[c-nsp] Understanding how ARP works

[Gert Doering]

Hi,

On Sat, Jun 07, 2008 at 04:46:44PM +0300, Joost greene wrote:
> 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.

Your assumptions are correct, with one small exception:

> 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.

The ARP *request* is actually sent as ethernet broadcast, so the switch 
just broadcasts the packet.  (Since the host does not know the target MAC
address, it *needs* to send it as broadcast...)

> *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.

Actually it will usually build a new MAC header, instead of just taking
the ethernet header and "rewrite fields" (the next hop could be a PPP
link, or such) - but effectively, for Ethernet-Ethernet, this is what 
happens.

(The IPv4 TTL will be also decremented, and if zero, the packet dropped)

> Src and Dst MAC has been changed along the way but never the Src and Dst IP
> addresses?

Correct.

gert
-- 
USENET is *not* the non-clickable part of WWW!
                                                           //www.muc.de/~gert/
Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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