show ip arp

[giacomo12]

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

[Tej Savsani]

hi giacomo12

 router is forward a packet on base of ip add so it will forward packet on 192.168.40.5 (fa0/2)...   

[Jellygrade]

+1 for Tej 

it will forward packet on 192.168.40.5

[Abhishek Soni]

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.

[jhed91166]

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.

[giacomo12]

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?

[giacomo12 · Accepted solution]

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

[jhed91166]

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.

[giacomo12]

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?

[jhed91166]

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.