I apologize about the confusion.
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