[c-nsp] Understanding how ARP works

[Wink]

http://www.tcpipguide.com/free/t_AddressResolutionandtheTCPIPAddressResolutionProto.htm


This might be the best explanation of ARP I have ever read.  This book 
is great!  And its on-line for free!

Derick Winkworth
CCIE #15672



Joost greene wrote:
> Hello everyone,
>
> Please help me confirm my understanding of how basic routing and ARP works,
> assuming the below setup, I'll replay how I think it works hoping for
> correction.
>
>
> If you know of an end-to-end example that is documented anywhere on the net
> or books like the below, please point me to it.
>
>
> HostA---SwitchA----(e0)Router(e1)-----SwitchB-----HostB
>
> *
> *
>
> *On Host A:*
>
>
> HostA wants to ping HostB and so it creates an ICMP packet with Src IP of A
> and Dst IP of B, looks up the routing table and finds a default gateway
> pointing to the Router e0 interface IP address.
>
>
> A second/recursive lookup is made to the routing table to find out how to
> reach the Router's e0 interface and HostA realizes it's directly connected
> because they are on the same subnet.
>
>
> So the Packet gets handed down to Layer 2 and the Src MAC of HostA is added
> to the frame and a lookup is made for the Dst MAC in the arp table but none
> found so an arp broadcast request is sent out.
>
>
> *On SwitchA:*
>
>
> SwitchA receives the request (arp who has) for the IP address of Host B and
> it checks its MAC table but none found so it will broadcast the request to
> all ports and changes the Src MAC to that of the switch port that is
> directly connected to e0 on the router.
>
>
> Router will receive the request and reply with its e0 MAC address and
> SwitchA adds the MAC to its table, so now it replies back to HostA with the
> MAC address of e0.
>
>
> *On Host A:*
>
> MAC address table is now updated and the frame is created and sent to the
> Router.
>
>
> *On SwitchA:*
>
> Frame destined to the router is received and the switch will read only the
> Dst MAC and looks up its table, finds which port to send it and there it
> goes.
>
> *On Router:*
>
> Router de-encapsulates the Layer 2 frame to find the destination IP of HostB
> and looks it up in its routing table, it finds that it's on the same subnet
> as the directly connected interface e1 and so it decides to send it out
> there.
>
> Router knows now where it needs to send this packet and wants to build a
> layer 2 frame for it; we will rewrite the Src MAC to be of e1 before
> sending.
>
> To find out the Dst MAC, Router checks its arp table for this MAC but none
> found so it sends out an ARP request.
>
> SwitchB receives the request and won't find it in its table so it will
> broadcast the request to all ports and then HostB responds and so SwithcB
> and correspondingly the Router has the MAC address of HostB and is now able
> to construct a Layer 2 frame with all needed information.
> Frame is sent out and de-encapsulated at HostB and a reply is sent back and
> thats it.
>
> Src and Dst MAC has been changed along the way but never the Src and Dst IP
> addresses?
>
>
> This is all an effort to understand this thread which is:
>
> http://puck.nether.net/pipermail/cisco-nsp/2005-April/019738.html
>
> Thanks,
> Joost
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