From: Ahsan Mohiuddin (ahsan.mohiuddin@gmail.com)
Date: Sun Sep 28 2008 - 14:48:52 ART
Hello Ali,
when IP Routing is enabled on R1, it looks up the route to 155.1.0.0/24 in
its IP Route table. Upon not finding the desired network in the routing
table, the ping packets to destination 155.1.0.4 are dropped by R1.
However, when you turn off IP Routing on R1, the router essentially behaves
like a Host (similar to a PC connected to a LAN). It assumes that
the destination in question (155.1.0.4) is a directly attached host out one
of its LAN interfaces. So, it sends out ARP requests out all FastEthernet
interfaces for resolving the unknown IP address to a Layer 2 MAC address.
You can see this in "debug arp" output:
*Mar 1 00:10:14.071: IP ARP: creating incomplete entry for IP address:
155.1.0.4 interface FastEthernet0/0
*Mar 1 00:10:14.075: IP ARP: sent req src 155.1.146.1 cc00.0f44.0000,
dst 155.1.0.4 0000.0000.0000 FastEthernet0/0
*Mar 1 00:10:14.079: IP ARP: sent req src 155.1.12.1 cc00.0f44.0020,
dst 155.1.0.4 0000.0000.0000 FastEthernet0/1
*Mar 1 00:10:14.099: IP ARP: rcvd rep src 155.1.0.4 cc02.0f44.0000, dst
155.1.146.1 FastEthernet0/0
You can see that no such activity takes place when IP Routing is enabled on
R1, because router is in fact behaving like a router and thinking, "Well, I
do not know the IP network to which this host belongs, why should I bother
looking up for its Layer 2 address?"
Of course, if R1 did have a route to 155.1.0.4, it will also know the
outbound interface for that route. So, accordingly the ping
packet's destination Layer 2 address will be set to that of the directly
attached neighbor on that link.
Hope this helps,
~Ahsan
On Sun, Sep 28, 2008 at 11:15 PM, faisal saleem <faisal2heaven@live.com>wrote:
> Hi Ali,
> You can check my this post
> ==> http://www.sadikhov.com/forum/index.php?showtopic=126696
> Here ^^ i made same scenario with 3 routers and there has been enough
> debate
> on these simple concepts...
>
> HTH.
>
> > Hello GS,
> >
> > I am testing proxy-arp with the following scenario: R1
> > fa0/0-------155.1.146.0--------fa0/0 R4 s0/0-------155.1.0.0------s0/0 R5
> >
> > R4 has proxy-arp enabled on int Fa0/0.I am able to ping 155.1.0.4 from
> R1
> > only if ip routing is disabled. Would you please explain how proxy arp
> works
> > ?
> >
> > Regards
> > Ali
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
> > _______________________________________________________________________
> > Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
> >
> >
> >
> >
> >
> >
> >
>
> _________________________________________________________________
> Invite your mail contacts to join your friends list with Windows Live
> Spaces.
> It's easy!
>
> http://spaces.live.com/spacesapi.aspx?wx_action=create&wx_url=/friends.aspx&m
> kt=en-us
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
Blogs and organic groups at http://www.ccie.net
This archive was generated by hypermail 2.1.4 : Sat Oct 04 2008 - 09:26:20 ART