IP Communication Question

[silvercas]

I have some questions on arp,mac and ip

if PC 3 is going to be communicating with Server and initates an ARP request how is it handled in the below diagram and why?


Router A ------ Serial Connection ------- Router B
| |
Switch A Switch B
/ \ \
Server PC 2 PC 3

 

[unclerico]

Basically the communication will be like this:

1) When PC3 attempts to communicate with Server it will do a binary calculation on the destination IP address.
2) If the destination IP address is not in PC3's local routing table it will arp for it's default gateway (assuming that the gateway's info is not it it's arp cache)
3) PC3 will send the packets to the gateway
4) Router B will de-encapsulate the layer 2 information and look at the layer 3 information (source/destination IP address).
5) Router B will consult its routing table to determine if it has a route to the destination IP address
6) If it does it will re-encapsulate the layer 2 information depending on what encapsulation is in use on the serial connection. It will forward the packets to Router A
7) Router A will receive the packets. It will de-encapsulate the layer 2 informaino and look at the layer 3 information (source/destination IP addrss)
8) Router A will consult its routing table to determine if it has a route to the destination
9) Since it does it will send an ARP request (if it is not cached) for Server.
10) Server will reply with it's MAC
11) Router A will re-encapsulate the layer 2 information with its local interface as the source MAC and Server as the destination MAC
12) Router A will forward the packet to Server
13) Rinse and repeat

This is assuming that the switches in question are Layer 2 switches.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[burtsbees]

The IP is stripped on the hops and replaced with the new destination IP. The MAC stays the same throughout. This is in the IP header.

Burt

 

[unclerico]

Layer 3 info isn't messed with, it's the layer 2 info that is de-encapsulated and re-encapsulated.

while the source and destination IP address of the packet remain unchanged from sending host to receiving host, the data link source and destination addresses change each time a router builds a new data link header when forwarding a packet

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[burtsbees]

Whoops...m'I sdrawkcab...yrros...

Turb

 

[unclerico]

now you owe me a Minnesota Wild vs. St. Louis Blues game ;-)

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[burtsbees]

Next time you're in St. Louis...be glad to go!

Burt

 

[silvercas]

doesnt the mac get changed at this point to the gateways mac on router a?

6) If it does it will re-encapsulate the layer 2 information depending on what encapsulation is in use on the serial connection. It will forward the packets to Router A