Layer 2 / Layer3 question

chinaman

PPl,

After failing the exam, I started reading the Sybex 5th edition.
However, I just want to know based on the book that when IP has a datagram to send, it must inform the network access protocol such as Ethernet, tokenring, of the destination's hardware address on the local network.

if IP does not found the des. MAC address 802.3 in it's ARP cache it will issue an ARP broadcast to find the information.

NOW I want to know does network layer has the right to know the MAC address, because I am confused that only the layer2 device deals with the 802.3 or MAC. and the L3 to know the destination of the packet it will check the IP destination on the packets and compare it's on it's arp cache if not found else issue arp cache if it's local else send it to default gateway.

AM I CORRECT and please answer my question.
THANKS FOR ALL THE HELP.

EdTheLad

Every layer 3 capable device will have an arp cache,this cache is a mapping from the layer 2 i.e. Mac address to the layer 3 i.e. ip address.
For one PC to talk to another it must know the MAC address, remember that frames are being transmitted.
How to know the remote mac address? ARP is used.ARP is basically a layer 2 broadcast with a built in autoreply.Basically a packet is sent with source and destination IP addresses and a source MAC address, the destination MAC address is all 1's therefore every device on the local broadcast segment will receive the frame unencapsulate it and sent it to its upper layer.Here the packet is analysed and if the destination IP address is matched the packet will be sent ack to the sender,if the IP address is not matched the packet is discarded.
So when the sending device receives the packet back it will record the source MAC address and enter the value in its arp cache corresponding to the IP address.

If at the beginning of this ping,the remote machine was on different subnet, found by comparing the source and destination IP addresses.The packet would be sent wih the MAC address of the default gatewy.If the DG
is unknown an ARP request is sent for the DG ip address and the same behaviour as above follows!

chinaman

Any idea what is proxy ARP? is this new to cisco routers ?

EdTheLad

chinaman wrote:

Any idea what is proxy ARP? is this new to cisco routers ?

http://www.cisco.com/warp/public/105/5.html

SV

ed_the_lad wrote:

If at the beginning of this ping,the remote machine was on different subnet, found by comparing the source and destination IP addresses.The packet would be sent wih the MAC address of the default gatewy.If the DG
is unknown an ARP request is sent for the DG ip address and the same behaviour as above follows!

Lets say the IP Address of PC1 and PC_2 which are in two different networks are as follows
PC_1 = 192.168.0.10
PC_2 = 192.168.1.21

When PC_1 sends a packet to PC_2, the PC_1's network layer will check if the destination IP address belongs to the same network? Since in this case its not in the same network, the packet will have to be forwarded to the default gateway.

I have a doubt. Is it the layer three protocols that makes the call, that is weather to send the packet to DG's ip or not?

If yes, will the layer three pass the packet down to layer 2 with a packet labeled with the DG's ip address so that layer 2 could find the MAC address of the DG?

EdTheLad

SV wrote:

ed_the_lad wrote:

If at the beginning of this ping,the remote machine was on different subnet, found by comparing the source and destination IP addresses.The packet would be sent wih the MAC address of the default gatewy.If the DG
is unknown an ARP request is sent for the DG ip address and the same behaviour as above follows!

Lets say the IP Address of PC1 and PC_2 which are in two different networks are as follows
PC_1 = 192.168.0.10
PC_2 = 192.168.1.21

When PC_1 sends a packet to PC_2, the PC_1's network layer will check if the destination IP address belongs to the same network? Since in this case its not in the same network, the packet will have to be forwarded to the default gateway.

I have a doubt. Is it the layer three protocols that makes the call, that is weather to send the packet to DG's ip or not?

If yes, will the layer three pass the packet down to layer 2 with a packet labeled with the DG's ip address so that layer 2 could find the MAC address of the DG?

Yes to both!